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Process regime, salinity, morphological, and sedimentary trends along the fluvial to marine transition zone of the mixed-energy Mekong River delta, Vietnam
... Although the sedimentary characteristics of the FTT have been investigated extensively in modern systems (Carling et al., 2015;Dalrymple et al., 2003;Gugliotta et al., 2017;Nittrouer et al., 2021;Prokocki et al., 2020;Roche, 2022;Smith, 1988) as well in ancient strata (Gugliotta et al., 2016;Jablonski & Dalrymple, 2016;La Croix et al., 2019;Smith, 1987), most studies have focused on characterizing either the hydrodynamic processes operating within the FTT (Jay et al., 2015;Keevil et al., 2015;Wu et al., 2022), or the resulting record of sedimentation Van den Berg et al., 2007). Few studies have attempted to link real-time quantitative measurements of flow parameters to their associated sedimentary deposits. ...
... One major focus of research on the FTT has been the development of generalized depositional models explaining the down-river transition in sedimentary characteristics (which record the time average of sedimentation processes across a range of time scales) from the landward extent of tidal effects into marine-dominated conditions at the seaward extent. Several trends have been identified as useful proxies for determining longitudinal position in the FTT including trends in facies characteristics (Choi et al., 2004;Gugliotta et al., 2017;Roche, 2022;Van den Berg et al., 2007), stratigraphic architecture (Choi et al., 2021), geomorphology (Dalrymple & Choi, 2007;Gugliotta & Saito, 2019), ichnology , sediment geochemistry (Czarnecki et al., 2014;Dashtgard et al., 2022;A. Wang et al., 2021), as well as calcareous microfossils (Armstrong & Brasier, 2005) and palynology (Czarnecki et al., 2014;Dashtgard et al., 2022). ...
... The trend in the proportion of mud in surface sediments along the FTT of the Waihou River is similar but less well-defined than other tidally influenced systems globally such as the Fraser River Delta (La Croix & Dashtgard, 2014), Mekong River Delta (Gugliotta et al., 2017), and Changjiang River Delta (Z. Wang et al., 2009). ...
The fluvial–tidal transition zone (FTT) is a critical interface where complex interactions between river flow, tides, and sedimentation shape geomorphic systems and influence the dynamics of aquatic environments. However, few previous studies have integrated real‐time hydrodynamic data with sedimentary deposits. In particular, the range of depositional conditions over which mud accumulates remains poorly constrained, and little is understood about how these deposits are preserved in the stratigraphic record. To address this knowledge gap, we examined co‐located hydrodynamic instrument data and sedimentary deposits from the lower Waihou River, Aotearoa New Zealand. Results reveal that “dynamic mud” events, including fluid mud and rapidly deposited mud, dominate the spatial and temporal record, with few “static mud” events in which mud is deposited through gravitational settling. We suggest that dynamic mud conditions with the potential for deposition may occur throughout the tidal cycle, although cyclic tidal successions are never fully preserved. Many of the trends in sedimentation observed in studies of larger systems are not present in this small muddy river system, indicating the significance of climatic and river‐flow characteristics on the sedimentary record. This work underscores the importance of studying systems of multiple sizes across diverse climatic regimes to establish holistic facies models to reconstruct geological history accurately.
... A species accumulation curve was approximated using the rational function, and the asymptote indi- Gugliotta et al., 2017; rose fill-zone of sand mining according to Le Meur et al., 2020. Sampling points are numbered following the data paper (Seleznev et al., 2023). ...
... According to generalized data considering April and December, 88% of group III communities were situated in zone IV of the frontal section, 65% of group II in the frontal section: zone III (24%) and IV (41%); but 60% of group I in the freshwater section (30% in zones I and II both) (Tables 1, 2). Thus, β-diversity groups of communities III and II are more typical for our frontal hydrological section (tide-dominated tract according to Gugliotta et al., 2017) and group I for our freshwater section (fluvial-dominated tract according to Gugliotta et al., 2017). ...
... According to generalized data considering April and December, 88% of group III communities were situated in zone IV of the frontal section, 65% of group II in the frontal section: zone III (24%) and IV (41%); but 60% of group I in the freshwater section (30% in zones I and II both) (Tables 1, 2). Thus, β-diversity groups of communities III and II are more typical for our frontal hydrological section (tide-dominated tract according to Gugliotta et al., 2017) and group I for our freshwater section (fluvial-dominated tract according to Gugliotta et al., 2017). ...
... Studies in which channel slope and depth are derived from datasets that cover the river path continuously over stretches longer than the backwater length are few (Nittrouer et al., 2011;Gugliotta et al., 2017). This reflects the efforts (e.g. ...
... Subsequently, slope and depth estimates from only one single location can't provide representative parameters. Therefore, the preferred method to estimate backwater length in modern rivers is to use datasets with channel slope and thalweg depth covering the river path over long enough distances to identify the point where the riverbed elevation intersects sea level (Nittrouer et al., 2011;Gugliotta et al., 2017). By doing so, the locally irregular riverbed profile is averaged over a longer section, and subjectivity and ambiguity in obtaining slope and depth from one or a few selected locations or a certain section of the river path are minimized. ...
... There are two fundamental approaches to estimating the backwater length in modern systems, i) the the intersection method in which the distance between the river mouth and the location where riverbed elevation intersects sea level provides the backwater length (Nittrouer et al., 2011;Blum et al., 2013;Fernandes et al., 2016;Gugliotta et al., 2017;Smith et al., 2020) and ii) indirect estimates of J o u r n a l P r e -p r o o f Journal Pre-proof backwater length (L b ) by calculating L b = h/S, with h is bankfull thalweg channel depth and S is slope (Paola & Mohrig, 1996;Hartley et al., 2016;Ganti et al., 2016;Brooke et al., 2020Brooke et al., , 2022Prasojo et al., 2022). ...
... These systems have considerably altered the natural hydrodynamics and sediment transport [11,12]. Recently, researchers have paid more attention to the whole domain through extensive monitoring networks [13,14] and satellite observations [7,15] and by applying 1D [16][17][18], semi/quasi 2D [19] or 1D-2D coupled flow simulation models [8,12]. Other studies concentrated on the impacts of dyke systems on hydrodynamics [20,21]. ...
... Nowadays, these structures comprise 7,000 km of main channels, 4,000 km of secondary on-farm channel systems, 193 spillways, 409 reservoirs, 528 junctions, 29 sluices and 749 compartments [37]. The system is concentrated on flood-prone areas to efficiently drain out floodwater from the LXQ and the PoR to the Gulf of Thailand (the West Sea of Vietnam) and to the Vam Co River before debouching into the East Sea of Vietnam [14]. The channel system in the VMD is fully interconnected, without separation between irrigation channels and drainage ones [42]. ...
... The VMD estuaries experience semidiurnal tides (M2, S2, N2, K2) originating from the East Sea of Vietnam with amplitudes of 1 -3.5 m and diurnal tides (O1 and K1) from the West Sea of Vietnam with amplitudes of 0.8 -1 m [8]. The tides also influence the hydrodynamic condition in the VMD, as identified by [14] through bed sample analysis and by [34] through coupled numerical and in situ/satellite observations. ...
The Mekong Delta has the world’s third-largest surface area. It plays an indisputable role in the economy and livelihoods of Vietnam and Cambodia, with repercussions at regional and global scales. During recent decades, the Vietnamese part of the Mekong Delta underwent profound human interventions (construction of dykes and multi-channel networks), which modified the hydrodynamic regime, especially cycles of field submersion. In this study, we first applied a full 2D numerical hydraulic model, TELEMAC-2D, to examine the effects of the complex channel and river networks on the spatial and temporal distribution of the flow in the 40,000 km2 of the Vietnamese Mekong Delta. Then, two scenarios of relative sea-level rise in 2050 and 2100 were implemented to simulate the future patterns of water fluxes in the delta. The results show that dykes and multi-channel networks would reduce the inundation area by 36% and lessen the peak water level by 15% and the discharge over the floodplains by 24%. Despite this protection, under a relative sea-level rise of 30 cm and 100 cm, the maximum flooded area could occupy about 69% and 85% of the whole delta in 2050 and 2100, respectively.
... Furthermore, the VMD may be classified as a wave-influenced, tidedominated delta (Wright, 1985). With an average tidal range of around 2.5 ± 0.1 m at its mouth (Gugliotta et al., 2017), the delta may be classified as mesotidal (Davies, 1964). Tidal variations can still be observed beyond the border to Cambodia, even though the tidal range diminishes in landward direction (Gugliotta et al., 2017). ...
... With an average tidal range of around 2.5 ± 0.1 m at its mouth (Gugliotta et al., 2017), the delta may be classified as mesotidal (Davies, 1964). Tidal variations can still be observed beyond the border to Cambodia, even though the tidal range diminishes in landward direction (Gugliotta et al., 2017). ...
... The sediment inventory is highly variable in the lower 100 km of the distributary channels, whereas sand dominates in the upstream parts of both the Tien and Hau River (Gugliotta et al., 2017). Estimates for the yearly sediment load at Kratie vary between 40-160 Mt/yr (Milliman & Meade, 1983;Nowacki et al., 2015). ...
... The VMD is located in the fluvial-to-marine transition zone, which is divided into two distinctive zones: the upstream, fluvial-dominated zone and the downstream, tide-dominated zone (Gugliotta et al., 2017). The boundary between these zones is at the My Thuan and Can Tho gauging stations (Fig. 1a). ...
... During the flood season, tidal influence is limited to the upper VMD (e.g., at Chau Doc) compared to the lower VMD (e.g., at Can Tho) ( Fig. 1c) due to high riverine fluvial discharges. However, tide-driven water level fluctuations are significant during the dry season (e.g., approximately 1 m at Tan Chau and Chau Doc and 2 m at My Thuan and Can Tho) (Gugliotta et al., 2017). The flow is bidirectional during the dry season because of the interaction between the semidiurnal tide from the East Vietnam Sea and the riverine discharge from the Mekong River. ...
... The SSL is dominated by silt and clay, accounting for 95 to 98 % of the total load (Koehnken, 2014;Binh et al., 2020b). Bedload, composed of fine sand, constitutes only 1 to 3 % of the total annual load (Gugliotta et al., 2017;Jordan et al., 2019;Hackney et al., 2020). Fig. 2 shows a methodological flowchart. ...
Flow, suspended sediment transport and associated morphological changes in the Vietnamese Mekong Delta (VMD) are studied using field survey data and a two-dimensional (2D) depth-averaged hydromorphodynamic numerical model. The results show that approximately 61–81 % of the suspended sediment load in the Hau River during the flood seasons is diverted from the Tien River by a water and suspended sediment diversion channel. Tidal effects on flow and suspended sediment load are more pronounced in the Hau River than in the Tien River. The results show the formation of nine scour holes in the Tien River and seven scour holes in the Hau River from 2014 to 2017. Additional six scour holes are likely to form by the end of 2026 if the suspended sediment supply is reduced by 85 % due to damming. Notably, the scour holes are likely to form at locations of severe riverbank erosion. In the entire study area, the simulated total net incision volume in 2014–2017 is approximately 196 Mm³ (equivalent to 65.3 Mm³/yr). The predicted total net incision volumes from 2017 to 2026 are approximately 2472 and 3316 Mm³ under the 18 % and 85 % suspended sediment reduction scenarios, respectively, thereby likely threatening the delta sustainability. The methodology developed in this study is helpful in providing researchers and decision-makers with one way to predict numerically the scour hole formation and its association with riverbank stability in river deltas. Of equal importance, this research serves as a useful reference on the role of water and suspended sediment diversion channels in balancing landforms in river-delta systems, particularly where artificial diversion channels are planned.
... Among challenges facing the Mekong Delta, especially over the past decade, SWI is perhaps among the highest in the list as the key indicator to land use and strategic development (Mekong Delta Plan, 2013). In principle, SWI is associated with the dry season (Gugliotta et al., 2017;Nowacki et al., 2015). The flood pulse limits SWI during wet season to only a few kilometers, versus tens of kilometers during the dry season (Gugliotta et al., 2017;Nowacki et al., 2015) -affecting 1.3 million ha of the VMD (Carew-Reid, 2008;Smajgl et al., 2015). ...
... In principle, SWI is associated with the dry season (Gugliotta et al., 2017;Nowacki et al., 2015). The flood pulse limits SWI during wet season to only a few kilometers, versus tens of kilometers during the dry season (Gugliotta et al., 2017;Nowacki et al., 2015) -affecting 1.3 million ha of the VMD (Carew-Reid, 2008;Smajgl et al., 2015). In the pristine MR, the flood pulse inundated the Tonle Sap Lake (TSL, see Figure 1.2) and Cambodian floodplains, functioning as retention areas that were drained during the dry season. ...
... The baroclinic dynamics of a deltaic system can be perplexing as it often demands a thorough analysis of tides, freshwater dynamics, 3D processes and riverocean interaction. In previous studies of the Mekong Delta, field measurements have limited temporal/spatial coverage (Gugliotta et al., 2017;Nguyen and Savenije, 2006;Nowacki et al., 2015), and numerical studies often suffer from level of detail, resolution and/or spatial coverage (Smajgl et al., 2015;Tran Anh et al., 2018a;Tran Quoc Dat et al., 2011;Vu et al., 2018) due to computational or data limitations (See Appendix-B). Therefore, this study was motivated, and based on a series of principles: a) it follows a total system approach, i.e., it incorporates the interactions between large, small, natural and human-made elements of the delta, b) integrates the upstream-downstream-coastal interactions, and c) reveals the role of anthropogenic drives -such as sand mining and extraction-induced land subsidence on past, present and future trends of salt intrusion -as well as their relative importance in the context of global climate change. ...
Natural resources of the transboundary Mekong River are existential to the livelihood of tens of millions of people. Numerous mainstream and tributary impoundments, overexploitation of groundwater resources as well as excessive sand mining are emblematic of rapid economic development in the region. These significantly strain the riverine resources, and reflect in hydrological regime shift, biodiversity decline and sediment starvation. The Mekong Delta, indispensable to Vietnam’s food and economic security, is impacted by both climatic and anthropogenic drivers of change, within and beyond the delta. Global sea level rise, river discharge anomalies, land subsidence, riverbed/bank and coastal erosion and saline water intrusion are among the most pressing challenges. Among them, increased saline water intrusion, not only has cost the delta millions of dollars yearly in freshwater shortage and crop loss, but is also identified as the key to its strategic land use planning. The present research aims to demonstrate the combined and isolated effects of climatic and anthropogenic drivers of change on past, present, and future dynamics of salt intrusion in the world’s 3rd largest delta. To fill the knowledge gaps, we addressed five topics in this research: 1) we developed a 1D-2D numerical model of the delta to study flow division and barotropic tidal dynamics within the multi-channel estuarine system; 2) we studied the historical trends of tides and salinity in the delta; 3) we developed the first 3D numerical model of the entire Mekong Delta and identified the physical processes leading to increased salinity in detail; 4) we used the 3D model to project saline water intrusion over the next three decades for the delta; 5) we expanded on policy implications of the main findings for the delta and the basin, applicable to similar systems worldwide.
... All rights reserved 2017; Levell et al., 2019), but in modern systems they show mud-dominated or highly heterolithic sediments, although they could be sandier in the upstream river-dominated portion of their channels or in locally wave-dominated beach ridges (e.g. Harris et al., 1994;Wang et al., 2009;Tamura et al., 2012;Gugliotta et al., 2017). Based on this difference in grain size, together with other incongruences and uncertainties regarding sedimentary structures as diagnostic indicators of specific processes (e.g. ...
... Furthermore, although studies of modern environments are also useful in this sense, core-based studies of Late Holocene successions allow a better stratigraphic control compared, for example, to surface grab samples or measurements of suspended-sediment concentrations in modern environments (e.g. Nowacki et al., 2015;Gugliotta et al., 2017). Finally, if modern environments are subjected to various modifications on the hydro-sedimentary regime and morphology because of human impacts through dams, sand mining, dykes and others (see Anthony et al., 2014;Li et al., 2017), in the Late Holocene successions these issues are much less relevant and more natural conditions can be expected. ...
... The shoreline area is mesotidal, with tides observed in channels more than 300 km upstream of the river mouths and brackish water intruding up to approximately 50 km during the dry season (Gugliotta et al., 2017). The delta plain is intersected by numerous beach ridges, seven active distributary channels and the abandoned Ba Lai palaeochannel (Fig. 1B), in addition to a dense network of smaller natural and artificial canals. ...
To better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as its pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record.
... Given the gravity of SWI as a threat to the livelihood of the VMD and other major deltas around the world that also face SWI such as Ganges-Brahmaputra, Indus, Mississippi, Chao Phraya, Orinoco, Ayeyarwady, Red, and Pearl River deltas (Bucx et al., 2014;Echezuría et al., 2002;Gong and Shen, 2011a;Hoeppner et al., 2008;Nguyen et al., 2017;Rabbani et al., 2013;Takeda et al., 2016;Zia et al., 2017), the available literature addressing SWI in the VMD is limited. Some field measurements (Gugliotta et al., 2017;Nguyen and Savenije, 2006;Nowacki et al., 2015) provide snapshots of specific periods or regions form a qualitative impression of salinity within the VMD. Modelling efforts are bound to analytical models (e.g. ...
... Spring tides are dominated by semi-diurnal tidal species (tidal range ∼ 3 m) and neap tides are determined by diurnal tidal components (tidal range ∼ 2 m), defining a sharp spring-neap variability. Morphologically, the lower ∼ 80 km of the chan-nels is tide-dominated and fluvially influenced, with widening and shallowing channels seaward and significant mud content in the seabed (Gugliotta et al., 2017). Between that lower ∼ 80 km and Phnom Penh, Cambodia, the channels are fluvially dominated and tidally influenced, with meandering channels that deepen seaward and little mud content (Gugliotta et al., 2017). ...
... Morphologically, the lower ∼ 80 km of the chan-nels is tide-dominated and fluvially influenced, with widening and shallowing channels seaward and significant mud content in the seabed (Gugliotta et al., 2017). Between that lower ∼ 80 km and Phnom Penh, Cambodia, the channels are fluvially dominated and tidally influenced, with meandering channels that deepen seaward and little mud content (Gugliotta et al., 2017). The VMD is characterized by strong monsoon-driven seasonal climate variations (Mekong River Commission, 2010), with the wet (July to October, SW monsoon) and dry (December-May, NE monsoon) seasons contrasting dramatically. ...
On the list of challenges facing the world largest deltas, increased saline water intrusion (SWI) in the surface water system and its role in jeopardizing freshwater supply are often ranked very high. Yet, detailed process-based studies of SWI at the whole delta scale are limited, and the trends are regularly associated with global sea level rise. Here, using field measurements and a sophisticated 3D model that integrates the riverine, rural, estuarine, and coastal dynamics within one numerical domain, we study SWI at the scale of the Mekong Delta in extensive detail. While many studies downscale the SWI problem to a topic within an estuary, we show that the physical processes on the continental shelf, such as monsoon-driven ocean surge, directly influence salinity dynamics within the delta. Typical values of 20–40 cm surge over the continental shelf contribute to up to 10 km of further SWI. The delta's estuarine system is also more sensitive than many other systems to variations of river discharge. Furthermore, spring–neap variability plays a key role in SWI in the delta. The estuarine variability from a stratified to a mixed system between neap and spring tides develops 3D processes such as estuarine circulation and tidal straining that become the main upstream salt transport mechanisms. The 3D nature of salinity dynamics, and the role of upstream and downstream processes, suggests that compromising on dimension or extent of the numerical domain can limit the accuracy of predictions of SWI in the delta. The study also showcases the fact that riverbed incision in response to anthropogenic sediment starvation in the last 2 decades has increased stratification and activated or magnified 3D salt transport subprocesses that amplify upstream salt transport. With all the external forces on the delta, namely climate change and an altered hydrological regime by the upstream dams, due to deeper estuarine channels (driven by sand mining and upstream impoundments) compared to its near past, the delta itself has become far more vulnerable to even mild natural events. This exemplifies the fundamental importance of preserving the sediment budget and riverbed levels in protecting the world's deltas against SWI.
... Brooke et al. (2022) note that many avulsion locations on deltas lay within the backwater zone, but 38% of delta avulsions occurred upstream of the backwater zone. Unfortunately, these simple approaches do not adequately account for the entire tidal-influenced fluvial zone (Gugliotta et al., 2017;Sandbach et al., 2018;Gugliotta and Saito, 2019), the impact of storm surge heights, or the seasonality of fluvial discharge levels on tidal duration asymmetry (Yu et al., 2020). ...
... Mekong river delta; Tamura et al., 2009;Ishii et al., 2021), or abandoned marine terraces surrounding fluvial plains caused by relative fall of sea level since the Middle Holocene (e.g., Yangtze, and Red river deltas: Tanabe et al., 2006;Funabiki et al., 2012;Song et al., 2013). The backwater of these river systems is landward of the delta apex and the upper delta plain because of their low gradients and channel depths of 20 m or more (~1200 km in the Yangtze case Wang et al., 2009; ~600 km in the Mekong case, Gugliotta et al., 2017). ...
Deltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ~1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ~4 mm/y.
At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km²) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10–1000 km²) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1–10 km²), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators.
... La Niña and El Niño have caused periodic occurrences of extreme floods (e.g., 1996, 2000, and 2011) and droughts (e.g., 1993, 1998, 2005, 2010, 2015, and 2020), resulting in tremendous damage to the delta. The flow regime in the delta is influenced by tides, with strong tidal effects in the dry season (peak in dry months) and fewer tidal effects in the flood season (Gugliotta et al., 2017). In dry seasons, tidal effects are observable at Phnom Penh, Cambodia, which is approximately 320 km from the river mouth (Gugliotta et al., 2017). ...
... The flow regime in the delta is influenced by tides, with strong tidal effects in the dry season (peak in dry months) and fewer tidal effects in the flood season (Gugliotta et al., 2017). In dry seasons, tidal effects are observable at Phnom Penh, Cambodia, which is approximately 320 km from the river mouth (Gugliotta et al., 2017). The semidiurnal tide in the East Vietnam Sea (Figure 1a) causes the discharge hydrograph to have two peaks and two troughs daily. ...
Key Points ML methods can reliably reconstruct missing daily discharge values. The MARS and RF models outperform other ML and RC methods. Data pre-processing reduces the simulation time and effort. Abstract In this study, six machine learning (ML) models, namely, random forest (RF), Gaussian process regression (GPR), support vector regression (SVR), decision tree (DT), least squares support vector machine (LSSVM), and multivariate adaptive regression spline (MARS) models, were employed to reconstruct the missing daily-averaged discharge in a mega-delta from 1980-2015 using upstream-downstream multi-station data. The performance and accuracy of each ML model were assessed and compared with the stage-discharge rating curves (RCs) using four statistical indicators, Taylor diagrams, violin plots, scatter plots, time-series plots, and heatmaps. Model input selection was performed using mutual information and correlation coefficient methods after three data pre-processing steps: normalization, Fourier series fitting, and first-order differencing. The results showed that the ML models are superior to their RC counterparts, and MARS and RF are the most reliable algorithms, although MARS achieves marginally better performance than RF. Compared to RC, MARS and RF reduced the root mean square error (RMSE) by 135% and 141% and the mean absolute error (MAE) by 194% and 179%, respectively, using year-round data. However, the performance of MARS and RF developed for the climbing (wet season) and recession (dry season) limbs separately worsened slightly compared to that developed using the year-round data. Specifically, the RMSE of MARS and RF in the falling limb was 856 and 1040 m 3 /s, respectively, while that obtained using the year-round data was 768 and 789 m 3 /s, respectively. In this study, the DT model is not recommended, while the GPR and SVR models provide acceptable results.
... The suspended sediment characteristics vary spatially in the delta. Grain sizes are generally coarser upstream, and finer downstream (Gugliotta et al., 2017). The median grain sizes 50 are 15 µm and 3.9 µm in the upstream (near Tan Chau) and downstream estuary reaches, respectively (Hung et al., 2014b;Wolanski et al., 1996). ...
... During the high flow season, the Mekong River supplies a substantial amount of sediment to the sea due to seaward residual velocity. During the low flow season, the tidal processes cause a small amount of landward sediment import (Gugliotta et al., 2017;Nowacki et al., 2015;Xing et al., 2017). The landward residual sediment flux is resulted by baroclinic effects (Nowacki et al., 2015). ...
... The suspended sediment characteristics vary spatially in the delta. Grain sizes are generally coarser upstream, and finer downstream (Gugliotta et al., 2017). The median grain sizes 50 are 15 µm and 3.9 µm in the upstream (near Tan Chau) and downstream estuary reaches, respectively (Hung et al., 2014b;Wolanski et al., 1996). ...
... During the high flow season, the Mekong River supplies a substantial amount of sediment to the sea due to seaward residual velocity. During the low flow season, the tidal processes cause a small amount of landward sediment import (Gugliotta et al., 2017;Nowacki et al., 2015;Xing et al., 2017). The landward residual sediment flux is resulted by baroclinic effects (Nowacki et al., 2015). ...
... The suspended sediment characteristics vary spatially in the delta. Grain sizes are generally coarser upstream, and finer downstream (Gugliotta et al., 2017). The median grain sizes 50 are 15 µm and 3.9 µm in the upstream (near Tan Chau) and downstream estuary reaches, respectively (Hung et al., 2014b;Wolanski et al., 1996). ...
... During the high flow season, the Mekong River supplies a substantial amount of sediment to the sea due to seaward residual velocity. During the low flow season, the tidal processes cause a small amount of landward sediment import (Gugliotta et al., 2017;Nowacki et al., 2015;Xing et al., 2017). The landward residual sediment flux is resulted by baroclinic effects (Nowacki et al., 2015). ...
... The suspended sediment characteristics vary spatially in the delta. Grain sizes are generally coarser upstream, and finer downstream (Gugliotta et al., 2017). The median grain sizes 50 are 15 µm and 3.9 µm in the upstream (near Tan Chau) and downstream estuary reaches, respectively (Hung et al., 2014b;Wolanski et al., 1996). ...
... During the high flow season, the Mekong River supplies a substantial amount of sediment to the sea due to seaward residual velocity. During the low flow season, the tidal processes cause a small amount of landward sediment import (Gugliotta et al., 2017;Nowacki et al., 2015;Xing et al., 2017). The landward residual sediment flux is resulted by baroclinic effects (Nowacki et al., 2015). ...
Deltas are low-lying plains which are formed when river sediments deposit in coastal environments. Deltas are nutrient-rich, and productive ecological and agricultural areas with high socio-economic importance. Globally, deltas are home to about 500 million people and are considerably modified by human activities. In addition, they are vulnerable to climate change and natural hazards like changing river flow and sediment supply, coastal flooding by storminess or sea level rise. To encourage better delta management and planning, it is of utmost importance to understand existing delta sediment dynamics. The objective of this study is to investigate the prevailing sediment dynamics and the sediment budget in the Mekong Delta by using a process-based model. Understanding sediment dynamics for the Mekong Delta requires high resolution analysis and detailed data, which is a challenge for managers and scientists. This study introduces such an approach and focuses on modeling the entire system with a process-based approach, Delft3D-4 and Delft3D Flexible Mesh (DFM). The first model is used to explore sediment dynamics at the coastal zone. The latter model allows straightforward coupling of 1D and 2D grids, making it suitable for analysing the complex river and canal network of the Mekong Delta. This study starts by generating trustworthy bathymetries based on limited data availability. It describes a new interpolation method for reproducing the main meandering channel topographies of the Mekong River. The reproduced topographies are validated against high resolution measured data. The proposed method is capable of reproducing the thalweg accurately. Next, this study describes the development of a Delft3D Mekong Delta model. The model is validated for hydrodynamics and sediment dynamics data for several years and focuses on describing near shore sediment dynamics. The model shows that sediment transport changes in the Mekong Delta are strongly modulated by seasonally varying river discharges and monsoons. The nearshore suspended sediment concentration (SSC) is significantly decreased due to a lack of wave-induced stirring when there is no monsoon. 3D Gravitational circulation effects limit the SSC field from expanding seaward in case of high river flow. In addition, the bed composition has an important role in reproducing sediment fluxes which were considerably decreased when a sandy bed layer is included. This happens due to effects of the initially mostly sandy mixing layer, where resuspension of the mud is proportional to the fraction of mud present. It takes time for an equilibrium bed composition to develop. Seasonally, the sediment volumes deposited in the river mouths increase regularly during the high flow season. During October they remain more or less constant and then, as wave action increases and discharges decrease, the deposited material is resuspended and transported southward along the coast. The DFM model explores the hydrodynamics and sediment dynamics in the fluvial reach of the Mekong River including the anthropogenic effect of dyke construction. After an extremely high flood in 2000 which caused huge damages, a dyke system has been built to protect agriculture in the Vietnamese Mekong Delta (VMD). These structures change hydrodynamic characteristics on floodplains by avoiding floodwaters coming into the floodplains. The DFM model shows that the high dykes slightly change hydrodynamics in the VMD downstream. These structures increase daily mean water levels and tidal amplitudes along the mainstreams. Interestingly, the floodplains protected by high dykes in Long Xuyen Quadrangle and Plain of Reeds influence water regimes not only on the directly linked Mekong branch, but also on other branches. Based on the validated hydrodynamic model, the model is validated against sediment data and used to derive a sediment budget for the Mekong Delta. For the first time, this study has computed sediment dynamics over the entire Mekong Delta, considering riverbed sediment exchange. The model suggests that the Mekong Delta receives ~99 Mt/year sediment from the Mekong River This is much lower than the common estimate of 160 Mt/year. Only about 23% of the modelled total sediment load at Kratie is exported to the sea. The remaining portion is trapped in the rivers and floodplains of the Mekong Delta. Located between Kratie and the entrance of the Mekong Delta, the Tonle Sap Lake receives Mekong River flow at increasing flow rates seasonally and returns flow when Mekong River flow rates decay. As a result Tonle Sap Lake traps approximately 3.9 Mt/year of sediments and explains the hysteresis relationship between water discharges and SSC at downstream stations. The VMD receives an amount of 79.1 Mt/year (~80 % of the total sediment supply at Kratie) through the Song Tien, the Song Hau and overflows. The model results suggest that the Mekong mainstream riverbed erodes in Cambodia and accretes in Vietnam. The results of this study advance understanding of sediment dynamics and sediment budget in the Mekong Delta. The model developed is an efficient tool in order to support delta management and planning. The validated model can be used in future studies to explore impact of climate change and human interference in the Mekong Delta.
... The present-day delta shows one of the world's largest delta plains, which is intersected by numerous beach ridges, seven active distributary channels and the abandoned Ba Lai channel (Fig. 1B). The delta is a mixed-energy system characterised by an upstream to downstream transition from river-dominated to tide-dominated distributary channels 13,14 and a wave-and tide-dominated shoreline 15,16 . The delta area and the lower drainage basin are characterised by a tropical savannah to monsoon climate of the Köppen-Geiger classification 17 , with a rainy summer generating high river discharge and a dry winter associated with low river discharge. ...
... Furthermore, an additional consideration can be made based on the MRD sediment supply. At quasi-natural conditions (i.e., before dams, sand mining, etc.), the sediment discharge of the MRD was estimated in 160 Mt/yr 26 , of which approximately 27% flows through the My Tho channel located north-eastward of the Ba Lai 14 . If assuming that the delta configuration and discharge were similar at 0.7 ka, it could be expected that the sediment exported by the My Tho channel would be transported predominantly south-westward due to the dominant current in the area; part of this sediment would have been reimported to fill the Ba Lai channel (Fig. 8). ...
The Ba Lai distributary channel of the Mekong River Delta was abandoned and infilled with sediment during the Late Holocene, providing a unique opportunity to investigate the sediment fill, timing and mechanisms of channel abandonment in tide-dominated deltaic systems. Based on analysis and age dating of four sediment cores, we show that the channel was active since 2.6 ka and was abandoned at 0.7 ka as marked by the abrupt disappearance of the sand fraction and increase in organic matter and sediment accumulation rate. We estimate that the channel might have been filled in a time range of 45–263 years after detachment from the deltaic network, with sediment accumulation rates of centimetres to decimetres per year, rapidly storing approximately 600 Mt of organic-rich mud. We suggest that the channel was abandoned due to a sediment buildup favoured by an increase in regional sediment supply to the delta. This study highlights that mechanisms for abandonment and infilling of tide-dominated deltaic channels do not entirely fit widely used models developed for fluvial-dominated environments. Their abandonment might be driven by autogenic factors related to the river-tidal and deltaic dynamics and favoured by allogenic factors (e.g., human impact and/or climate change).
... Accurately locating the bpt distance (L) from the river mouth is crucial for understanding riverine and deltaic dynamics. Traditional linear empirical slope models, such as L = h/S, estimate the backwater length (L) using the river depth (h) and channel slope (S) [3,13,14]. While simple, these models often fail to capture the spatial and temporal variability of typical river systems. ...
The study of river backwater points (bpts) is pivotal for understanding the interactions between riverine and coastal systems, including brackish water dynamics, coastal flooding, and ecosystem processes. Despite extensive research, the global spatio-temporal dynamics of bpts, particularly in rivers with minimal human intervention, remains underexplored. This study investigates backwater lengths and shifts in 18 major global rivers (discharge > 5000 m³/s) from 2000 to 2020, uncovering significant hydrological and geographical patterns. In 2000, backwater lengths ranged from 113.16 km (Salween) to 828.75 km (Amur), with bpts consistently positioned upstream of apex points. By 2020, all rivers exhibited upstream retreats of their bpts, ranging from 10.43 km (Salween) to 132.51 km (Amazon), and retreat ratios typically falling between 0% and 20%. The Salween, Niger (60%), and Irrawaddy (38%) demonstrated the most significant proportional shifts. Geographical transitions of bpts varied widely: rivers such as the Ganges and Amur shifted toward urbanized areas, while the Amazon and Orinoco remained in remote regions, reflecting the differential impact of human activity and natural processes. There was a general correlation between backwater length and river discharge, with exceptions like the Amur indicating the influence of other factors such as geomorphic settings and sediment dynamics. While sea-level rise (0.019–0.115 m) affected estuarine conditions, it showed no consistent relationship with bpt retreat at the global scale, but a regional-scale analysis indicates that sea-level rise can lead to the retreat of bpts for those rivers with macro-tidal environments and high sediment yields with less human intervention, suggesting localized interactions dominate backwater dynamics. These findings highlight the complex interplay of environmental and anthropogenic pressures on global river systems. They provide a critical foundation for advancing hydrological modeling, improving river management strategies, and understanding the broader implications of spatio-temporal bpt dynamics under changing climatic and human influences.
... The higher sand content in subunit B 1 ( Figure 4B) indicates stronger hydrodynamic conditions, while the bimodal grain size frequency curves ( Figure 5G) suggests contributions from both fluvial and tidal processes. Such deposition is analogous to that in the distributary channels of the Mekong Delta plain, where strong freshwater discharge during the flood season removes fine grains, leaving coarse bedload, while tidal processes dominate during the dry season, depositing fine sediments (Gugliotta et al., 2017;Ogston et al., 2017;Jiang et al., 2020). Alkaline earth metal contents and ratios indicate that tidal influence was stronger in unit B compared to unit A ( Figure 6C). ...
The response of ports and navigation channels in tide-dominated or tide-influenced estuaries to climate warming is of significant practical relevance. However, studies utilizing sedimentary records to understand these dynamics remain limited. This study investigates the rapid siltation of the palaeo-Qinglong channel during the Song dynasty and its relationship to climate change. Three cores were drilled in the lower reaches of the palaeo-Wusong River, and surface sediment samples were collected from the Huangpu River in the southern plain of the Yangtze Delta. Using AMS ¹⁴C and optically stimulated luminescence (OSL) dating, sedimentological and alkaline earth metal analyses, this study explores the formation and silting history of the palaeo-Qinglong channel. The results indicate that the palaeo-channel was formed about 2,000 years ago as a fluvial-dominated channel but transitioned into a tide-dominated environment, as evidenced by the prevalence of muddy sediments. Rapid siltation occurred during the 11th and 12th centuries, characterized by increased Sr content and Sr/Ba ratio, which indicate intensified salinity intrusion. This rapid infilling is attributed to the increased storm frequency during the Song dynasty, which enhanced the mud import into the channel. A brief period of strengthened fluvial processes, characterized by the occurrence of sandy bedload in the sediments near Qinglong Town, likely reflects channel regulation projects undertaken along the lower reaches of the palaeo-Wusong River during the Northern Song dynasty. The findings suggest that the climate warming and relative sea-level rise during the Medieval Climate Anomaly (MCA) amplified tidal processes in the lower palaeo-Wusong River. This study provides valuable insights into fluvial-marine interactions and their implications for managing ports and navigation channels in tidally influenced estuaries.
... However, increased freshwater extraction during dry periods has resulted in the inland expansion of saltwater intrusion by several kilometers in recent years [60]. Gugliotta et al. [61] report variable saltwater intrusion extents in the Mekong Delta, ranging from 15 to 50 km inland, contingent upon seasonal precipitation patterns. ...
In light of observed climate change dynamics, including intensified precipitation events, prolonged arid spells, and elevating sea and ocean levels, water supply infrastructures face escalating challenges. Some regions are facing significant damage, with failures in network components leading to losses of up to 30%, while globally, can escalate to tens of millions of cubic meters of water. The spatial analysis of energy consumption in abstraction, transmission, and treatment processes per cubic meter of water carried out in this article highlights the impact of climate change on the choice of specific abstraction and treatment methods. Consequently, clarifying the intricate interplay between water and energy assumes paramount importance in ecological and economic water supply network planning.
... Its average elevation ranges from 0.5 m to 1.5 m above mean sea level, with a coastline length of 254 km with 154 km bordering the Gulf of Thailand in the west 130 and 100 km facing the East Sea in the east. In the East Sea, a semi-diurnal tidal pattern with an amplitude ranging from 2.5 to 4.0 m is observed, whereas the Gulf of Thailand experiences a diurnal tidal regime with amplitudes varying from 0.8 m to 1.2 m (Gugliotta et al., 2017). This region is characterized by a dense network of natural and human-made rivers, canals and is currently undergoing infrastructure development, including expressways, coastal embankments, and water control works along the West Sea and East Sea coasts. ...
The Ca Mau Peninsula plays a critical role in the agricultural and aquacultural productivity of the Vietnam Mekong Delta (VMD), central to regional food security and the population’s economic and social welfare. Unfortunately, this region has also historically been a hotspot for natural disasters, particularly from flooding, which is initiated by seasonal river flux upstream and heightened sea levels downstream, but also exacerbated by global climate change (e.g., increased rainfall and sea-level rise, tropical storm surges) and human activities (e.g. river bed lowering, land subsidence). The potential risks associated with rising inundation levels is important information for the future sustainability of the region and its ability to adapt to both current and forthcoming changes. The research around the influence of such drivers on future flood risk, in the Ca Mau Peninsula, is incomplete, primarily due to the absence of a quantitative coastal inundation map corresponding to future compounded scenarios. In this study, we therefore evaluate flooding dynamics in the Ca Mau peninsula using a fully calibrated 1D model, to represent a range of anthropogenic and climate change compound scenarios. Our findings indicate that factors such as increased high-flows upstream, alterations in the riverbed of the main Mekong channel, and occurrences of storm surges effecting the mainstream Mekong River, are unlikely to significantly affect inundation dynamics in this region. However, land subsidence, rising sea levels, and their combined effects emerge as the primary drivers behind the escalation of inundation events in the Ca Mau peninsula, both in terms of their extent and intensity, in the foreseeable future. These results serve as vital groundwork for strategic development and investment as well as for emergency decision-making and flood management planning, providing essential insights for shaping development policies and devising investment strategies related to infrastructure systems in an area which is rapidly developing.
... Observed daily water level amplitude is higher in dry periods (November-May) than in times of monsoon rains [6]. Oppositive changes in water flow direction have a place each day in river branches with high morphological differences of maximal channel depth in places exceeding 30-35 m [7]. The banks of the river canals are abundantly colonized by water hyacinth mats with variable surface and density, regardless of hydrotechnical transformation degree. ...
This work is licensed under Creative Commons Attribution 4.0 License AOMB.MS.ID.000571. Abstract Evaluation of short-time freshwater habitat fluctuations was provided in the period October 2019-February 2020 in the Mekong River delta in Vietnam. Three phases of water flow direction were documented in the diurnal tide cycle. In the middle-"stagnation" state, a daily maximal oxygen concentration in the river occurs, regardless of the sun's position above the horizon. Each tide cycle also creates a periodic water mass friction zone in the nearshore part of the riverbed and increases the sedimentation rate within the opposite direction of water mass flow. The biotic effects of freshwater habitat modification in the river branches are discussed.
... Much of the research investigating estuarine response to climate change focuses on individual river systems undergoing surface water salinization. Some locations, such as the Mekong River estuary (Vietnam), have experienced significant increases in salinization during dry periods, with the surface water saline zone expanding to over three times its extent during wet periods (Gugliotta et al., 2017). Similarly, salinity measurements in the Mississippi River have shown brackish water reaching as far inland as 100 km from the river's mouth during several droughts (Yuill et al., 2023;McAnally and Pritchard, 1997). ...
Estuaries are vulnerable to oceanic and atmospheric climate change. Much of the research investigating climate
change impacts on estuaries is focused on saltwater intrusion within surface water due to drought and rising sea
levels, with implications for ecosystems and humans. Groundwater and soil near estuaries may also be influenced,
as estuary salinity and hydraulic head changes can impact soils and aquifers not previously at risk of
salinization. This study was conducted to address knowledge gaps related to present and future groundwater
salinity distribution in a groundwater system connected to a macro-tidal estuary. The studied estuary experiences
a tidal bore due to its hydraulic connection to the Bay of Fundy in Nova Scotia, Canada. A parcel of agricultural
land adjacent to the estuary was selected to assess the groundwater response to episodic fluctuations in estuary
water levels and salinity. Groundwater monitoring and electromagnetic surveys were conducted to map soil and
groundwater salinity patterns. A numerical model of groundwater flow and solute transport informed by field
data was used to investigate how varying estuary salinity due to droughts and sea-level rise could impact
groundwater salinity. Results showed that, in contrast to salt wedges observed along marine coasts, the saline
groundwater existed as a plume immediately around the estuary. Model simulations showed that short-term
droughts had an insignificant impact on the adjacent groundwater salinity. However, permanent increases in
salinity caused by sea-level rise increased the plume volume by 86 %, or an additional ~11 m horizontally and ~
4.5 m vertically. Our results suggest that increased river salinity in this setting would not result in widespread
salinization of porewater and agricultural soils, but more extensive salinization may be experienced in permeable
aquifers or along more saline estuarine zones. Findings may inform land management decisions in regions
exposed to increased salinity in the future.
... The increase in the islet area decreased the annual river width between 1988 and 2020, as illustrated by the inverse correlation in Figure S10. The river width variation along the Mekong and Bassac Rivers is typical in tide-affected rivers, where it is relatively constant upstream and rapidly increases toward the sea [27]. Many other delta river systems, such as the Changjiang River delta [66] and the Fly River delta [67], show similar trends. ...
The evolution of delta and riverbank erosion within the river basin can significantly impact the environment, ecosystems, and lives of those residing along rivers. The Vietnamese Mekong Delta (VMD), counted among the world’s largest deltas, has undergone significant morphological alterations via natural processes and human activities. This research aims to examine these morphological alterations and their impacts on local economic and social conditions in the VMD. This study utilized satellite data from 1988 to 2020, coupled with population density and land use/land cover (LULC) maps from 2002, 2008, and 2015. The findings reveal that the VMD experienced widespread erosion over the past three decades, covering an area of 66.8 km2 and affecting 48% of the riverbank length (682 km). In contrast to riverbanks, islets showed an accretion trend with an additional area of 13.3 km2, resulting in a decrease in river width over the years. Riverbank and islet erosion has had a profound impact on the LULC, population, and economy of the provinces along the VMD. From 2002 to 2020, eight different land use types were affected, with agricultural land being the most severely eroded, constituting over 86% of the total lost land area (3235.47 ha). The consequences of land loss due to erosion affected 31,273 people and resulted in substantial economic damages estimated at VND 19,409.90 billion (USD 799.50 million) across nine provinces along the VMD. Notably, even though built-up land represented a relatively small portion of the affected area (6.58%), it accounted for the majority of the economic damage at 70.6% (USD 564.45 million). This study underscores the crucial role of satellite imagery and GIS in monitoring long-term morphological changes and assessing their
primary impacts. Such analysis is essential for formulating effective plans and strategies for the sustainable management of river environments.
... Before the flow from the upper of the Mekong River movements into the East Sea, the Tien River splits into five branches (the Tieu, Dai, Ham Lu on g, Co Chi en , an d Cu ng H au estuaries) while the Hau River flows into two branches, namely the Dinh An and Tran De estuaries (Nguyen et al., 2008;Vu et al., 2016). The Mekong delta has a tropical monsoon climate with a wet season from May to October and a dry season from November to April (Gugliotta et al., 2017;Lee and Dang, 2020). The area is dominated by the semi-diurnal tide from the East Sea and diurnal tide from the Gulf of Thailand (Dang et al., 2018;Phan et al., 2019). ...
The Mekong Delta is one of the most vulnerable regions to the impacts of climate change (CC). Prolonged drought causes salinity intrusion, which has become more and more complicated and seriously affects agricultural production and residential daily life. Therefore, studies on salinity intrusion play a critical role in socio-economic development. This study applies the remote sensing techniques to predict salinity intrusion at the main estuaries across the Mekong Delta based on comparing with observed salinity data for the years 2014, 2016, and 2020, at 22 hydrology stations along the Tien River and the Hau River. The results show that salinity tends to decrease gradually, inland from the estuary; it gradually increases from January, peaking in February, March, or April, and then gradually decreases. The farthest salinity intrusion extended approximately 80 km inland, at the Ham Luong branch, and 56-61 km inland from the other branches. Overall, salinity intrusion penetrated the deepest at four main river branches in 2020. Salinity intrusion can be predicted through multiple regression models.
... Previous modeling efforts have also suggested that tides in deltas promote more symmetrical and stable bifurcations (Iwantoro et al., 2020(Iwantoro et al., , 2022, but also that they increase seaward sediment transport efficiency that results in deeper and more stable distributary channels with less frequent bifurcation and avulsion events (Rossi et al., 2016). Tides have also been suggested to promote distributary channels that are low sinuosity and relatively stable; whereas river processes promote constant channel width, higher sinuosity, and relatively high channel mobility (Gugliotta & Saito, 2019;Gugliotta et al., 2017;Nienhuis et al., 2018). Tides have further been suggested to increase sediment retention and nutrient and pollutant residence times in deltas thanks to flux convergence and reduced bed shear stress (Glover et al., 2021;McLachlan et al., 2017;Ralston & Geyer, 2017). ...
Plain Language Summary
River deltas build extensive coastal lands that are one of the most economically and ecologically valuable environments on Earth. While many deltas lose land globally due to climate change and urbanization, deltas significantly influenced by tides are suggested to gain land. Here we utilize numerical modeling to explore the relative formative roles of river and tidal flows, and quantify how the land loss and gain processes differ in river‐ and tide‐dominated deltas. We show that deltas where tidal currents dominate throughout the whole delta build land with a low efficiency, because they form narrow elongate bars and seaward widening distributary channels. In contrast, deltas where tidal currents only dominate in seaward reaches are more efficient at gaining land as they build lobate deltas through mouth bar formation. We propose morphological criteria to distinguish river‐ and tide‐dominated deltas, so more accurate predictions of future land loss and gain can be made. We successfully test these criteria on 40 modern deltas, and show that identification of delta types and understanding the relative roles of river and tidal processes are critical for accurately predicting future change in deltas.
... In the study area, the tidal currents generally follow the direction of tidal currents in the East Sea. The dominant tide in the area is irregular semidiurnal, and the tidal range, which is the difference between high tide and low tide, ranges from 2 m to 4 m during the day [24,42]. The flow velocities during the spring tide and neap tide phases were extracted at 13:00 and 19:00 on 15 February 2017, respectively. ...
Kim Tran Thi 1 , Huy Dinh Ngoc 1 , Huy Nguyen Dam Quoc 2 , Phuoc Nguyen Van 3 , Phung Nguyen Ky 1 , Bay Nguyen Thi 4,5*
... den La Croix and Dashtgard, 2015;Prokocki et al., 2015;La Croix et al., 2019) and rarely the ancient rock record have been used to characterize the FMTZ, despite it being a crucial component of both present and ancient coastal rivers (Martinius et al., 2015;Gugliotta et al., 2016;Jablonski and Dalrymple, 2016). Gugliotta et al., 2017 proposed a comprehensive model covering all the upstream and downstream sub-zones of the FMTZ from the Meokang River delta in Vietnam. Using integrated facies analysis and sequence stratigraphy the possibility to determine how depositional processes interact with one another along the FMTZ and how these interactions affect the morphology and sedimentary processes must be explored. ...
Gondwana basins in peninsular India are riftogenic, half-graben basins preserving thick succession of syn-rift and post-rift sedimentary deposits. The rocks of the Gondwana Supergroup (Late Carboniferous – Early Cretaceous) are scattered across several isolated basins in palaeo-rift valleys in Peninsular India. One such rift valley is the Pranhita-Godavari Valley, where a thick repository of the entire Lower and Upper Gondwana succession is preserved. The Middle Permian Barren Measures Formation of the Lower Gondwana Group is relatively poorly understood from a sedimentological point of view. An integrated approach using facies analysis, geochemical analysis and sequence stratigraphic analysis of the Barren Measures Formation can potentially provide significant insights into the palaeoenvironment, palaeogeography, and palaeoclimate existing during Middle Permian time. The Barren Measures Formation is traditionally interpreted as fluvio-lacustrine deposit at its type area, Jharia coalfield, and other Gondwana basins in peninsular India.
The present research focuses on outcrop based sedimentological and sequence stratigraphic analysis supplemented by laboratory-based investigations of diagenesis. The in-depth facies analysis from selected outcrops, recorded eight distinct facies types, categorized into three facies associations, namely, (i) conglomerate-sandstone facies association (BM-CS); (ii) sandstone-mudstone facies association (BM-SM); and (iii) mudstone facies association (BM-M). The BM-CS facies association comprises pebbly, coarse-grained trough cross-stratified sandstone facies (BM-CS1), planar tabular cross-stratified sandstone facies (BM-CS2), pebbly coarse-grained plane-bedded sandstone facies (BM-CS3), red, ferruginous pebbly cross-stratified sandstone facies (BM-CS4) and lensoidal conglomerate facies (BM-CS5). The sandstones of the BM-CS facies association show lenticular geometry, concave-up basal contacts, large scale cross-stratifications with pebbles concentrated near the base and that are vertically stacked in nature. The BM-SM facies association comprises fine-grained yellow sandstone facies (BM-SM1) and fine-grained cross-stratified heterolithic facies (BM-SM2). Each facies preserves feature such as: (i) tidal bundles characterized by laterally accreting foresets draped with mud and separated by reactivation surfaces; (ii) foresets with laterally variable thickness occurring within a single cross-strata set; (iii) mutually opposite foreset bundles in vertically adjacent beds; (iv) sigmoidal strata bundles; and (v) abundant flaser beddings. The BM-M facies association is composed of ferruginous siltstone/mudstone facies (BM-M1) and calcareous mudstone facies (BM-M2). The facies types bear signatures of pedogenic overprinting in the form of rhizocretions, root tubules and colour mottling effects. Petrographically, the sandstones and mudstones are dominantly composed of quartz and orthoclase, remnant plagioclase, heavy minerals, biotite, muscovite, clay minerals, matrix, ferruginous, silica and calcite cement. The entire succession of the Barren Measures Formation is characterized by multiple vertically stacked, fining-upward cycles. Each fining-up cycle is defined by a coarse-grained pebbly trough cross-stratified sandstone near the base and fine-grained, heterolithic sandstone-mudstone or a mudstone near the top. The overall facies architecture reveals deposition under high energy unidirectional currents. The vertically stacked, multi-storeyed nature of the cross-stratified sandstones of the BM-CS facies association in the lower part of the successions with paucity of fine-grained sediments of the BM-M facies association points to a high-energy river deposit. The occurrence of tidal bundles, reactivation surfaces, spring-neap-spring tidal cyclicity, and systematic changeover of different types of foresets, oppositely directed cross-strata sets, and flaser bedding signify tidally influenced fluvial channels and overbanks. The mudstone facies association (BM-M) indicates deposition in a low energy calm environment, which underwent post-depositional pedogenic modifications resulting in the formation of hardpan calcrete horizons. A nearby low-grade metamorphic source rock with a short transport path is inferred from the petrographic study of the sandstones. The pebbly coarse-grained sandstone beds of the BM-CS present near the base of most of the fining-upward cycles record various soft-sediment deformation structures (SSDS). SSDS such as: (i) complexly deformed layers; (ii) pseudonodules; (iii) load and flame structures; (iv) various water escape structures including sedimentary dikes; and (v) syn-sedimentary faults are generally sandwiched between two undeformed beds. The deformed beds are present in the close vicinity of several intrabasinal faults. The SSDS are unique because of their (i) extremely large size, (ii) complex pattern with normal grading and (iii) predominance in coarse-grained sandstone. In the absence of triggering agents like storms/pounding waves, rapid dumping (massive beds), slumps, or sediment gravity flows in the study area, frequent palaeo-earthquake shocks are possibly the only trigger responsible for liquefaction of the sediments. The extensive liquefaction along the sediment-water interface is evidenced by the complex character and large size of the deformation structures. The coarse-grained sandstone beds bearing large SSDS, characteristically present at the basal part of each fining-up cycle, are thus identified as seismites, indicating possible liquefaction by frequent palaeo-seismic events.
The facies architecture reveals the control of autogenic and allogenic processes on the cyclical sedimentation pattern and the development of different stratal stacking patterns within the basin. A stratal stacking pattern with a high channel to overbank ratio resulted in a High Amalgamation Systems Tracts (HAST), and a stratal stacking pattern with a low channel to overbank ratio, resulted in the development of Low Amalgamation Systems Tracts (LAST). There are two High Amalgamation Systems Tracts (HAST-1 and 2), each represented by coarsening-up succession of the vertically stacked, multi-storeyed channel-fill deposits of BM-CS facies association, with rare overbank deposits (BM-M). There are also two Low Amalgamation Systems Tracts (marked as LAST-1 and LAST-2, respectively) characterized by fining-up succession of single-storeyed sandstone with large SSDS and tidal signatures, embedded in thick overbank deposits.
The deposition of the HAST sediments took place under autogenic upstream-controlled low-accommodation condition. The deposition of LAST sediments indicate low rates of channel amalgamation under higher accommodation space caused by an interplay of varying rates of tectonic subsidence within a rift basin along with flooding by the encroaching tidal currents. Such encroachment of tidal currents in a syn-rift basin may be related to basin tectonism, evident from the frequent occurrences of seismite beds. The varied stratal stacking patterns observed from the rock record in the study area manifest modifications of the river equilibrium profile (base level), with changing accommodation, as a result of interplay of the different autogenic and allogenic controls. The landward encroachment of the tidal limit, causes frequent flooding and modification of the river equilibrium profile, leading to a changing stratal stacking pattern. The allogenic processes such as encroaching tidal currents and syn-depositional tectonism, influenced the sequence stratigraphic architecture in the upstream controlled settings apart from the common autogenic processes. Thus, the prevalent sequence architecture is attributed to the alternate phases of autogenic mechanisms followed by the allogenic mechanisms including the syn-rift tectonic subsidence and the encroaching tidal currents, in the upstream controlled fluvial depositional systems. The facies and sequence stratigraphic architecture reveal a river deposit, with minor tidal-influences in the upper part, indicating a fluvial-tidal depositional system, during Middle Permian sedimentation.
Post-depositional early diagenetic changes in the sandstones and mudstones include: (i) partial dissolution of detrital grains; (ii) biotite kaolinization; (iii) kaolinite formation and clay infiltration; (iv) localized extensive ferruginous cementation; (v) vadose and phreatic zone calcite cementation; and (vi) formation of glauconite pellets, which are observed at different stratigraphic levels. Geochemical signatures reveal low-Mg calcite as the common cement composition of the sandstone and mudstone in the study area. The micrite, pendant and microspar coating morphology of the calcite cement in the mudstones point to vadose zone cementation. The poikilotopic, blocky, bladed and prismatic morphologies of the calcite cement in the sandstones point to phreatic zone cementation. The presence of silica cementation in the form of quartz overgrowth and development of secondary porosity in the sandstones and mudstones indicate a post-burial mesogenetic change. The abundances of the diagenetic features show a definite link with the grain size and sorting of the sandstones in the study area. During the early diagenesis stage, the action of meteoric water resulted in the development of biotite kaolinization, kaolinite, alteration of the feldspars and phreatic zone calcite cementation within the sandstones and mudstones. With the onset of pedogenic (calcretization) processes vadose zone cementation took place within the fine-grained overbank mudstones. The calcite cement chemistry changed under the influence of marine (tidal) water, interacting with the fluvial sediments under a semi-arid climate. Increasing marine influences, recorded in the form of abundant authigenic glauconites, suggest a sustained marine invasion during deposition of the Barren Measures sediments. These modifications occurring due to early diagenetic (near-surface) changes appear to be predictive in nature within a sequence stratigraphic framework. The changes taking place during early diagenesis significantly impacted the development of silica cementation and secondary porosity during the burial diagenetic (mesogenetic) stage. Integrating all observations, a predictive model is established to understand the effects of syn- to post-depositional changes on the development of heterogeneity within the Barren Measures Formation of the Pranhita-Godavari Valley.
... The fluvial-to-marine transition zone is the area with the most complex sedimentary environments controlled by the continuous interaction of fluvial flows, tide currents, and waves [1][2][3][4]. Among which, estuaries are typical sediment systems [5,6], comprising predominantly sand components and minor mud components and salt marshes [7]. ...
Estuaries are important sediment facies in the fluvial-to-marine transition zone, are strongly controlled by dynamic interactions of tides, waves, and fluvial flows, and show various changes in depositional processes and sediment distribution. Deep investigations on the sediment dynamic processes of the sand component of estuaries have been conducted; however, the understanding of how mud supply affects estuaries’ sedimentary characteristics and morphology is still in vague. Herein, the effects of mud concentration, mud transport properties, fluvial discharge, and tidal amplitude on the sedimentary characteristics of an estuary were systematically analyzed using sedimentary dynamic numerical simulation. The results show that the mud concentration has significant effects on the morphology of tidal channels in estuaries, which become more braided with a lower mud concentration, and straighter, with reduced channel migration, with a higher mud concentration. The mud transport properties, namely, setting velocity, critical bed shear stress for sedimentation, and erosion, mostly affect the ratio between the length and width (RLW) of the sand bar; a sheet-like sand bar with a lower RLW value develops in the lower settling velocity, while there are obvious strip shaped bars with a high RLW value in the higher settling velocity case. Moreover, the effects of hydrodynamic conditions on sedimentary distribution were analyzed by changing the tidal amplitudes and fluvial discharges. The results show that a higher tidal amplitude is often accompanied by a stronger tidal energy, which induces a more obvious seaward progradation, while a higher fluvial discharge usually yields a higher deposition rate and yields a greater deposition thickness. From the above numerical simulations, the statistical characteristics of tidal bars and mud interlayers were further obtained, which show good agreement with modern sedimentary characteristics. This study suggests that sedimentary dynamic numerical simulation can provide insights into an efficient quantitative method for analyzing the effects of mud components on the sediment processes of estuaries.
... They play an important role in global water cycles and sediment budget, and offer resources for human settlement, transportation, fisheries and agriculture. For these large low-lying estuaries, the width generally converges landward, and overall the longitudinal bed level has a seaward decrease-increase trend, i.e., a scour pattern (Zheng et al., 2018;Gugliotta et al., 2017;Canestrelli et al., 2014;Gallo and Vinzon, 2005;Kästner et al., 2017). Apart from the decrease-increase trend in the longitudinal bed level, a closer look shows that the downstream bed level can actually decrease again, forming a 'scour-bar' pattern ( Fig. 1). ...
A seaward decrease-increase-decrease trend in the longitudinal bed level, defined as scour-bar pattern, is observed in several large estuaries on low-lying coastal plains such as the Amazon, Yangtze and Fly. Why this pattern forms has not yet been well explained. The mechanism for the formation of this pattern is explored here using a one-dimensional morphodynamic model, with varied geometry, fluvial water discharge, sediment flux and tide. Results show that for those estuaries with bed slopes in the order of 10⁻⁵ and sediment concentration in the order of 0.1 kg m⁻³, bars are likely to occur for channels with relatively weak convergence and wide mouth, while scours are nearly ubiquitous. Scours are classified as fluvial and tidal based on the relative strength of river and tide, which can be distinguished by bed slope and water depth. A balance between river and tide can lead to no scours or bars. Sediment reduction and sea level rise enhances the relative strength of tide, and transition of a fluvial scour to a tidal scour may occur.
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... Hơn nữa, hệ số thấm của đất cũng gây ra nguy cơ sạt lở bờ sông: hệ số thấm của đất càng thấp thì nguy cơ sạt lở bờ sông càng cao nếu mực nước giảm và ngược lại [10]. Hiện tượng sạt lở trên sông rạch ở ĐBSCL đã được nghiên cứu khá nhiều [11][12][13][14] và chỉ ra 7 nguyên nhân ảnh hưởng đến diễn biến sạt lở bờ sông như sau: địa chất; địa hình-hình thái sông; chế độ thủy lực; chế độ phù sa bùn cát; khai thác cát; giao thông thủy; xây dựng cơ sở hạ tầng. Sạt lở bờ biển, sông đang xảy ra và có chiều hướng ngày càng gia tăng. ...
... In combination with additive processes, the low-lying MD is threatened in its very existence. Those include natural compaction of the upper Holocene sediment layers (Zoccarato et al., 2018), a decline of fluvial sediment supply transported by the Mekong River due to upstream dam constructions (Allison et al., 2017;Karlsrud et al., 2017;Kummu and Varis, 2007), salinization of surface water (Ferguson and Gleeson, 2012;Gugliotta et al., 2017) as well as sea-level rise due to global climate change (Smajgl et al., 2015;Woodroffe et al., 2007). Some parts of the MD are additionally facing heavy coastal and riverbank erosion (Anthony et al., 2015;Karlsrud et al., 2017;Liu et al., 2017;Luijendijk et al., 2018;Tamura et al., 2020). ...
Study region: Ca Mau Province (CMP), Mekong Delta (MD), Vietnam.
Study focus: Groundwater from deep aquifers is the most reliable source of freshwater in the MD but extensive overexploitation in the last decades led to the drop of hydraulic heads and negative environmental impacts. Therefore, a comprehensive groundwater investigation was conducted to evaluate its composition in the context of Quaternary marine transgression and regression cycles, geochemical processes as well as groundwater extraction.
New hydrological insights for the region: The abundance of groundwater of Na-HCO3 type and distinct ion ratios, such as Na⁺/Cl⁻, indicate extensive freshwater intrusion in an initially saline hydrogeological system, with decreasing intensity from upper Pleistocene to deeper Miocene aquifers, most likely during the last marine regression phase 60–12 ka BP. Deviations from the conservative mixing line between the two endmembers seawater and freshwater are attributed to ion-exchange processes on mineral surfaces, making ion ratios in combination with a customized water type analysis a useful tool to distinguish between salinization and freshening processes. Elevated salinity in some areas is attributed to HCO3⁻ generation by organic matter decomposition in marine sediments rather than to seawater intrusion. Nevertheless, a few randomly distributed locations show strong evidence of recent salinization in an early stage, which may be caused by the downwards migration of saline Holocene groundwater through natural and anthropogenic pathways into deep aquifers.
... Recent research on modern tidal environments highlights the complexity of tidal systems (Dalrymple et al., 2012;Fan, 2012;Gugliotta et al., 2017;Cosma et al., 2020;Finotello et al., 2020). This increased understanding of modern systems is one way to improve the recognition and the interpretation of ancient tides (see discussion in Gugliotta & Saito, 2019). ...
Simulating hydrodynamic conditions in palaeo‐ocean basins is needed to better understand the effects of tidal forcing on the sedimentary record. When combined with sedimentary analyses, hydrodynamic modelling can help inform complex temporal and spatial variability in the sediment distribution of tide‐dominated palaeo‐ocean basins. Herein, palaeotidal modelling of the epicontinental Upper Jurassic (160 Ma, lower Oxfordian) Sundance and Curtis Seas of North America reveals possible regional‐scale variations in tidal dynamics in response to changes in ocean tidal forcing, physiographic configuration and bottom drag coefficient. A numerical model forced with an M2 tidal constituent at the open boundary shows that the magnitude and location of tidal amplification, and the variability in current velocity and bed shear stress in the basin, were controlled by palaeophysiography. Numerical results obtained using a depth of 600 m at the ocean boundary of the system enable the prediction of major facies trends observed in the lower Curtis Formation. The simulation results also highlight that certain palaeophysiographic configurations can either permit or prevent tidal resonance, leading to an overall amplification or dampening of tides across the basin. Furthermore, some palaeophysiographic configurations generated additional tidal harmonics in specific parts of the basins. Consequently, similar sedimentary successions can emerge from a variety of relative sea‐level scenarios, and a variety of sedimentary successions may be deposited in different parts of the basin in any given relative sea‐level scenario. These results suggest that the interpretation of sedimentary successions deposited in strongly tide‐influenced basins should consider changes in tidal dynamics in response to changing sea level and basin physiography.
... Several studies have investigated depositional processes operating in this region, including documentation of the gravity-flow deposits of the Fraser River delta front associated with fluvial and tidal interaction (Ayranci et al. 2012), and remote-sensing analysis of the outward delta plume of the Mekong River (Loisel et al. 2014). In recent years, a growing number of studies have examined the detailed sedimentology of FMTZ-related deposits, from both modern systems (e.g., La Croix and Dashtgard 2014Prokocki et al. 2015;Gugliotta et al. 2017Gugliotta et al. , 2019 and from ancient successions (e.g., Van den Berg et al. 2007;Shiers et al. 2014;Martinius et al. 2015;Gugliotta et al. 2016;La Croix et al. 2019a). ...
The interaction of fluvial, tidal, and wave processes in coastal and paralic environments gives rise to sedimentary successions with highly varied styles of facies architecture; these are determined by the morphology and evolutionary behavior of the range of coastal sub-environments, which may be difficult to diagnose in subsurface sedimentary successions with limited well control.
This study presents depositional models to account for stratigraphic complexity in a subsurface fluvial to shallow-marine succession, the Middle Jurassic Dhruma Formation, Saudi Arabia. The study achieves the following: i) it examines and demonstrates sedimentary relationships between various fluvial, nearshore, and shallow-marine deposits, ii) it develops depositional models to account for the stratigraphic complexity inherent in fluvial to shallow-marine successions, and iii) it documents the sedimentology and the stratigraphic evolutionary patterns of the lower Dhruma Formation in the studied area of Saudi Arabia. The dataset comprises facies descriptions of 570 m of core from 14 wells, 77 representative core thin sections, 14 gamma-ray logs, and FMI image logs from 4 wells. These data are integrated with quantitative information from > 50 analogous systems from a wide range of modern and ancient settings, stored in a relational database. Stratigraphic correlations reveal the internal anatomy of the succession.
Facies associations are representative of fluvial channels, intertidal flats, pedogenically modified supratidal flats or floodplains, river-influenced tidal bars, weakly storm-affected shoreface and offshore-transition zones, storm-dominated delta-front and prodelta settings, and an open-marine carbonate-dominated shelf. These sub-environments interacted in a complex way through space and time. The vertical succession of the studied interval records an overall transition from coastal-plain deposits at the base to marine deposits at the top. As such, the succession records a long-term transgressive, deepening-upward trend. However, this general trend is punctuated by repeated progradational events whereby coastal sand bodies of fluvial, wave, and tidal origin prograded basinward during stillstands to fill bays along a coastline. The nature of juxtaposition of neighboring sub-environments has resulted in a sedimentary record that is highly complex compared to that generated by morphologically simple shoreface systems that accumulate more regularly ordered stratal packages.
... The ichnological suite is interpreted to reflect brackish-water conditions, but with lower levels of physico-chemical stress (i.e., more marine) than is interpreted for the FA1 channels (e.g., Howard and Frey, 1975;Gingras et al., 1999;Johnson and Dashtgard, 2014;Gugliotta et al., 2016Gugliotta et al., , 2017Collins et al., 2019;Weleschuk and Dashtgard, 2019). The combined sedimentological and ichnological observations of FA2 support the interpretation that these channel-belt deposits represent distributary channels, with the two fining-upward facies successions recording distinctive channel-related deposition (Château et al., 2020). ...
Depositional environments are typified as low-to high-accommodation space settings based on the sediment accumulation rate. In low-accommodation settings, persistent top truncation of regressive units renders it challenging to accurately determine the rate of accommodation space creation. Herein, we analyze a nearly continuous succession of the Lower Cretaceous McMurray Formation, which was deposited during the foreland development of the Western Canada Sedimentary Basin in northeast Alberta, Canada (the McMurray Depocenter). Across the study area, the McMurray Formation consists of a vertical succession of 6 discrete regressive depositional units (DU) bounded by flooding surfaces and/or transgressive surfaces of erosion. The sedimentology, architecture and stratigraphy of these DUs are investigated and statistical analysis is used to quantitatively assess variations in DU thickness. Additional statistical analysis is used to quantitatively assess the vertical distribution of transgressive mudstone subfacies in the McMurray Formation.
The mean thickness of each DU is 10.4 m, and each DU represents approximately 1.7 million years of deposition. This indicates that the McMurray Formation preserves deposition in a low-accommodation space setting, and that creation of accommodation space was primarily controlled by transgression of the Boreal Sea. An abrupt decrease in DU thicknesses is interpreted to result from concomitant increased erosional truncation of DUs via wave action during transgression and changes in basin morphology. DU thickness variations correlate to a change in the marine expression of the transgressive mudstone underlying each DU. Together, these data suggest an acceleration in the rate of the Boreal Sea transgression, and we hypothesize that the duration of still-stands during which DUs accumulated diminished in the latter stages of McMurray Fm deposition. This study demonstrates that detailed sedimentologic and stratigraphic research tied to statistical analysis can be used to quantify changes in the rate of transgression, especially in low-accommodation space settings.
... The FTTZ may be located in single-thread rivers (e.g., Torres, 2017) or at several locations within deltaic distributary systems (e.g., Kastner et al., 2017;Hoitink et al., 2017;Sassi et al., 2012). Researchers have identified the FTTZ as a storage site for sand-sized bed material (Dalrymple & Choi, 2007;Gugliotta et al., 2017;Wang et al., 2009) due to a reduced transport capacity (Dalrymple & Choi, 2007;Lamb et al., 2012;Nittrouer et al., 2011), with corresponding high sediment accumulation rate (Dalrymple & Choi, 2007;Ensign et al., 2014;Lawrence et al., 2015). Others have shown how these sedimentation effects translate to FTTZ channel morphology (Gurnell, 1997;Ensign et al., 2013;Sassi et al., 2012), and Torres (2017) referred to sharp along-channel changes in tide-influenced river geomorphology as the "fluvial-tidal discontinuity" that appears to be the result of locally enhanced tidal energy dissipation (Yankovsky et al., 2012). ...
Rivers that traverse the terrestrial‐marine interface may have lower reaches that are influenced by both terrestrial and marine processes. However, only a handful of studies have focused on how the interactions of fluvial and tidal processes translate to channel geomorphology, and those are largely from delta/distributary systems. Here we quantify channel properties along the fluvial‐tidal transition reach of a coastal plain river and provide insight into their origins. The study site is a 47 km long tidal, single‐thread freshwater section of a river at 29 to 76 river kilometers inland of the estuary mouth, upstream of the delta/distributary system, and with average riverbed slope of 10⁻⁴. Results show that a tidal wave approaching the study reach loses 15%–17% of its incident energy (per horizontal area) per kilometer of channel, and at 51 km upstream of the mouth the incident energy is reduced to <1%. Also, at or near 51 km we observed breaks in along‐channel trends of channel cross‐section geometry, bed grain size, sinuosity, channel bed and water surface slopes. We propose that fluvial‐tidal flow processes and corresponding geomorphic feedbacks are apparent as abrupt changes in channel properties that highlight the influence of tides, and these discontinuities may be endemic to fluvial‐tidal transition zones in general. How these transition reaches self‐adjust in response to climate change remains largely unexplored but these reaches are likely to become important geomorphic hotspots.
... The large (inter)annual variability in rainfall, river discharge and tidal regime, in combination with human interventions, makes the VMD a physically dynamic delta (Gugliotta et al., 2017;Unverricht et al., 2013). From a biophysical point of view, the VMD is divided into three zones: downstream, midstream, and upstream (see Figure 1). ...
The need for explicitly considering equity in climate change adaptation planning is increasingly being recognized. However, evaluations of adaptation often adopt an aggregated perspective, while disaggregation of results is important to learn about who benefits when and where. A typical example is adaptation of rice agriculture in the Vietnam Mekong Delta (VMD). Efforts focused on flood protection have mainly benefitted large‐scale farmers while harming small‐scale farmers. To investigate the distributional consequences of adaptation policies in the VMD, we assess both aggregate total output and equity indicators, as well as disaggregated impacts in terms of district‐level farming profitability. Doing so requires an adequate representation of the multisectoral dynamics between the human and biophysical systems which influence farming profitability. We develop a spatially explicit integrated assessment model that couples inundation, sedimentation, soil fertility and nutrient dynamics, and behavioral land‐use change and farming profitability calculation. We find that inter‐district inequality responds in a non‐linear way to climatic and socio‐economic changes and choices of adaptation policies. The patterns of who wins and who loses could change substantially when a different policy is implemented or if a slightly different uncertain future materializes. We also find that there is no simple ranking of alternative adaptation policies, so one should make trade‐offs based on agreed preferences. Accounting for equity implies exploring the distribution of outcomes over different groups over a range of uncertain futures. Only by accounting for multisectoral dynamics can planners anticipate the equity consequences of adaptation and prepare additional measures to aid the worse‐off actors.
Context Understanding chemical properties and biogeochemical changes can help us answer difficult ecological questions. Water chemistry is often dynamic in large tropical rivers, particularly in deltas where sea tides and river hydrological regimes are extremely influential. Aims This study assessed the spatial variability of water chemistry by measuring the elemental concentrations of seven trace elements and strontium isotopes (87Sr:86Sr) in the Ayeyarwady River Basin in Myanmar. Methods Inductively Coupled Plasma Optical Emission Spectroscopy and multi-collector inductively coupled plasma mass spectrometry were used to quantify concentrations of trace elements and strontium isotopes at 50 sampling sites, covering 1700 km of the Ayeyarwady River. Data was grouped into regions for statistical analyses. Key results Three elements (Sr, Ca, and Mg) showed distinct longitudinal concentration profiles, which were higher at the coast but consistently lower in freshwater. For example, elemental concentrations at coastal areas were 22-, 63-, and 150-times higher than in freshwater further upstream for Ca, Sr, and Mg, respectively. Although, longitudinal concentration profiles of 87Sr:86Sr ratios varied along the Ayeyarwady River, they were not significantly different among regions. Conclusions Longitudinal profiles of dissolved elements varied significantly. In particular, three elements (Sr, Ca, and Mg) are good indicators to differentiate between marine and freshwater in the Ayeyarwady River. Implications Our findings provide important baseline information on water chemistry for future fish otolith (inner ear of bony fish) chemistry studies to track fish migrations in the basin.
The erosion crisis in the Mekong Delta, spurred by upstream impacts, climate change, and human activities, manifests in extensive riverbank degradation. This study employs a comprehensive investigation to unveil seven primary factors and four mechanisms contributing to this degradation. Utilizing the Mike 21 HD model, the study accurately predicts flow rates, demonstrating strong alignment with observed data from 2017 to 2019 (R2>0.72). Despite slightly elevated Root Mean Square Error (RMSE) values due to modeling intricacies, this method showcases the model’s ability to approximate flow rates. Spatial analysis reveals Tan Chau and Can Tho as hotspots with the highest flow rates, highlighting crucial regional patterns. Leveraging these findings, surveys and fieldwork pinpoint key factors influencing riverbank erosion, crucial for devising sustainable management strategies. Insights into unstable slope formation, gradual erosion, sand mining impacts, and ship-generated waves inform pivotal erosion control approaches. Despite financial constraints, the study advocates for cost-effective, nonstructural solutions—such as legal reinforcement, community awareness campaigns, strict regulations, and intensified monitoring. Effective governance strategies for sand mining and collaborative sediment management, coupled with ongoing surveys, further enrich adaptive erosion control strategies, significantly contributing to the pursuit of sustainable riverbank management in the dynamic Mekong Delta.
Fluvial sediment supply towards the coast has been the subject of extensive research. Important aspects relate to the impact of sediment retaining hydropower dams, potential delta progradation, coastal sediment supply and delta vulnerability to sea level rise. Once validated, process-based models provide a valuable tool to address these aspects and offer detailed information on sediment pathways, distribution and budget in specific systems. This study aims to advance the understanding of the sediment dynamics and sediment budget in the Mekong Delta system. We developed a process-based model (Delft3D FM) that allows coupling 2D area grids to 1D network grids. The flexible mesh describes both wide river sections and channel irrigation and drainage networks present in the Mekong Delta. We calibrated the model against observed discharge, salinity, suspended sediment concentration (SSC) and sediment flux. The model was able to skillfully describe seasonal variations of SSC and hysteresis of SSC and water discharge caused byTonle Sap Lake induced flow patterns and seasonally varying bed sediment availability in the channels. Model results suggest that the Mekong River delivers ~99 Mt/year of sediment at Kratie, towards the delta which is much lower than the common estimate of 160 Mt/year. About 23% of the modeled total sediment load at Kratie reaches the sea. Our modelling approach is a useful tool to assess sediment dynamics under strategic anthropogenic interventions or climate change scenarios.
The backwater effect (i.e. channel flow influence by a body of standing water) is used to predict down-dip changes in fluvial morphodynamics and consequent sediment distribution on delta plains. These changes include downstream fining, decrease in sinuosity, and deepening and narrowing of channel belt deposits. This study reviews existing methods for estimating backwater length in ancient and modern settings and proposes workflows to minimize ambiguity in resultant estimates. The proposed workflows are tailored to both modern and ancient settings and are prioritized based on practicality, accuracy, smallest uncertainty ranges and allow different types of data as input parameters. In modern river systems, we recommend using direct field measurements of bankfull thalweg channel depth and river water elevation to determine the location where riverbed elevation intersects sea level (i.e. the upstream limit of the backwater zone). Alternatively, the backwater length (Lb) can be estimated indirectly by Lb = h/S, with h is bankfull thalweg channel depth and S is slope. In ancient settings, bankfull thalweg depth and grain size representative of bedload transport are the most reliably measurable parameters, obtained at one or a few locations. For the first time, the application of multiple methods to obtain backwater length estimates are tested on a single modern and ancient river system. In the modern case study, the riverbed intersection with sea level matches previously documented major changes in sedimentary trends, such as decreasing channel-belt width/thickness ratios, decreasing meander-bend migration rates, and coarsening grain size followed by distinct downstream fining. However, backwater lengths based on h/S plot downstream of this zone characterized by major changes, when input parameters are derived from discharge and grain size. Therefore, we recommend obtaining bankfull thalweg channel depth from a cross-sectional profile if backwater length is estimated based on h/S. In the ancient case study, bankfull thalweg channel depth derived from fully preserved single story channel fill and slope based on Shields’ empirical relation with grain size, match changes in fluvial architectural style interpreted as a result of backwater effects. Although uncertainty management is improved with the proposed workflows, a degree of uncertainty remains in the resulting backwater length estimates, due to inherent scatter in previously established relationships (e.g. Shields stress relation to obtain slope estimates). This review is a critical step forward in discussing the shortcomings, and listing and acknowledging the uncertainties and ambiguity in obtaining the necessary input parameters to estimate backwater lengths. The proposed workflows facilitate comparability and applicability of future backwater length estimates and their corresponding influence on the hydrodynamic environment and ultimately the stratigraphic record. Potential scaling relationships between the backwater length, sedimentary trends and avulsion nodes makes this of key importance as the latter two also play a crucial role in devastating floods when rivers change course.
Destructive coastal floods are commonly increasing in frequency and may be caused by global precipitation intensification. Such connections through climate, watershed, and river processes are poorly understood because of complex interactions in transitional fluvial‐marine environments where flooding is caused by rivers, marine storm surge, or both in compound events. To better understand river floods along the fluvial‐marine transition, we study watersheds of the northeastern Gulf of Mexico using long‐term observations. Results show intensifying precipitation decreased precipitation‐discharge lag times, increasing river‐flood frequency and the likelihood of compound events in fluvial‐marine transitions. This reduction in lag time occurred when the Atlantic Multidecadal Oscillation and El Niño Southern Oscillation began strongly affecting river discharge through the advection of moist air, intensifying precipitation. Along the fluvial‐marine transition, compound events were largest in inland reaches. However, for inland reaches, compound event water levels did not exceed the floods caused solely by river flooding, the largest flood hazard in these systems. Our results demonstrate precipitation and river discharge play critical roles in coastal flooding and will likely escalate flooding as the climate continues to warm and intensify precipitation.
Fine sediment systems involve mud supply from both large and small rivers, mud concentration processes within estuaries and sometimes on the shoreface, and alongshore mud supply to adjacent muddy coasts. Fine sediment systems are ecologically important. Thick muddy shoreface deposits are extremely efficient in dissipating wave energy, but this process can also involve alongshore streaming of mud. Shear stresses on muddy substrates are associated with both waves and tidal currents, but winds can also play a role. Muddy sedimentation is commonly strongly aided by flocculation, and muddy systems are commonly subject to significant biological mediation.
Rising temperatures, rapid urbanization and soaring demand for natural resources threaten deltas worldwide and make them vulnerable to rising seas, subsidence, droughts, floods, and salt intrusion. However, climate change projections in deltas often address climate-driven stressors in isolation and disregard parallel anthropogenic processes, leading to insufficient socio-political drive. Here, using a combination of process-based numerical models that integrate both climatic and anthropogenic environmental stressors, we project salt intrusion within the Mekong mega-Delta, in the next three decades. We assess the relative effects of various drivers and show that anthropogenic forces such as groundwater extraction-induced subsidence and riverbed level incisions due to sediment starvation can increase the salinity-affected areas by 10–27% compared to the present-day situation, while future sea level rise adds another 6–19% increase. These projections provide crucial input for adaptation policy development in the Mekong Delta and the methodology inspires future systemic studies of environmental changes in other deltas. Human activities, such as groundwater extraction and sediment starvation, are projected to add to climatic factors like sea level rise to exacerbate saline water intrusion into the Mekong Delta, Vietnam over the next 30 years, according to process-based model simulations.
There is no abstract to this Chapter contribution. The five principal conclusions are:
(1) OSL dating of LCR braid bars within the Cathlamet and Grays Bay regions (from rkm 51.1 to 29.3) shows they began forming at statistically identical times (ca. 2.2–2.0 ka), thus displaying no temporal delay in the onset of deposition in the downstream direction. This finding, coupled with the alluvial architecture of these bars, suggest they are more diagnostic of late Holocene fluvial barforms, and not late Holocene progradational “bay-head deltaic foreset and bottomset” deposits, and thus represent the vertical construction of the late-Holocene (ca. 2.0 ka–present) LCR “fluvial top-set.”
(2) From ca. 4.3 to 2.0 ka, a net deficit existed between the rate of Qbl supplied to the LCRV and the rate of Qbl lost at the mouth of the LCR to the CRLC and shallowshelf sediment sink that was driven by intense ocean wave energy, and potentially coseismic-subsidence events. This condition forced the LCR to “entrench,” thus causing the bypassing of the majority of fluvial–deltaic sediments of this age through the active channelized regions of the LCRV, and into to the CRLC, continental shelf, barrier beach plains, and Willapa Bay and Grays Harbor.
(3) From ca. 2.0 ka to present, the rate of accommodation created within the LCRV driven by sea-level rise lowered to less than 1.0 mmyr-1, thus allowing the rate of upstream-derived LCR Qbl versus the rate of Qbl lost to the CRLC and shallowshelf sediment sink to find a new balance, which is manifested today by lower magnitude channel “entrenchment” relative to that during the mid-Holocene.
(4) The late-Holocene LCR (ca. 2.0 to present) products of channel “entrenchment” are: (a) a weakly developed subaqueous ebb tidal delta at its mouth, (b) the development of a shallow-water central bay region (i.e., Cathlamet and Grays Bay) within the LCRV, extending from its mouth to ca. rkm 37, whose upper bar sediments reside below the intertidal environment (i.e., nearly always subaqueous), and (c) the present day occurrence of estuarine processes extending to ca. rkm 37.
(5) The late-Holocene LCR channel “entrenchment” has formed a distinctive sedimentological signature that is preserved within upper bar sediments located in the mixed tidal–fluvial, hydraulic regime, and which is defined by stacked successions (approx. 0.5 to 1.5 m thick) of dirty and/or clean small-scale current ripple cross-laminae formed via: (a) shallow water flows operating over bars during tidal cycles combined with slackwater periods and ETM activity, (b) oscillatory currents derived from shallow intrabasinal wind-waves modified by slackwater periods and ETM activity, or (c) combined flows constructed from both (a) and (b).
(6) In summary, from a longer-term geological perspective, the large-scale geomorphic/morphodynamic state of the LCR from the mid-Holocene (ca. 4.3 ka) to present is that of an “entrenched” fluvial, and not an estuarine, depositional setting.
The Mekong River is ranked as the 8th in terms of water discharge and as the 10th in terms of sediment load in the world. During the last 4500 years, its delta prograded more than 250 km to the south due to a tremendous amount of sediments deposited, and turned from a “tide-dominated” delta into a “wave-and-tide dominated” delta. This study aims at completing our knowledge on the fate of sediments that may be stored in estuarine or coastal systems, or dispersed over the continental shelf and slope. Sediment transport in the Mekong River Delta (MRD) coastal area was studied by numerical simulations using the Delft3D model. The model configuration was calibrated and validated from data collected in situ during 4 periods from 2012 to 2014. Then, 50 scenarios corresponding to different wave conditions (derived from the wave climate) and river discharge values typical of low flow and flood seasons enabled us to quantify the dispersal patterns of fluvial sediments close to the mouths and along the coast. Sediments mostly settled in the estuary and close to the mouths under calm conditions, and suspended sediment with higher concentrations extend further offshore with higher waves. Waves from the Southeast enhanced the concentration all along the MRD coastal zone. Waves from the South and Southwest induced coastal erosion, higher suspended sediment concentrations in front of the southern delta, and a net transport towards the Northeast of the delta. Because of episodes of Southern and Southwestern waves during the low flow season, the net alongshore suspended sediment transport is oriented Northeastward and decreases from the Southwestern part of the coastal zone (~960 × 103 t yr−1) to the Northeastern part (~650 × 103 t yr−1).
The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone (FMTZ), which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the FMTZ are preserved in the rock record, large-scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial-tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward-oriented cross-stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the FMTZ. Heterolithic deposits with dm-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial-tidal interactions with overall fluvial dominance, or of purely fluvial processes. No purely tidal or tide-dominated facies were recognized in the studied deposits. Moreover, fluvial-tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off-axis (interdistributary) parts of the system. Along major channel axes, the interpreted FMTZ is mainly represented by the fluvial-dominated section, whereas little or no tide-dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial-dominated, tide-influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient FMTZs, is fundamental to refining interpretations of ancient deltaic successions.
Sedimentation in the fluvial–marine transition is governed by the interaction of river and tidal currents. Tidal currents act continuously, albeit with small variations in strength as a result of neap–spring cyclicity and modulation by changes in river discharge. River currents, by contrast, commonly change more dramatically because of the presence of river floods. The superposition of river floods on the tides causes depositional conditions to vary temporally from less fluvially influenced/dominated (and more tidally influenced/dominated) during the times between river floods (the interflood periods) to more fluvially influenced/dominated (and less tidally influenced/dominated) during river floods. These temporal variations in the relative importance of river and tidal currents are recorded within individual beds in the point-bar and mouth-bar deposits of the fluvial–marine transition, creating a spectrum of possible deposit types depending on the longer-term ratio of river flood to tidal energy. In areas of fluvial dominance, tidal action is only present in the interflood deposits, if present at all, whereas in areas of tidal dominance, river-flood sedimentation can become cryptic and is indicated by intervals with coarser sand and a greater abundance of fluid-mud deposits. The ichnological character of the deposits preferentially reflects interflood conditions. A detailed analysis of the deposit characteristics allows deductions to be made about the strength of the tidal currents, the intensity of river floods, and the relative position of a given deposit within the fluvial–marine transition. Discrepancies between the proximality indicated by the nature of the river-flood deposits and the ichnology of the interflood intervals can give an indication of the relative magnitude of river-discharge fluctuations.
Tidal rivers are a vital and little studied nexus between physical oceanography and hydrology. It is only in the last few decades that substantial research efforts have been focused on the interactions of river discharge with tidal waves and storm surges into regions beyond the limit of salinity intrusion, a realm that can extend inland hundreds of kilometers. One key phenomenon resulting from this interaction is the emergence of large fortnightly tides, which are forced long waves with amplitudes that may increase beyond the point where astronomical tides have become extinct. These can be larger than the linear tide itself at more landward locations, and they greatly influence tidal river water levels and wetland inundation. Exploration of the spectral redistribution and attenuation of tidal energy in rivers has led to new appreciation of a wide range of consequences for fluvial and coastal sedimentology, delta evolution, wetland conservation and salinity intrusion under the influence of sea level rise and delta subsidence. Modern research aims at unifying traditional harmonic tidal analysis, nonparametric regression techniques and the existing understanding of tidal hydrodynamics, to better predict and model tidal river dynamics both in single-thread channels and in branching channel networks. In this context, this review summarizes results from field observations and modeling studies set in tidal river environments as diverse as the Amazon in Brazil, the Columbia, Fraser and St Lawrence in North America, the Yangtze and Pearl in China, and the Berau and Mahakam in Indonesia. A description of state-of-the-art methods for a comprehensive analysis of water levels, wave propagation, discharges and inundation extent in tidal rivers is provided. Implications for lowland river deltas are also discussed in terms of sedimentary deposits, channel bifurcation, avulsion and salinity intrusion, addressing contemporary research challenges.
An outcrop of the McMurray Formation along the Christina River (Alberta, Canada) has been investigated to better understand depositional processes and setting. The succession is formed by large-scale tabular sets of unidirectional trough cross-stratification. Many of these sets are characterized by profusely ripple-laminated and thick, laterally persistent bottomset intervals at their base. Additionally, reactivation surfaces and infrequent set climbers occur in the foresets. The bottomsets almost entirely consist of backflow cross-lamination. Available knowledge indicates that this points to a rather strong vortex circulation and related strong and persistent main flow velocity. The observed bottomset succession is discussed within the range of variation in bottomset architecture that results from the structure and strength of the flow in the wake behind dunes and related strength of the main flow. Sets descend along a gentle slope, suggesting that dunes filled a preexisting depression, thus representing conditions of a vertically expanding and decelerating flow. This means that aggradation rate was high, which is in accordance with the thickness of the preserved sets. Systematic changes in flow strength are documented by downstream cyclic variations in organic debris, bottomset thickness, and foreset dip. The periodic increase of flow velocity is interpreted as being produced by the increased strength of the river flow during the ebbing tide on the days around spring tide. Apart from these subtle variations, the area experienced large changes in flow strength due to seasonal differences in fluvial discharge. The turbidity maximum zone was located downstream of the study site since thick slackwater mud drapes that characterize the seaward part of the fluvial to tidal transition zone are not present; only a few thin mud drapes are found at the study locality. Therefore, it is concluded that deposition took place in the most landward part of this zone. This new interpretation of this facies in the Christina River area is in line with the inferred depositional setting of the transition to the overlying thick point bar units formed by inclined heterolithic stratification.
Optically stimulated luminescence dating of lower Columbia River (LCR) sediments reveals only a small fraction of mid-Holocene (ca. 4.3–2.0 ka) sands are preserved within depositionally active bars/floodplains in the Lower Columbia River Valley (LCRV). These LCR sands thus bypassed through the LCRV, and were driven by channel “entrenchment” that was forced by rapid rates of bedload loss to the Columbia River Littoral Cell sediment dispersal system. The magnitude of LCR channel “entrenchment” then relaxed during the late-Holocene (ca. 2.0 ka-present) allowing bars located across the mixed tidal–fluvial, hydraulic regime (~ river kilometer (rkm) 20–56) to initiate vertical aggradation at identical times (ca. 2.2 ka-present). This contradicts the hypothesis that these bars represent the progradation of a late-Holocene “bay-head delta” into the LCRV. Instead, these bars represent the vertical building of the late-Holocene LCR “fluvial top-set.” The products of continued late-Holocene LCR channel “entrenchment” are: (a) the absence of a subaerial delta, (b) a shallow-water central bay within the LCRV, extending from its mouth to ca. rkm 37, whose upper bar sediments are exposed to intertidal processes, and (c) the present day occurrence of estuarine processes extending to ca. rkm 37. These conditions preserve a sedimentological signature in upper bar deposits composed of dirty and/or clean stacked successions (~ 0.5–1.5 m thick) of current ripple cross-laminae, which are the result of shallow flows over bars during tidal cycles and/or oscillatory currents derived from intrabasinal wind-waves modified by slackwater periods and activity of the estuarine turbidity maximum.
The delta of the Fly River is tide dominated, both morphologically and sedimentologically: the spring tidal range reaches 5 m, with near-bed tidal currents that commonly exceed 1 m/s, whereas wave influence is minimal, except during the monsoon period, when winds blow onshore. The delta plain displays a classic funnel-shaped geometry, with three main distributaries that flare seaward from a common bifurcation point 110 km inland. Of these, the southernmost is the main route for sediment export; the northernmost channel is effectively abandoned at present and is experiencing widespread erosion by tidal currents. Despite the net export of mud and sand, mutually evasive tidal-current patterns have created a series of elongate tidal bars within both active and abandoned distributary channels and in the distributary-mouth-bar area. The later is a zone of bedload convergence, with net seaward transport on its landward side and net landward transport on its seaward flank. This limits the offshore movement of sand.
The deposits of the Fly delta are dominated by mud, because the river flows along the low-gradient axis of the foreland basin before reaching the sea, causing deposition of most sediment coarser than fine to very fine sand farther inland. Distributary-channel bases are floored by a thin unit of cross-bedded and rippled sand and mud-pebble conglomerate. These are typically overlain abruptly by mud deposits formed by dense fluid-mud bodies that form in the channel bottoms during spring tides. The mud layers in this facies are anomalously thick (commonly > 1 cm), and channel-floor deposits are characterized by interbedding of the coarsest and finest sediments. Above these muds, the sediments are pervasively heterolithic and show a net upward coarsening to about the mid-depth level on the tidal bars because of the thinning of the mud layers. These bars may contain 50% (or more) mud and display lateral-accretion bedding. Bioturbation is scarce to absent. The sediments then fine upward into the intertidal zone. Clear indications of a tidal origin for the heterolithic stratification are relatively uncommon, although tidal rhythmites are present locally, including within active channels. The mouth-bar deposits are predominantly sand and also contain lateral-accretion bedding. The delta-front facies are heterolithic, with both millimeter- and decimeter-thick sand/mud alternations, all with a limited degree of bioturbation. The prodelta consists of biologically homogenized mud. The stratigraphic organization of the deposits reflects the fact that the delta plain aggraded vertically during the last part of the postglacial sea-level rise, producing a complex stack of channel deposits, followed by progradation of the mouth bars by as much as 40–50 km.
As international concern for the survival of deltas grows, the Mekong River delta, the world's third largest delta, densely populated, considered as Southeast Asia's most important food basket, and rich in biodiversity at the world scale, is also increasingly affected by human activities and exposed to subsidence and coastal erosion. Several dams have been constructed upstream of the delta and many more are now planned. We quantify from high-resolution SPOT 5 satellite images large-scale shoreline erosion and land loss between 2003 and 2012 that now affect over 50% of the once strongly advancing >600 km-long delta shoreline. Erosion, with no identified change in the river's discharge and in wave and wind conditions over this recent period, is consistent with: (1) a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment, (2) large-scale commercial sand mining in the river and delta channels, and (3) subsidence due to groundwater extraction. Shoreline erosion is already responsible for displacement of coastal populations. It is an additional hazard to the integrity of this Asian mega delta now considered particularly vulnerable to accelerated subsidence and sea-level rise, and will be exacerbated by future hydropower dams.
Most existing ichnological models predict an increase in burrow density and diversity from the zone of persistent brackish-water into the realm of persistent freshwater. Herein we compare the neoichnology of five tidal–fluvial channels with varying tidal magnitudes, saltwater incursion distances, and river discharge. The results indicate that there is a noticeable diminution in the sizes of marine traces and a corresponding decrease in their distribution (reduced abundance of burrowed versus unburrowed beds) with decreasing salinity. From the landward limit of saltwater incursion into the wholly freshwater tidal backwater, there is no concurrent increase in terrestrial or freshwater burrow forms; rather, burrow diversities remain low (range: 10 to 35% of the " open marine " signature) and burrow densities decrease from BI 0–3 to BI 0–1 in both sand and mud units. The five modern systems described in this study do not support the hypothesis that there is an increase in bioturbation from the brackish-water reaches of channels into freshwater reaches. In fact, freshwater channels, including those in the tidal backwater zone, are characterized by very low trace-fossil diversities (1 to 2 forms) as well as low and sporadically distributed bioturbation intensities (BI 0–1).
A better understanding of flow and sediment dynamics in the lowermost portions of large tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean. Flow velocity, salinity, and suspended-sediment concentration were measured for 25 hours at three cross-sections in the tidal Song Hau distributary of the Mekong River, Vietnam. Two campaigns took place during comparatively high and low seasonal discharge, and estuarine conditions varied dramatically between them. The system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow. The changing freshwater input, sediment sources, and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow. The Dinh An channel of the Song Hau distributary exported sediment to the coast at a rate of about 1 t s−1 during high flow and imported sediment in a spatially varying manner at approximately 0.3 t s −1 during low flow. Scaling these values results in a yearly Mekong sediment discharge estimate about 65% smaller than a generally accepted estimate of 110 Mt y−1, although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate. Fluvial advection of sediment was primarily responsible for the high-flow sediment export. Exchange-flow and tidal processes, including local resuspension, were principally responsible for the low-flow import. The resulting bed-sediment grain size was coarser and more variable during high flow and finer during low, and the residual flow patterns support the maintenance of mid-channel islands. This article is protected by copyright. All rights reserved.
Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-energy successions. Large-scale outcrops from the Middle Jurassic Lajas Formation (Argentina) expose a well-constrained stratigraphic succession of marginal-marine deposits with a strong fluvial influence and well-known tidal indicators. The studied deposits show decimetre-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities. The alternation of these two types of beds forms non-cyclic successions that are interpreted to be the result of seasonal variation in river discharge, rather than regular and predictable changes in current velocity caused by tides. Seasonal bedding is present in bar deposits that form within or at the mouth of minor and major channels. Seasonal bedding is not preserved in channel thalweg deposits, where river flood processes were too powerful, or in floodplain, muddy interdistributary bay, prodelta and transgressive deposits, where the river signal was weak and sporadic. The identification of sedimentary facies characteristic of seasonal river discharge variations is important for accurate interpretation of ancient deltaic process regime.
Sedimentological, neoichnological, palynological, and geochemical trends from upper subtidal and intertidal positions on channel bars in the lower Fraser River, Canada are synthesized into a single, coherent framework. From these data we define criteria for determining depositional position in shallow water depths in tide-influenced rivers. Three sedimentological trends are observed from the river mouth, through the locus of mud deposition (within the turbidity maximum zone (TMZ)), and into the freshwater-tidal zone. (1) The recurrence (per meter) and thickness of mud beds increase towards the TMZ and tapers in both the landward and seaward directions. (2) Muddy current ripples and graded current ripples are most abundant in the TMZ; they are less common with decreasing brackish-water influence and are absent in the freshwater river reach. (3) Heterolithic bedding (i.e., flaser, wavy, and lenticular) is common in the TMZ, less common seaward, and absent from the freshwater realm. In addition to the sedimentological trends, four ichnological trends parallel decreasing water salinity. With decreasing salinity, there is: (1) a decrease in bioturbation intensity from BI 2-3 to BI 0-1; (2) a decrease in the abundance of bioturbated beds; (3) a marked decrease in the diversity of traces from 5-6 forms to 1-2 forms; and, (4) a decrease in the diameter and length of traces. Traces are rare to absent in the tidal freshwater zone. Palynological and geochemical trends generally follow ichnological trends but are less obvious. Neither dinocyst abundance nor geochemical signature can be used to determine relative position in a tide-influenced river channel, although dinocyst abundances greater than 1% indicate significant marine influence, and 0-1% marine dinocysts indicate tidal influence. Although it is not feasible to determine exact depositional position within the tidal-fluvial transition, our results suggest that it is possible to determine where sediments were deposited relative to the TMZ. In turn, predicting relative depositional position can assist in unraveling stratigraphy and in recognizing nested channels in architecturally complex sedimentary successions.
Sand and gravel mining from rivers has increased throughout the developed countries since at least the mid-1900s, for construction (concrete) and for landfill (railways, motorways, land reclamation in flooded areas, offshore reclamation). Over the past 30 years, this phenomenon has spread due to the economic development of countries such as China, India and other fast-growing economies of South-East Asia. The detrimental impacts of aggregate extraction have been well documented in Europe and in the U.S. since the early 1980s and studies are now available on extraction in S.E. Asia, notably in China. The lower Mekong River and its tributary watersheds are hotspots for biodiversity, but have seen considerable dam construction, particularly along the Lancang, the upper course of the Mekong in China. Currently, the most important issue in the Mekong basin is certainly the threat to the delta coastline because of its importance in terms of food security for the growing populations of Vietnam and Cambodia. The reasons most often cited to explain the retreat of the delta are the trapping of sediment in the existing upstream reservoirs and climate change. Sand and gravel mining in the Mekong River bed and in the tributary channels has not been taken into consideration to date. This paper provides the first assessment of sediment extraction along the 2 400-kilometre channel of the main stem of the lower Mekong River, from the Chinese border to the delta. The World Wide Fund for Nature ran a survey in 2011-2012. Systematic interviews assessing the types of aggregates, extraction techniques, extracted volumes, trends over the past years and the expected changes in the geography of extraction were carried out in the four countries drained by the Lower Mekong. The survey results reveal that the yearly volume of sand and gravel extracted, despite its undervaluation, exceeds the transported volume of sand and gravel in the Mekong sediment load. The results also highlight the extreme importance of this factor in the geomorphologic and ecological changes of the Mekong reaches and the delta.
Numerous research efforts have been devoted to understanding estuarine morphodynamics under tidal forcing. However, the impact of river discharge on estuarine morphodynamics is insufficiently examined. Inspired by the Yangtze Estuary, this work explores the morphodynamic impact of river discharge in a 560-km long tidal basin based on a 1D model (Delft3D). The model considers total load sediment transport and employs a morphodynamic updating scheme to achieve long-term morphodynamic evolution. We analyze the role of Stokes drift, tidal asymmetry and river discharge in generating tidal residual sediment transport. Model results suggest that morphodynamic equilibrium is approached within millennia by vanishing spatial gradients of tidal residual sediment transport. We find that the interaction between ebb-directed Stokes return flow/river flow with tides is an important mechanism that flushes river-supplied sediment seaward. Increasing river discharge does not induce continuously eroded or accreted equilibrium bed profiles because of the balance between riverine sediment supply and sediment flushing to the sea. An intermediate threshold river discharge can be defined which leads to a deepest equilibrium bed profile. As a result, the shape (concavity or convexity) of the equilibrium bed profiles will adapt with the magnitude of river discharge. Overall, this study reveals the significant role of river discharge in controlling estuarine morphodynamics by supplying sediment and reinforcing ebb-directed residual sediment transport.
Spatio-temporal patterns of suspended particulate matter, SPM, in coastal waters under the Mekong's influence are examined through remote sensing data collected from January 2003 to April 2012 by the MEdium Resolution Imaging Spectrometer (MERIS) at full spatial resolution (300 × 300 m2). The first SPM climatology over this region is provided and the SPM temporal variation schemes (irregular variability, seasonal variability, and long term trend) are described using the Census-X-11 time series decomposition method. The different spatio-temporal patterns are then analyzed with regard to regional oceanographic and hydrologic conditions. The origin of the processes controlling the seasonality of the Mekong Delta plume is characterized. The increase of turbidity observed from June to December, starts with the Mekong sediment inputs which are maximum during the summer monsoon (the water discharge reaches its maximum in September/October). While the Mekong water discharge decreases, the concentration of suspended sediment keeps increasing in coastal waters during the following two/three months (November to January). This increase is explained by resuspension effects occurring in the shallow coastal areas. Due to higher wave energy and oblique orientation of the waves breaking near the coast, the winter monsoon triggers a high level of agitation and high value of resuspended material concentration which are submitted to a longshore current directed towards the South–West. Deposition (in front of the Delta) and erosion (northern and southern areas of the delta) areas are identified in good agreement with recent results obtained from a prognostic model. While the temporal variability is strongly dominated by the seasonal component, a long term trend of about − 5% SPM concentration per year is observed in the pro-delta area and is attributed to the decrease of the Mekong river sediment output during the high flow season.
The thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty-six vibracores totalling approximately 114 m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time-averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30 cm) and thin mud beds (up to 5 cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish-water transition zone comprises thin sands (up to 10 cm) and thicker muds (up to 19 cm), and the sustained brackish water tidal zone consists of thin muds (up to 6 cm) with relatively thicker sands (up to 25 cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish-water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo-turbidity maximum.This article is protected by copyright. All rights reserved.
An operational satellite-based approach was implemented to monitor turbidity and organic absorption in the Mekong river system. Using physics-based algorithms linked together in a fully automated processing chain, more than 300 Landsat Enhanced Thematic Mapper (ETM) scenes and 1000 MODIS scenes, representing five years of data, were used to produce standardized, quantitative time series of turbidity and organic absorption across Vietnam, Thailand, Cambodia, Laos, and China. To set up this system, the specific inherent optical properties (SIOPs) of the Mekong river system were determined through three separate field campaigns, laboratory analysis, and subsequent optical closure calculations. Following this, a range of satellite data types was tested using the derived Mekong-specific inherent optical properties, including Moderate Resolution Imaging Spectroradiometer (MODIS) 500 m data, Landsat ETM, Medium Resolution Imaging Spectrometer (MERIS), Satellite Pour l’Observation de la Terre (SPOT) 5, RapidEye, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and QuickBird. The satellite-based turbidity estimates were coincident with available field data, and comparisons showed them to be in good agreement. Overall, the derived SIOPs were suitable for water-quality monitoring of the Mekong, and the MODIS, MERIS, Landsat, and RapidEye sensors were found to be the most radiometrically stable and thereby suitable for ongoing operational processing. The implemented system delivers consistent results across the different satellite sensors and over time, but is limited to where the spatial resolution of the sensor is still able to resolve the river width. The system is currently applicable for the entire Mekong river system, both for near-real-time monitoring and for analysis of historical data archive.
The Mekong Basin in Southeast Asia is facing rapid development, impacting its hydrology and sediment dynamics. Although the understanding of the sediment transport rates in the Mekong is gradually growing, the sediment dynamics in the lower Mekong floodplains (downstream from Kratie) are poorly understood. The aim of this study is to conduct an analysis to increase the understanding of the sediment dynamics at the Chaktomuk confluence of the Mekong River, and the Tonle Sap River in the Lower Mekong River in Cambodia. This study is based on the data from a detailed field survey over the three hydrological years (May 2008 - April 2011) at the two sites (the Mekong mainstream and the Tonle Sap River) at the Chaktomuk confluence. We further compared the sediment fluxes at Chaktomuk to an upstream station (i.e. Mukdahan) with longer time series. Inflow sediment load towards the lake was lower than that of the outflow, with a ratio on average of 84%. Although annually only a small amount of sediment load from the Tonle Sap contributes to the delta (less than 15%), its share is substantial during the February-April period. The annual sediment load transport from the confluence to the delta in 2009 and 2010 accounted for 54 and 50 Mt, respectively. This was on average only 55% of the sediment fluxes measured at Mukdahan, more upstream station. And when compared to sediment loads further downstream at Cambodia-Vietnam border, we found that the suspended sediment flux continued to decline towards the South China Sea. Our findings thus indicate that the sediment load to the South China Sea is much lower than the previous estimate 150-160 Mt/yr. This article is protected by copyright. All rights reserved.
Tidal rivers feature oscillatory and steady gradients in the water surface, controlled by interactions between river flow and tides. The river discharge attenuates the tidal motion, and tidal motion increases tidal-mean friction in the river, which may act as a barrier to the river discharge. Time series of tidal water level amplitudes at five gauge stations along the River Mahakam in Indonesia, and tidal flow velocity amplitudes at a discharge monitoring station were obtained applying wavelet analysis. Temporal variations in tidal damping coefficients for quaterdiurnal, semidiurnal, and diurnal tidal species were quantified from the observed amplitude profiles. The analysis shows that tidal damping during the rising limb of a discharge wave differs from damping during the falling limb. Wavelet cross-correlations between surface levels yielded empirical estimates of wave numbers. An empirical relation between tidal damping and river flow is derived to describe subtidal bottom friction along an idealized tidal river resembling the Mahakam. The subtidal friction is decomposed into contributions from the river flow only, from river-tide interaction, and from tidal asymmetry. Even for high river flow and low tidal velocity, river flow enhances friction attributed to river-tide interaction, causing subtidal water level setup. A simple multilinear regression model using subtidal bottom friction is employed to predict subtidal water levels at locations upstream, with a relatively good agreement between predictions and observations. The explicit expression shows the nonlinear dependence of subtidal friction on river flow velocity, explaining the complex behavior of tidal-mean surface level profiles.
Three anomalous events (AEs) were recorded off the Fraser River, Canada during the freshet of 2008, and are ascribed to quasi-continuous gravity flows. These flows transport warm, low-salinity Fraser River waters to the upper delta front of the Fraser delta to at least 50 m water depth. All flows occurred during the freshet and spring ebbing tide, and were characterized by deposition of fine-grained sand and silt beds with likely high-water contents. These sediments were susceptible to resuspension by the subsequent flood-tide current, producing very high near bed suspended sediment concentrations (SSC) with enough excess density to generate hyper-concentrated flows. Our observations show that these episodic events result from the combination of high river discharge, high SSC, and strong tides, and therefore, are likely to occur on the delta fronts of other large tide-dominated and tide-influenced rivers. In shallow-water basins (< 50 m water depth), these tide-supported gravity flows can affect sediment deposition on both the delta front and prodelta.
Trends and interannual variability of the surface winds (SW), sea surface height (SSH), and sea surface temperature (SST) of the South China Sea (SCS) in 1993–2003 are analyzed using monthly products from satellite observations. Time series are smoothed with a 12-month running mean filter. The east and north components of the SW, SSH, and SST have linear trends of 0.53 ± 0.35 ms−1 decade−1, −0.04 ± 0.17 ms−1 decade−1, 6.7 ± 2.7 cm decade−1, and 0.50 ± 0.26 K decade−1, respectively. The sea level rising rate and sea surface warming rate are significantly higher than the corresponding global rates. An Empirical Orthogonal Function (EOF) analysis is performed to evaluate the interannual variability. Results show that the first EOF of the SW is characterized by a basin-wide anticyclonic pattern. The corresponding time coefficient function (TCF) correlates with the Nino3.4 index at the 99% confidence level, with a lag of 3 months. The first EOF of the SSH is characterized by a low sea level along the eastern boundary. The corresponding TCF correlates with the Nino3.4 at the 99% level, with a lag of 2 months. The first EOF of the SST is characterized by a basin-wide warming with the highest anomalies in the north deep basin. The corresponding TCF correlates with the Nino3.4 index at the 95% level, with a lag of 8 months. Based on the EOF analysis, the ENSO-associated correlation patterns of the SW, SSH, and SST are presented.
NGUYEN, T.V. AND TANAKA, H., 2007. Study on the Effect of Morphology Change on Salinity Distribution in the Dinh An Estuary, Lower Mekong River of Vietnam. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 268 - 273. Gold Coast, Australia, ISSN 0749.0208 Salinity intrusion into estuaries, in general, is mostly affected by river discharge and tidal level. Additionally, the change of salinity can also be attributed to other external forces such as wave height and river mouth morphology. However, there has been very few investigations conducted so far, taking into account these kind of aspects. Therefore, the present paper focused on examining the influence of morphology change on salinity concentration. The question that should be paid attention here is: how is the interaction between morphology change and salinity distribution in the Dinh An estuary, Vietnam? In this research, a three-dimensional model has been used to figure out the effects of morphology change on salinity distribution. The drawn conclusion is that the change of salinity is influenced by not only usual parameters as river discharge and tidal level but also another external force such as changes in the estuarine morphology. Furthermore, river mouth width is found to be directly proportional to the averaged salinity concentration. Therefore, the influence of morphology changes on salinity distribution should not be ignored when simulating salinity.
Deltaic sediments of the Mekong River delta sampled from a tidal beach in Vietnam during the wet season (late October 2011) showed strong Cs-137 and Cs-134 activities reflecting radionuclides released from the Fukushima Dai-ichi nuclear power plant (FDNPP) after the March 2011 Tohoku-oki earthquake and tsunami, but samples from the same site taken about three months later during the dry season (early February 2012) showed weak activities. This finding indicates that soil from the Mekong drainage basin was deposited along the delta front in the wet season and then removed in the dry season during the winter monsoon. Thus, seasonal changes in the sedimentary environment of sediment supply and accumulation were elucidated by Cs isotopes from the FDNPP accident. This finding may expand the usefulness of radionuclides for obtaining important information about geochemical events.
The geometry of river deltas is considered to reflect the interplay of coastal processes, river discharge, and sediment supply. As similar to 25% of the world's population lives on deltaic lowlands, prediction of delta growth is critical. Knowledge of processes responsible for delta geometries is not well established, although such knowledge is critical for the risk management of land use and settlements on deltas. The Mekong River delta of southern Vietnam is one of the largest deltas in the world and offers a unique opportunity to understand the sedimentary evolution of mixed tide- and wave-dominated large river deltas. We constrained the three-dimensional sedimentary evolution of the delta plains, based on optically stimulated luminescence dating of beach ridges coupled with radiocarbon-dated sediment cores. Results show that the beach shoreline in the lower delta plain initiated ca. 3.5 ka by aggradation on basement shoals. The delta plain propagated laterally during the late Holocene, evolving from bars that resulted in asymmetric bifurcation of the river mouth. Asymmetry was caused by the southwestward longshore sediment drift enhanced by the winter monsoon. Bars were successively formed on the wider side of the bifurcated river mouths, and subsequently accreted seaward, being stabilized by tide effects to result in shore-perpendicular elongate delta plains. Our work, based on the Mekong delta, demonstrates for the first time that bar emergence is a key process in the long-term evolution of mixed tide- and wave-dominated deltas.
In a sedimentological sense a fluvial to tidal transition zone can be defined in rivers as a zone that separates the upstream fluvial from the downstream estuarine zone. Characteristic sedimentary structures within this zone are notoriously difficult to recognize. This study demonstrates the influence of tidal modulation within the most proximal part of the fluvial-tidal transition zone (i.e. the ‘backwater zone’) of an ancient fluvial system. Criteria have been established to differentiate between purely fluvial facies and those modulated by tidal energy. The stratigraphic interval from which the data were derived is the Lourinhã Formation (Late Jurassic) of the Lusitanian Basin, Western Portugal. An analysis of sedimentary features at four key localities has identified a temporal spectrum of tidal influence ranging from the daily modulation of fluvial flows to the effects of tidal bore passage. A combination of quantitative and qualitative data reveals systematic changes in the internal architecture of dune-scale bedforms deposited in a channel-floor setting. The key co-occurring features are: (i) increasing-decreasing organic particle concentration; (ii) increasing-decreasing bottomset thickness; (iii) increasing-decreasing foreset dip and shape (from convex to concave); and (iv) increasing-decreasing brinkpoint height. Collectively, these features are interpreted as having been produced by successive fluctuations in flow regime conditions from lower (during flood tidal retardation) to higher (during ebb tidal drawdown) current velocities. Bedforms showing these features occur in both meandering fluvial channels and straighter distributary systems. In addition, several examples of a specific type of stepped erosion surface and draping sediment have been recognized, the interpretation of which strongly suggests generation by the passage of tidal bores. If this interpretation is correct, then it represents one of the first published examples of tidal bore propagation in ancient fluvial systems. Palaeoclimatic evidence (cellular analysis of woody tissue, palaeosol character and plate reconstruction) indicates a warm, seasonal, winter wet to summer dry climate during deposition of the Lourinhã Formation. From this evidence it is suggested that tidal modulation and tidal bore effects are more likely to develop in the ‘dry season’, when fluvial flow in the main river channels was reduced.
Two- and three-dimensional Delft3D Flow and Morphology models were constructed for the lower Song Hau distributary channel of the Mekong River in Vietnam to provide insights into the hydrodynamics and sand transport of the channel system. The models were calibrated and validated with data for observed water level, water discharge, velocity, and suspended-sand concentration during the high- and low-flow seasons of 2014 and 2015. The water and sand budgets of the Dinh An and Tran De channels, the two sub-distributaries of the lower Song Hau channel, were calculated, showing that ~73% of the fluvial water discharge and ~90% of suspended sand were transported through the Dinh An channel, while the rest was transported through the Tran De channel in the high-flow season. In the low-flow season, the total fluvial water discharge was < 20% of that in the high-flow season; and ~96% of it was transported through the Dinh An channel and the rest was transported through the Tran De channel. Sand was transported from channels into the ocean in both the Dinh An and Tran De channels during the high-flow season, but from the ocean into the two channels during the low-flow season. The reversed sand flux from ocean into river channels during low-flow season was not observed in field studies. The different behaviors between models and field observations might be caused by the absence of baroclinic effects in the model set up. The Dinh An channel was found to be dominated by ebb tide, in favor of transporting water and sediment to the ocean, and the Tran De channel was dominated by flood tide. In the high-flow season, the residual currents were directed toward the ocean for both the Dinh An and Tran De channels. In the low-flow season, the magnitude of the residual currents decreased in the river channels. Under these conditions, the residual-current direction changed to be ambiguous in the Tran De channel, but remained seaward in the Dinh An channel. The low fluvial discharge combined with strong tides under very low-flow conditions can probably generate a net clockwise water circulation between the Dinh An and Tran De channels, which transports water from the ocean to the river through the Tran De channel, then back into the ocean through the Dinh An channel. The clockwise circulation pattern is likely to influence the channel evolution and impact saltwater intrusion issue in the lower Song Hau channel.
River gauging stations do not accurately characterize sediment flux (magnitude and timing) through river-mouth distributaries and, thus, are not suitable for predicting changes to coastal morphology. This is because they are often upriver of tidal propogation where widely varying ratios of marine to freshwater influence have implications on sediment transport processes and storage that have yet to be thoroughly understood. To characterize sediment transport dynamics in this understudied zone, flow velocity, salinity, and suspended-sediment properties (concentration, size, and settling velocity) were measured within the tidal Sông Hậu distributary of the lower Mekong River, Vietnam. Fine-sediment aggregation, settling, and trapping rates were promoted by seasonal and tidal fluctuations in near-bed shear stress as well as the intermittent presence of a protective salt wedge and estuary turbidity maximum. Beginning in the tidal river, fine-grained particles were aggregated in freshwater. Then, in the tidal river - estuary interface zone, a region upstream of estuarine processes with relatively little study, impeded near-bed shear stress and particle flux convergence promoted settling and trapping. Finally, in the estuary, enhanced particle aggregation paired with estuarine circulation further encouraged sediment retention. These patterns promote mud export (~1.7 t s⁻¹) from the entire study area in the high-discharge season when fluvial processes dominate and mud import (~0.25 t s⁻¹) into the estuary and interface zone in the low-discharge season when estuarine processes dominate. Within the distributaries’ lower region, morphological change in the form of channel abandonment was found to be promoted within minor distributaries by feedbacks between channel depth, vertical mixing, and aggregate trapping. In effect, this field study coupled with collaborative remote sensing and modeling efforts has shed light on how fine-sediment dynamics and morphology of large tropical deltas such as the Mekong will respond to changing fluvial and marine influences.
The Mekong River Delta is one of the largest in Asia. To understand its sediment distribution, thickness, mass budget, stratigraphic sequences and sediment-transport process, extensive geophysical and geochemical surveys were conducted on the inner and portions of the adjacent continental shelf. Analyses of >80 high-resolution Chirp-sonar profiles show the Mekong River has formed a classic sigmoidal cross-shelf clinoform, up to 15 m thick, with topset, foreset and bottomset facies, but constrained to water depths of <20 m. Beyond this depth, the East Sea/western South China Sea shelf is dominated by relict silt, sand and gravel with patches of early to middle Holocene mud deposits. Parallel to shore, the Mekong-derived sediment has extended >250 km southwestward to the tip of the Ca Mau Peninsula, forming a distal mud depocenter up to 22 m thick, and extending into the Gulf of Thailand. A large erosional trough or channel (up to 8 m deeper than the surrounding seafloor and parallel to the shore) was found on the top of the clinoform, east of the Ca Mau Peninsula.
Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02 m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths <14 m, as expected from observed cross-shelf gradients of sedimentation. Sediment accumulation rates were greatest on the upper and lower foreset beds (>4 cm/yr at <10 m depth, and 3–8 cm/yr at ~10–20 m depth) and lowest on the bottomset beds. Physically laminated sediments transitioned into mottled sediments between the upper foreset and bottomset regions. Application of a simple wave-stress model to the Mekong and several other clinoforms illustrates that shallow systems are not necessarily energy-limited, and thus rollover depths cannot be predicted solely by bed-stress distributions. In systems like the subaqueous Mekong Delta, direction of transport may have a key impact on morphology.
Field studies in the Song Hau distributary of the Mekong Delta in Vietnam conducted at high (Sept.-Oct 2014) and low (March 2015) Mekong River discharge are utilized to examine channel bottom morphology and links with sediment transport in the system. Multibeam bathymetric mapping surveys over the entire channel complex in the lower 80 km of the distributary channel, and over 12- to 24-hour tidal periods at six transect locations in the reach are used to characterize bottom type and change on seasonal and tidal timescales, supplemented by bottom sampling. The results of this study indicate that the largest proportion of channel floor (up to 80% of the total area) is composed of substratum outcrops of relict sediment units deposited during the progradation of the delta in the last 3.5 ka. These take the form of outcrops that are either (1) steep-sided, tabular channel floor, (2) steep-sided sidewall, or (3) relatively flat channel floor. Flatter outcrops of channel floor substratum are identified by the presence of sedimentary furrows (<0.5 m deep) incised into the channel bottom that are exposed at high discharge and oriented along channel and laterally continuous for kilometers. These furrows are persistent in location and extent across tidal cycles and appear to be incised into relict units, sometimes with a thin surficial layer of modern sediment observable in bottom grabs. The extent of substratum exposure, greater than that observed previously in low tidal energy systems like the Mississippi River, may relate both to a relatively low sand supply from the catchment, and/or to an efficient transfer of both sand and mud through this tidally energetic channel. Sand bottom areas forming dunes, comprise about 19% of the channel floor over the study area and are generally less than a few meters thick except on bar extensions of mid-channel islands. Both sandy and substratum areas are mantled by soft muds 0.25–1 m thick during low discharge in the estuarine section of the study area. This mud mantling appears to be a key control on bottom sourcing of sand to suspension. An understanding of channel bottom morphology, particularly mobility and erodibility of sediments, is valuable for setting up morphodynamic models of channel evolution that can be used to test system response to anthropogenic alterations in the catchment and rising sea levels.
Ichnologic data reveal the complexity of the fluvial–tidal transition in terms of the interaction of riverine and marine processes and changes in the salinity conditions. The ichnofaunas of the fluvial–tidal transition are mainly controlled by the salinity limit that separates two environmental zones, seaward of this limit brackish-water ichnofaunas occur, whereas landward terrestrial/freshwater ichnofaunas are present. The ichnology of the fluvial–tidal transition is summarized based on a series of case studies, spanning the Carboniferous to the Miocene. Freshwater trace-fossil assemblages are widespread in the fluvial–tidal setting and are characterized by moderate to relatively high diversity of surface trails and meniscate trace fossils typically present in continental environments, whereas brackish-water trace-fossil assemblages are typified by common monospecific to low-diversity suites of diminutive simple marine trace fossils. In addition, a comparison of freshwater ichnofaunas at the fluvial–tidal transition through time reveals a remarkable evolutionary control. Whereas Paleozoic ichnofaunas are dominated by shallow-tier grazing trails and arthropod trackways, post-Paleozoic examples are dominated by deeper-tier meniscate trace fossils, which significantly contributed to a remarkable increase in intensity of bioturbation of deposits formed in fluvial–tidal transition. Based on information from the stratigraphic record of freshwater ichnofaunas, these changes could be linked to the Mesozoic Lacustrine revolution, an evolutionary event representing a breakthrough for freshwater biotas. The increase in depth and extent of bioturbation obliterated shallow-tier traces and allowed overprinting of previously emplaced brackish-water suites, resulting in the formation of composite ichnofabrics.
1) The velocity of the ebb flow upstream from Taipingzhou and downstream from Jiangxinsha below Zhenjiang is higher than that of the reaches between them. 2) The sediments below Zhenjiang become gradually finer downstream. This distribution disagrees with the velocity change of the flow in the river mouth area. 3) The marine microfossils go upstream as far as the flood current (to Taipingzhou).-from English summary
This paper provides some meteo-hydro-dynamical features in Mekong River Mouth area which were based on statistical results from longtime measurement data such as wind, wave, sea water level, and river discharge. Study results show that distribution features of wind and wave characteristics are corresponding to seasonal variation. Predominant wind and wave directions during NE Monsoon period is NE, and in W, SW directions during SW Monsoon period. NE Monsoon period is from November to April (strongest in December and January), SW Monsoon period is from June to September (strongest in August). During NE Monsoon period the wind velocity and wave height were higher than that of SW Monsoon period. Variation of SWL at Vungtau Station shows that the tide is semi-diurnal with maximum value occurred during NE monsoon period (highest in November), and minimum value occurred during SW Monsoon period (lowest in June). Variation of river discharge is similarly at two stations (Mythuan and Cantho). During flood season (from July to December) average monthly discharge was ranged from 4,000 to 15,800 m³/s. During dry season (from January to May) average monthly discharge at both stations was less than 4,000 m³/s. Statistical data show that Mekong River Mouth area is a region of strong and complicated hydro-dynamical interaction processes between river and sea.
Key words: Monsoon, North-East (NE), South-West (SW), Sea Water Level (SWL), River discharge, Mekong River.
Field studies of fine sediment dynamics carried out in the Mekong River estuary, Vietnam, during the low flow season in April 1996 show semidiurnal macrotides and shallow water effects result in a tidal asymmetry with peak flood tidal currents 10% stronger than peak ebb tidal currents. The salinity intrusion extended 50 km up-river with vertical stratification in salinity occurring around slack tidal currents. The suspended sediment was mainly fine silt, with flocculation occurring in the saline region. The asymmetry of tidal currents, along with the baroclinic circulation, pump sediment upstream. The saline water region of the estuary was more turbid than the freshwater region and the location of the turbidity maximum varied spatially with the tides. Our study suggests that the proposed construction of about 100 hydroelectric dams and water diversion schemes on the Mekong River and tributaries will impact negatively on the Mekong delta.
Current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments. There are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides. As a consequence, the ability to predict bedform response to variable flow is rudimentary. Using high-resolution multibeam bathymetric data, this study explores the dynamics of a dune field developed by tidally-modulated, fluvially-dominated flow in the Fraser River Estuary, British Columbia, Canada. The dunes were dominantly low lee angle features characteristic of large, deep river channels. Data were collected over a field approximately 1.0 km long and 0.5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet, yielding the most comprehensive characterization of low-angle dunes ever reported. The data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs, then increasing as the tide rises and the flow velocities decrease. Bedform lengths do not appear to respond to the changes in velocity caused by the tides. Changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity, indicating that changes in the bedform geometry lag changes in the flow. The data reveal that lee angle slope responds directly to suspended sediment concentration, supporting previous speculation that low angle dune morphology is maintained by erosion of the dune stoss and crest at high flow and deposition of that material in the dune trough.This article is protected by copyright. All rights reserved.
Inclined heterolithic stratification in the Lower Cretaceous McMurray Formation, exposed along the Steepbank River in north-eastern Alberta, Canada, accumulated on point bars of a 30 to 40 m deep continental-scale river in the fluvial–marine transition. This inclined heterolithic stratification consists of two alternating lithologies, sand and fine-grained beds. Sand beds were deposited rapidly by unidirectional currents and contain little or no bioturbation. Fine-grained beds contain rare tidal structures, and are intensely bioturbated by low-diversity ichnofossil assemblages. The alternations between the sand and fine-grained beds are probably caused by strong variations in fluvial discharge; that are believed to be seasonal (probably annual) in duration. The sand beds accumulated during river floods, under fluvially dominated conditions when the water was fresh whereas the fine-grained beds accumulated during the late stages of the river flood and deposition continued under tidally influenced brackish water conditions during times of low river flow (i.e. the interflood periods). These changes reflect the annual migration in the positions of the tidal and salinity limits within the fluvial–marine transition that result from changes in river discharge. Sand and fine-grained beds are cyclically organized in the studied outcrops forming metre-scale cycles. A single metre-scale cycle is defined by a sharp base, an upward decrease in sand-bed thickness and upward increases in the preservation of fine-grained beds and the intensity of bioturbation. Metre-scale cycles are interpreted to be the product of a longer-term (decadal) cyclicity in fluvial discharge, probably caused by fluctuations in ocean or solar dynamics. The volumetric dominance of river-flood deposits within the succession suggests that accumulation occurred in a relatively landward position within the fluvial–marine transition. This study shows that careful observation can reveal much about the interplay of processes within the fluvial–marine transition, which in turn provides a powerful tool for determining the palaeo-environmental location of a deposit within the fluvial–marine transition.This article is protected by copyright. All rights reserved.
The Mekong delta, in Vietnam, is the world's third largest delta. Densely populated, the delta has been significantly armoured with engineering works and dykes to protect populations and infrastructure from storms, and shrimp farms from saltwater intrusion. Considerable development pressures in Vietnam and in the upstream countries have resulted in the construction of several dams in China and in important channel-bed aggregate extractions especially in Cambodia. The effects of these developments impact the delta dynamics in various ways. In this study, changes in the channel morphology of the Mekong proper and the Bassac, the two main distributaries in the 250 km-long deltaic reach from the Cambodian border to the coast, were analysed using channel depth data for 1998 and 2008. The channels display important and irregular bed changes over the 10-year comparison period, including significant incision and expansion and deepening of numerous pools. The mean depth of both channels increased by more than 1.3 m. Both channels also showed correlative significant bed material losses: respectively 90 million m3 in the Mekong and 110 million m3 in the Bassac over the 10-year period. These important losses over a relatively short period, and weak correlations between bed incision and hydraulic parameters suggest that the marked morphological changes are not in equilibrium with flow and sediment entrainment conditions, and are therefore not related to changes in river hydrology. We claim that aggregate extraction, currently practised on a very large scale in the Mekong delta channels and upstream of the delta, is the main cause of these recent morphological changes. These changes are deemed to contribute actively to rampant bank erosion in the delta as well as to erosion of the Mekong delta shoreline. Other contributory activities include the numerous dykes and embankments. The role of existing dams in bed losses remains unclear in the absence of reliable data on the Mekong sediment load. Future large-scale hydropower dam development on the Lower Mekong main stem will have cumulative effects with aggregate extraction, leading to increased morphological changes that will further impact the delta, especially in terms of sediment supply and coastal erosion.
A quantitative analysis of the depositional processes on the Mahakam Delta indicates that it is presently subsiding and is, in essence, a drowned delta that is being transgressed and modified by marine processes. Calculations of sediment transport rate indicate that most, if not all, fluvially derived sand is being stored onshore in the distributaries, whilst finer-grained sediment moves offshore. A fining-upward and increasingly marine-upward succession is being deposited in the distributaries, which is analogous to nearby outcropping and subsurface successions that have previously been interpreted as progradational. The mixed fluvial and tide-dominant shoreline morphology is not solely a product of the deltas present-day processes. The fluvial component is a relict feature from a phase of progradation that preceded the ongoing transgression and is now being modified by tidal processes. Facies distribution is a much better indicator of modern depositional processes than delta morphology on the Mahakam Delta, suggesting that facies-based delta classifications are more accurate than morphology-based classifications. All the apparently anomalous components of the sedimentology and morphology are reconciled by a transgressive interpretation, including the overly deep distributaries, gently dipping subaqueous delta plain, penetration by benthic marine organisms far into the distributaries, widespread Nypa palm on the lower delta plain, and the long, mud-filled gap between sand in the distributaries and on offshore bars.
We use a one-dimensional morphodynamic model to analyze the long-term evolution of the lower reaches of the Fly River, Papua New Guinea, from the Everill Junction to the delta mouth. The model shows how the break in the exponential trend of river width triggers deposition, thus producing a tidal region characterized by a higher bed elevation with respect to the river-dominated one. Numerical simulations indicate that the river attains a dynamic equilibrium configuration in which the amount of sediment entering upstream is flushed seaward. A sensitivity analysis is performed, in which the effect of varying solid discharge, tidal harmonics, and initial conditions is discussed. The model shows that an equilibrium configuration results from a delicate balance between the aggrading effect associated with channel divergence (acting mainly during neap tide and at slack water) and the opposite effect of tidal flushing driven by residual water discharge. A physically meaningful morphodynamic equilibrium occurs only for a small range of values of sediment discharge prescribed at the upstream boundary. In particular, an increase in sediment discharge leads to aggradation, while a decrease triggers extensive scour and a deepening of the estuary.
A Coupled Wave–Ocean–Sediment Transport Model was used to hindcast coastal circulation and fine sediment transport on the Mekong shelf in southeastern Asian in 2005. Comparisons with limited observations showed that the model simulation captured the regional patterns and temporal variability of surface wave, sea level, and suspended sediment concentration reasonably well. Significant seasonality in sediment transport was revealed. In summer, a large amount of fluvial sediments was delivered and deposited near the Mekong River mouth. In the following winter, strong ocean mixing, and coastal current lead to resuspension and southwestward dispersal of a small fraction of previously deposited sediments. Model sensitivity experiments (with reduced physics) were performed to investigate the impact of tides, waves, and remotely forced ambient currents on the transport and dispersal of the fluvial sediment. Strong wave mixing and downwelling-favorable coastal current associated with the more energetic northeast monsoon in the winter season are the main factors controlling the southwestward along-shelf transport.
An occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40-m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea. The channel bank demonstrates a convex-upward profile with intermittent presence of wave-formed scarps and terraces near the low-water level. The vertical succession of IHS is approximately 25 m thick and dips into the channel with angles reaching 14degrees. The IHS overlies 15 m of trough cross-bedded sand deposited in the channel thalweg and lower point bar. Even though the channel as a whole is ebb dominated, the preserved cross bedding is predominantly flood directed because the mutually evasive nature of the ebb and flood currents causes the point-bar surface to be flood dominated. This pattern may be a common feature of tidal point bars. The IHS itself consists of interstratified fine sand, sandy silt, and silt with an upward-fining textural trend. Tidal rhythmites are well developed in the middle and upper intertidal zone, and may also be present in the subtidal zone, but are poorly developed near the low-water level because of wave action. Seasonal discharge variations of the Han River are not obvious in the deposits, because the large size, distal location, and energetic tidal environment of the studied channel reduces the impact of river-stage fluctuations. Despite the moderate salinity levels, bioturbation is rare, except in the upper intertidal zone, because of the rapid sedimentation and energetic conditions.
Most tide-dominated estuarine and deltaic deposits accumulate in the fluvial-to-marine transition zone, which is one of the most complicated areas on earth, because of the large number of terrestrial and marine processes that interact there. An understanding of how the facies change through this transition is necessary if we are to make correct paleo-environmental and sequence-stratigraphic interpretations of sedimentary successions. The most important process variations in this zone are: a seaward decrease in the intensity of river flow and a seaward increase in the intensity of tidal currents. Together these trends cause a dominance of river currents and a net seaward transport of sediment in the inner part of the transition zone, and a dominance of tidal currents in the seaward part of the transition, with the tendency for the development of a net landward transport of sediment. These transport patterns in turn develop a bedload convergence within the middle portion of all estuaries and in the distributary-mouth-bar area of deltas. The transport pathways also generate grain-size trends in the sand fraction: a seaward decrease in sand size through the entire fluvial–marine transition in deltas, and through the river-dominated, inner part of estuaries, but a landward decrease in sand size in the outer part of estuaries. A turbidity maximum (i.e., a zone of significantly elevated suspended-sediment concentrations) is developed within estuaries and the delta-plain region of deltas as a result of flocculation and density-driven water-circulation patterns. This leads to an area within the estuary or delta plain where the abundance and thickness of the mud drapes are greatest, including the potential for the development of fluid-mud deposits (i.e., structureless mud layers more than 0.5–1 cm thick that were deposited in a single slack-water period). A monotonic seaward increase in salinity characterizes both estuaries and deltas. The brackish-water conditions in the transition zone, accompanied by the high turbidity and physically harsh conditions, produce a biologically stressed environment, in which bioturbation is generally not pervasive. The ichnofossil assemblage in this zone is characterized by the low diversity of ichnogenera, small size of the individual burrows (typically smaller than their open-marine counterparts), and highly variable population densities, ranging from unbioturbated to very high-density mono-specific assemblages in local areas.This review begins with a survey of how and why each depositional process varies through the fluvial-to-marine transition and then examines the sedimentological responses to these processes, focussing on the observable, longitudinal variations in the development and/or abundance of each deposit characteristic (e.g., sand grain size, paleocurrent patterns, mud drapes, and biological attributes). The review ends with a summary of the characteristics of each major facies zone through the transition, with separate discussions for both estuaries and deltas. It must be noted that any attempt to generalize, as is done here, will undoubtedly contain statements that are inappropriate for individual deposits or systems. Exceptions must be expected and the concepts must be applied with due consideration for the local context.
Rivers provide the primary link between land and sea. Utilizing the world’s largest database, this book presents a detailed analysis and synthesis of the processes affecting fluvial discharge of water, sediment and dissolved solids. The ways in which climatic variation, episodic events, and anthropogenic activities — past, present and future — affect the quantity and quality of river discharge are discussed in the final two chapters. The book contains 26 tables and more than 165 figures — many in full color — including global and regional maps. The book’s extensive appendix presents the 1534-river database as a series of 44 tables and 132 maps that provide quantitative data regarding the discharge of water, sediment and dissolved solids. The complete database is also presented within a GIS-based package available online at www.cambridge.org/milliman. River Discharge to the Coastal Ocean provides an invaluable resource for researchers, professionals and graduate students in hydrology, oceanography, geology, geomorphology and environmental policy.
Communicated by J.T. Wells Keywords: delta Mekong River monsoon OSL dating sediment discharge Vietnam Empirical understanding of decadal-to centennial-scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries. We tested the effectiveness of quartz optically-stimulated luminescence (OSL) dating of beach ridges to constrain shoreline changes of the Mekong River delta over the last 3500 years. Forty-seven OSL ages have been analyzed from the Tra Vinh delta plain, cen-tral Mekong River delta. The Tra Vinh beach ridges are recurved and branching, showing the hierarchy of shore-line changes, which include discontinuous shifts c. 5 km seawards caused by the emergence and elongation of a delta-front bar/island, and subsequent downdrift accretion of spits. The spit accretion is interrupted by changes of shoreline orientation. Each of the discontinuous shifts resulted in a cluster of beach ridges, which is referred to as ridgeset. The beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c. 5%. The OSL chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges. The chronology clearly illustrates the coastal progradation from 3500 years (relative to AD 2010) onwards. Two main types of beach ridge are present: recurved and trunk ridges. OSL ages of recurved ridges are consistently younger both downdrift and seawards, documenting decadal-to centennial-scale shoreline migration especially over the last 1500 years. Trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves. Ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset, representing the extensive erosion and resedimentation of the updrift coast. Major changes in shoreline orientation occurred around 400–500 years, suggesting strengthening of the northeasterly winter monsoon at the beginning of the Little Ice Age. The rate of progradation of the Tra Vinh delta plain is described in two ways: frequency of the discontinuous shift, and growth rate of the delta plain area. Both suggest the coast has expanded regularly over the last 1000 years. A decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred, possibly affecting the behavior of the modern and future shorelines, which can be compared with the less human-influenced, centennial-to decadal-scale past shoreline changes reconstructed in this study.
