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Impacts of dams on the sediment flux of the Pearl River, Southern China

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Abstract

Draining toward the South China Sea, the Pearl River is the third largest river in China. Based on datasets begun in the 1950s of water and sediment flux at the main gauging stations and measured deposition rates in some reservoirs, this paper examines the impact of dam construction on the sediment flux of the Pearl River. The results and conclusions are as follows: (1) The total storage capacity of reservoirs in the Pearl River basin had reached 65 km3 by 2005, which is 23% of the annual water discharge of the Pearl River. (2) The total deposition rate in the reservoirs has now probably reached 600 Mt/yr, one order of magnitude higher than the annual sediment flux into the sea (40 Mt/yr in 2000–2005). (3) The sediment flux of the Pearl River has shown a drastically decreasing trend since the mid-1980s, which is attributed mainly to deposition in the reservoirs. (4) A further decrease in sediment flux into the sea is expected to occur in the Pearl River in future decades because of the construction of new dams. This drastic decrease in sediment flux may be very import for the environments of the river channel, the estuary and the coastal areas, which need to paid considerable attention in scientific research and management.

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... In the twentieth century, mechanical erosion was amplified by intense human activities and precipitous relief which would increase the flux of SPM in Zhujiang River theoretically (Li et al., 2019). For instance, rock desertification induced by deforestation is present in around 11% of the whole basin, which accelerates the denudation of surface rocks and soils (Dai et al., 2008). In contrast, the SPM fluxes of the study basin has drastically reduced due to the construction of more than 9000 dams since the 1950s, which causes some potential adverse influence on the ecosystem of the local region (Dai et al., 2008). ...
... For instance, rock desertification induced by deforestation is present in around 11% of the whole basin, which accelerates the denudation of surface rocks and soils (Dai et al., 2008). In contrast, the SPM fluxes of the study basin has drastically reduced due to the construction of more than 9000 dams since the 1950s, which causes some potential adverse influence on the ecosystem of the local region (Dai et al., 2008). The land use patterns of the basin are mainly composed of cropland, forestland, grassland and urban land ( Fig. 1) (Liu and Han, 2020a;Zeng and Han, 2020a). ...
... With the construction of more than nine thousand dams across the Zhujiang River since the 1950s, the flux of sediments to the South China Sea has dramatically decreased (Dai et al., 2008). Since more than 99% of fluvial iron is exported to the sea in the form of suspended materials, only iron in SPM was calculated in this study. ...
Article
Understanding the environmental iron cycle influenced by natural and anthropogenic processes is significant to obtain the key information on earth-surface evolution. Iron isotope compositions and elemental compositions of the suspended particulate matter (SPM) in Zhujiang River were investigated to provide key insights for the earth-surface iron cycle. The δ56Fe values of SPM display the range from -0.05‰ to 0.34‰ (averaged 0.19‰) while the iron contents range from 0.73 wt% to 7.63 wt% (averaged 4.15 wt%). The Chemical Index of Alteration (CIA) shows that the main weathering type of SPM is intermediate weathering (mean CIA value: 79.12). While the main chemical weathering types are similar, the δ56Fe values vary dramatically, indicating that chemical weathering is not the governing factor of δ56Fe values of SPM. Furthermore, evidence from the enrichment factors (EF, 0.78 to 1.29) of iron and Zinc isotopes reveals that the iron input from anthropogenic activities is quite limited. The significant correlation between (Fe/Ca)SPM and (Na/Ca)SPM, (Mg/Ca)SPM, (Al/Ca)SPM and (K/Ca)SPM (0.73 < r < 0.99, p<0.01, n=22) and the A–CN–K diagram could confirm that clay minerals (especially smectite and illite) are important components of SPM. Meanwhile, the higher enrichment rates of La, Pr and Nd with the increasing iron contents denote heavy minerals (mainly Fe oxides/oxy-hydroxides) are also important compositions of SPM. Based on the isotopic mass balance, it shows that the iron flux of SPM to the oceans during the wet season would reduce the δ56Fe values of the bulk ocean by 0.3%. The present study reports the iron isotope and elemental compositions of Zhujiang SPM and identifies its influencing factors (weathering, anthropogenic inputs, and minerals effect), and also provides a quantitative reference for the intriguing question on the distinct δ56Fe values of oceans, which is beneficial for understanding iron cycle in earth-surface system.
... The spatial and temporal variation of TSS concentrations in estuaries and coasts are not only the issues of economic production activities closely related to estuarine coast, such as trade, transportation and fishery production, but also the focus of sustainable development of the human society, such as coastal zone planning and ports and waterways construction Mao et al., 2012). This topic has been paid great attention by many scholars, and the associated research works have been conducted in many regions, such as the foreign French Guyana, Scheldt, Gironde and Rio de la Plata estuaries (Dogliotti et al., 2015;Doxaran et al., 2002), the Mekong and Bassac river (Loisel et al., 2014), the Amazon River (Park and Latrubesse, 2014), the Mississippi River and its tributaries (Olmanson et al., 2013), and the domestic Yangtze River Shen et al., 2013;Yang et al., 2014;Zhang et al., 2010;Chen et al., 2012), Yellow River (Peng et al., 2010;Zhang et al., 2014;Zhang et al., 2010;Liu et al., 2011), Pearl River (Fang et al., 2010;Chen et al., 2005;Dai et al., 2008;Wu et al., 2014). ...
... Based on long time (more than 50 years) series of measured data (hydrological, meteorological and statistical data), many studies (Golosov et al., 2017;Yang et al., 2014;Dai et al., 2008;Liu et al., 2011;Wu et al., 2014) analyzed the change law and trends of total sediment load/transport in the whole area. However, these works have limit to show the spatial distribution of TSS concentrations, and could not know the difference of different spatial distribution and the corresponding changes. ...
... The annual mean surface runoff is 3.26 · 10 8 m 3 and sediment load is about 7.53 · 10 7 ton/year on average. There is a decreased trend for water discharge and sediment load in Pearl River basin in nearly 30 years (Dai et al., 2008;Wu et al., 2014). ...
... Heise et al. (2010) assumed that a larger amount of sediment was deposited in the PREA than was eroded [49]. Furthermore, dams also impact upon sediment flux [50]. Dai et al. (2008) found that the sediment flux of the Pearl River had decreased since the 1980s [50] and suggested that more sediment was deposited in the reservoirs. ...
... Furthermore, dams also impact upon sediment flux [50]. Dai et al. (2008) found that the sediment flux of the Pearl River had decreased since the 1980s [50] and suggested that more sediment was deposited in the reservoirs. As more dams were established in the PREA, less sediment flux would be transported into the sea. ...
... Furthermore, dams also impact upon sediment flux [50]. Dai et al. (2008) found that the sediment flux of the Pearl River had decreased since the 1980s [50] and suggested that more sediment was deposited in the reservoirs. As more dams were established in the PREA, less sediment flux would be transported into the sea. ...
Article
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The Pearl River Estuary Area was selected for this study. For the past 40 years, it has been one of the most complex coasts in China, yet few studies have analyzed the complexity and variations of the area’s different coastlines. In this investigation, the coastlines of the Pearl River Estuary Area were extracted from multi-temporal Landsat remote sensing data from 1978, 1988, 1997, 2008, and 2018. The coastline of this area was classified into mainland, island, and estuarine. To obtain more detailed results of the mainland and island, we regarded this area as the main body, rezoned into different parts. The box-counting dimension was applied to compute the bidimensional (2D) fractal dimension. Coastline length and the fractal dimension of different types of coastline and different parts of the main body were calculated and compared. The fractal dimension of the Pearl River Estuary Area was found to have increased significantly, from 1.228 to 1.263, and coastline length also increased during the study period. The island and mainland showed the most complex coastlines, while estuaries showed the least complexity during the past forty years. A positive correlation was found between length and 2D-fractal dimension in some parts of the study area. Land reclamation had the strongest influence on fractal dimension variations.
... Base on the combined effects of climate change and human activities, the WD and SL of the PR have changed significantly over time. Many studies have been carried out to understand the variability of WD and SL of the PR [18][19][20][21][22] . Some studies have investigated the spatial and temporal variations of WD and SL in the Pearl River basin (PRB). ...
... Since the Soil Preservation Law of the Republic of China was enacted in 1991, great efforts have been carried out to prevent soil erosion in Pearl River. According to the remote sensing images of the National Land Use Survey, the area of soil erosion in the PRB in 1995 and 2004 were 62.70 × 10 3 km 2 and 62.73 × 10 3 km 2 , respectively 18 year). These observations implies that although efforts at soil conservation were responsible for the decrease in SL, dam construction was the main factor causing the reduction of SL. ...
... Although there are other factors contributing to these changes, such as in-channel sediment mining and unreasonable coastal construction, the decline of SL is a dominant factor. Furthermore, given the large number of reservoirs in the PRB and their enormous storage capacity, the SL in the main river is expected to remain low on a century timescale 16,18 . Therefore, in future scientific research and management projects, increased attention must be paid to the long-term effects of a reduction in sediment flux on environmental changes. ...
Article
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Global climate change and human activities have important effects on the water discharge and sediment load of the Pearl River. In this study, the water discharge and sediment load were investigated by using hydro-meteorological data from 1954 to 2018. The linear regression, Mann–Kendall abrupt test and double mass curve were employed to detect trends and abrupt change-points in water discharge and sediment load and to quantify the effects of climate change and human activities on water discharge and sediment load. The results revealed that the annual sediment load exhibited a significant decreasing trend at a rate of − 2.24 × 10⁴ t/year, regardless of water discharge, and an abrupt change occurred in 1998. Human activities, especially dam construction contributed 96% to this change, while 4% was due to climate change. El Niño/Southern Oscillation (ENSO) events are often associated with low precipitation, resulting in low water discharge and sediment load, indicating that changes in ENSO periodicity could affect the inter-annual periodic variations of water discharge and sediment load. As population and economy boom, more dams are being built in the Pearl River basin, and special attention should be paid to the management and mitigation of the effects of dams on sediment load.
... However, with the implementation of the national reform and opening policy in the 1980s, the population and economy of the Pearl River Basin began to develop rapidly. Human activities, including dam construction in the river basin [20][21][22][23] , land reclamation, channel dredging, and sand extraction in the estuary [24][25][26] , have greatly accelerated in the Pearl River Basin and the PRE. Consequently, the natural changes in the sediment load from the river and the morphological evolution of the estuary have been severely disturbed. ...
... However, the sediment load exhibited a significant decreasing trend since the 1990s, regardless of the water discharge. Dam construction in the drainage basin is the main reason for the drastic reduction in sediment load 20,22,23,37 . Statistics indicate that over 9000 dams and reservoirs have been constructed in the Pearl River Basin since the 1950s. ...
Article
Full-text available
The evolution of the Pearl River Estuary (PRE), China in recent decades has been dominated by human activities. Historical admiralty charts and remote sensing images indicated that from 1936 to 2017, the tidal flat area and water area decreased by 23.6 × 10 ⁷ m ² and 60.7 × 10 ⁷ m ² , respectively. The average advancing rate of the coastline of the PRE to the sea from 1972 to 2017 reached approximately 64.8 m/year, which is several times or even dozens of times that since the mid-Holocene. Land reclamation was the main reason for the dramatic changes in the water area and coastline. Although the water volume of the PRE showed a decreasing trend from 1936 to 2017, the water volume reduction rates for 1996–2005 and 2005–2017 were only 29% (1.27 × 10 ⁷ m ³ /year) and 12% (0.53 × 10 ⁷ m ³ /year), respectively, of that for 1936–1972. The combined influences of channel dredging, sand mining, and sediment load reduction caused by dam construction have contributed to this change. From the perspective of the filling up of the estuary, channel dredging, sand mining, and dam construction in the river basin are beneficial for prolonging the life of the estuary.
... Water samples for sediment concentrations (SCs) are collected from each depth, and these samples are dried and weighed in the lab. Finally, daily sediment loads are computed by multiplying the SCs by the streamflow [46,47]. Reservoir. ...
... Water samples for sediment concentrations (SCs) are collected from each depth, and these samples are dried and weighed in the lab. Finally, daily sediment loads are computed by multiplying the SCs by the streamflow [46,47]. Table S1 reports the brief statistics of the used streamflow (Q) and sediment (S) data. ...
Article
Full-text available
The accurate estimation of suspended sediments (SSs) carries significance in determining the volume of dam storage, river carrying capacity, pollution susceptibility, soil erosion potential, aquatic ecological impacts, and the design and operation of hydraulic structures. The presented study proposes a new method for accurately estimating daily SSs using antecedent discharge and sediment information. The novel method is developed by hybridizing the multivariate adaptive regression spline (MARS) and the Kmeans clustering algorithm (MARS–KM). The proposed method’s efficacy is established by comparing its performance with the adaptive neuro-fuzzy system (ANFIS), MARS, and M5 tree (M5Tree) models in predicting SSs at two stations situated on the Yangtze River of China, according to the three assessment measurements, RMSE, MAE, and NSE. Two modeling scenarios are employed; data are divided into 50–50% for model training and testing in the first scenario, and the training and test data sets are swapped in the second scenario. In Guangyuan Station, the MARS–KM showed a performance improvement compared to ANFIS, MARS, and M5Tree methods in term of RMSE by 39%, 30%, and 18% in the first scenario and by 24%, 22%, and 8% in the second scenario, respectively, while the improvement in RMSE of ANFIS, MARS, and M5Tree was 34%, 26%, and 27% in the first scenario and 7%, 16%, and 6% in the second scenario, respectively, at Beibei Station. Additionally, the MARS–KM models provided much more satisfactory estimates using only discharge values as inputs.
... Water samples for sediment concentrations (SCs) are collected from each depth, and these samples are dried and weighed in the lab. Finally, daily sediment loads are computed by multiplying the SCs by the streamflow [46,47]. Reservoir. ...
... Water samples for sediment concentrations (SCs) are collected from each depth, and these samples are dried and weighed in the lab. Finally, daily sediment loads are computed by multiplying the SCs by the streamflow [46,47]. Table S1 reports the brief statistics of the used streamflow (Q) and sediment (S) data. ...
Preprint
Full-text available
The presented study proposes a new method for accurately estimating suspended sediments us-ing antecedent discharge and sediment information. The method is developed by hybridizing multivariate adaptive regression spline (MARS) and Kmeans clustering algorithm (MARS-KM). The efficacy of the proposed method is established by comparing its performance with adaptive neuro-fuzzy system (ANFIS), MARS and M5 tree (M5Tree) models in predicting suspended sediment.
... The Pearl River, a large river in Asia, drains an area of 4.5×10 5 km 2 in South China and flows into the northern South China Sea under a (sub)tropical monsoon climate (Tong, 2007). The river-derived sediments (75.3 Tg yr −1 , Dai et al., 2008) are mostly deposited in the estuary and on the adjacent inner shelf, forming a mudbelt towards the Leizhou Peninsula in the southwest (Ge et al., 2014). The export of OC terr by the Pearl River is controlled by intensive weathering and soil erosion of the world's largest karst area in the upper reaches and intense human activities in its lower reaches (Luk et al., 1997). ...
... iment fluxes are calculated to be 32.2 ± 5.6 and 38.3 ± 6.6 Tg yr −1 for the two zones (Table 2), yielding a total sediment flux of 70.5 ± 8.7 Tg yr −1 for the entire proximal area. According to the estimation by Dai et al. (2008), the annual mean particulate export flux from the Pearl River is 75.3 Tg yr −1 . ...
Article
Tropical and subtropical rivers deliver large quantities of terrestrial organic carbon (OCterr) to the ocean, acting as a crucial part of the global carbon cycle, but little is known about the timescale and efficiency of its transport to and in the adjacent coastal sea. Here we examined source-specific biomarker (fatty acids, FAs) contents and isotope compositions in surface sediments in an alongshore transect southwestward from the Pearl River mouth. The C28+30, rather than other long-chain saturated FAs, were found to be the most representative for OCterr, and a plant wax mean age of 3060 ±90 yr (resulting from protracted storage) was estimated in the Pearl River watershed from the 14C age of C28+30FA in a river mouth sample. A compilation of plant wax mean ages in global (sub)tropical river systems including this study suggests that regional differences in climate and morphology may have a limited impact on plant wax mean ages in (sub)tropical regions. A four-source mixing model based on bulk OC and biomarker isotope compositions demonstrated that surface sediments in the Pearl River-derived mudbelt consist of 0.15–0.36 wt.% marine OC, 0.03–0.13 wt.% riverine primary production-derived OC, 0.18–0.49 wt.% soil OC, and 0.07–0.16 wt.% fossil OC. The mean burial efficiency of fossil and soil OC is ∼85% and 49%, respectively, indicating the refractory nature of fossil OC but a significant loss of soil OC due to remineralization during transport in the marine environment before final burial. Over longer timescales, the OCterr loss experienced during transport may, thus, to some extent reduces the capacity of terrestrial ecosystems (particularly soils) as CO2 sinks.
... The Pearl River is China's second largest river in terms of annual freshwater discharge, and has three main branches, the West River, North River, and East River (Hu et al., 2011), as displayed in Figure 1a. The average annual loads of freshwater and riverine sediment are 2.86 × 10 11 m 3 and 3.04 × 10 7 tons, respectively (Dai et al., 2008;Zhang et al., 2019). The freshwater and sediment discharges of Pearl River exhibit strong seasonal variations; for example, approximately 80% of the freshwater discharge occurs and 95% of the sediment load is generated during the wet season between April and September (Xia et al., 2004). ...
... The future morphological evolution of the PRE depends on riverine sediment discharge and the conditions of the tides and waves. The extensive damming of the Pearl River basin in recent decades has caused a considerable decrease in sediment discharge from the Pearl River watershed, which is mainly attributed to deposition in reservoirs (Dai et al., 2008;Tan et al., 2017). In addition, the global mean sea level and the sea level in the PRE have been rising in recent years at an increasing rate (Church & White, 2006;Hong et al., 2020). ...
Article
Full-text available
Sediment dynamics have great effects on the morphology of estuaries and deltas. As riverine sediment input has decreased in many estuaries in recent years, the removal and dispersion of sediments by currents and waves inside estuaries has assumed a more dominant role; however, the overall effects of waves are less well understood. In this study, we used the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) system to model the effects of waves on sediment transport in China’s Pearl River Estuary. The results indicate that the presence of waves increased the landward current and sediment transport at the bottom of the channel and the seaward fluxes of water and sediment at the West Shoal, which is the main shallow area that accepts riverine sediment. Relative to the case without waves, the effects of waves increased the estuary’s total sediment export by 45% to 9.14 megatons in one typical year, which even exceeded the annual riverine load in the year. The dry winter season shows the highest wave effects on sediment budget, and the sediment export was increased by 86% to 2.59 megatons with waves when compared with that without waves. This increase export was mainly concentrated at the surface near the western shore. Moreover, the waves increased lateral sediment entrapment in the Southwest Shoal, which explains the local geomorphologic evolution in recent decades. This study is of implication for the study on sediment exchange between shoal and channel in estuaries and on the fates of riverine sediment from source to sink.
... He pointed out that reduction in sediment discharge depends upon the design, purpose and operation of the dam. Another example of the effects of dams on coastal geomorphology may be cited from the study on the Pearl river in China by Dai et al. (2008). 13 Sediment discharge from this river has decreased significantly since the mid-1980s with the construction of large dams and consequent deposition of sediment behind those dams. ...
... Another example of the effects of dams on coastal geomorphology may be cited from the study on the Pearl river in China by Dai et al. (2008). 13 Sediment discharge from this river has decreased significantly since the mid-1980s with the construction of large dams and consequent deposition of sediment behind those dams. ...
Article
Full-text available
Terrestrial sediment is a major source of sediment to all coasts. Suspended sediment is carried away by the rivers and supplied to the coast to maintain sediment budget. The construction of dams across the rivers arrest sediment behind it and affect the sediment budget of littoral cells along the coast. Reduction in sediment supply induces ecological as well as geomorphological changes along the shoreline. Coastal erosion may accelerate due to reduced sediment influx. With the growing number of cross-river dams and water diversion projects, it has become a major concern before the scientific community to measure, understand and find solutions to multi-fold geo-environmental problems that are arising out of river damming. The present study aims to find out the impact of dams on the coast. It examines how the changes in the suspended sediment supply of an Indian river impact the coast in terms of loss of area due to erosion. Temporal analysis of geomorphological changes along the shoreline in relation to sediment influx holds immense importance to coastal management essential for the sustainable life and livelihood of coastal communities. Scientific investigation into the impact of river dams on the coastal environment is likely to provide a strong ground to reconsider the way present basin development projects function. Areal changes in littoral sediment cells adjacent to the river mouth have been quantified and correlated with changes in sediment influx. Changes along the shorelines have been detected through multispectral satellite images of Landsat belonging to different dates. Image processing and quantification of changes have been performed in QGIS 3.14 “Pi” platform. Virtual raster, raster calculator, field calculator and other required tools in QGIS were used during image processing.
... With a large volume of dams operating in karst watersheds, the sediment discharge has decreased sharply. Dam construction was likely the principal cause of the significant decreases in sediment discharge in karst watersheds (Dai et al., 2008;Wu et al., 2012;Zhang et al., 2008;Li et al., 2018b;Ye et al., 2020). Large dams can reduce the amount of sediment by intercepting large quantities of eroded sediments. ...
... For example, with the completion of the Yantan reservoir in the Xijiang watershed (Fig. 1), a large amount of sediment was trapped behind the dam. The sediment flux in the lower reaches of the watershed decreased by 18%-66%, which eventually affected the sediment discharge of the whole Pearl River watershed (Dai et al., 2008). In 2006, with the completion of the Longtan and Baise reservoirs in the Xijiang watershed ( Fig. 1), the amount of sediment discharged decreased to 30 Mt yr − 1 , which was far lower than the sediment discharge amount simulated by precipitation data. ...
Article
Quantifying the relative contributions of climate change and human activities to changes in sediment discharge is closely related to regional water resources, soil management, and healthy ecosystem function. However, few studies have used different methods to decompose influences of climatic variability and human activities on changes to sediment discharge, especially in karst regions of southwest China which are experiencing severe soil erosion. This study systematically reviewed four methods (simple linear regression, double mass curve, sediment identify factor analysis, and elastic coefficient method) to separate the contribution of climate change and human activities to sediment discharge variation in four karst watersheds of southwest China during 1955 to 2015. The trend and abrupt change year of sediment discharge in the four watersheds were obtained using a nonparametric Mann-Kendall trend test and sequential test method, respectively. Result showed that the sediment discharge in all four watersheds all decreased significantly (P < 0.05), and the abrupt change years were 2003 or 2004 for these watersheds. The simple linear regression method, double mass curve method, and elastic coefficient method all indicated that the sediment discharge was mainly influenced by human activities, which contributed more than 73% of the observed change. For the sediment identify factor analysis method, sediment concentration was the principal factor influencing sediment discharge variation. The results provide a reference for evaluating the influence of climate and human activities on sediment discharge, and can aid in preparing regional soil conservation policy in karst watersheds.
... With respect to the turbid Pearl River estuary, a number of remote sensing related studies for TSS focused on algorithm development and assessment (Dai et al., 2008;Fang et al., 2010;Wu et al., 2013;Xing et al., 2013;Ye et al., 2014;Zhao et al., 2018). Only a few studies concentrated on long time series of TSS based on remote sensing data Zhan et al., 2019). ...
... Understanding factors causing this trend behind the scene is crucial for better interpreting the transport of sediment in the PRE, the remediation of local marine ecosystems related to potential damage caused by contaminated sediment, and the protection of marine protected areas. Different factors affect TSS in the PRE, such as soil erosion (Dai et al., 2009;Liu et al., 2018), meteorological and hydrological conditions (Chen et al., 2017;Zhao et al., 2018), dam constructions (Dai et al., 2008;Wu et al., 2012), and sand mining (Hu et al., 2010). These effects have already been comprehensively deciphered in previous studies. ...
Article
Full-text available
Total suspended solids (TSSs) can affect primary production through influencing light penetration and nutrient availability in aquatic environments. In this study, novel algorithms were developed for TSS in the Pearl River estuary using Landsat‐derived reflectance corrected for Rayleigh scattering (Rrc). Comparisons between in situ and estimated TSS using the calibration and validation data sets resulted in R²s > 0.8 and mean absolute percentage errors < 35%. Landsat‐derived annual TSS between 1987 and 2018 in the Pearl River estuary ranged from 16.5 to 41 mg L⁻¹. It indicated a statistically significant decreasing trend with an annual rate of 0.26 mg L⁻¹ and decreased by 58.5% from 1987 to 2018. It showed a higher decreasing rate after than before the construction of the Hong Kong‐Zhuhai‐Macau Bridge. Nonetheless, TSS 2 km upstream from the Hong Kong‐Zhuhai‐Macau Bridge on the western side increased after the construction of the bridge related to the change of flow influenced by piers and artificial islands.
... Approximately 40.0% of the total water discharge and sediment load are released via the outlets of the Hengmen, Hongqimen, and Jiaomen into the west part of the PRE. In the past decades, the output sediments from the Pearl River Basin have experienced a dramatic decrease from nearly 90 million tons in the 1990s to the present 25 million tons only [22]. In the monsoon season, due to the large runoff and high SSC from the Pearl River, the SSC near the west coast is relatively high, with an SSC of 0.2-0.4 ...
... Therefore, the estuarine SSC in summer and autumn is significantly higher than that in winter and spring, indicating the response of SSC to the changes in sediment load. Many studies [13,22,34] have shown that the sediment loads in the PRE from the Pearl River have decreased significantly in recent decades. ...
Article
Full-text available
Monitoring and quantifying suspended sediment concentrations (SSC) in estuaries such as the Pearl River Estuary (PRE) provide crucial information for environmental processes, hydrological infrastructure, and navigation. Traditional SSC mapping based on in situ investigations lacks the spatial coverage necessitated by detailed analysis. In this study, based on in situ spectral data and SSC measurements, we developed models to quantify SSC based on Landsat TM/OLI and Sentinel-2 imagery. The models were then used to map the SSC distribution in the PRE for the period 1986–2020. The results indicated that SSC in the entire PRE displayed an overall decreasing trend over the past 35 years, but an abnormal increase was also observed in shallow waters near Hengmen. Geographically, the concentration along the west coast is higher than on the east coast, and the central part of Lingding Bay underwent a slight increase. The decreased sediment output from the Pearl River Basin is the major cause of the decreasing changes in SSC. However, the natural factors and human activities such as channel dredging and the construction of artificial facilities can also disturb the spatial distribution of SSC. Regions with a significant decrease in SSC should have special concerns about retaining the PRE’s health and sustainability.
... Aggravated by natural crustal movements and sea level rise, this development poses a severe threat to the delta population (Chen and Zong, 1999). Subsidence of 1.76 m was observed in the city between 1921 and 1965, and subsidence has continued at a similar rate during the subsequent years (Bo et al., 2010;Chen and Zong, 1999;Chu et al., 2006;Dai et al., 2008;Zhang et al., 2019). ...
Article
River deltas and estuaries are disproportionally-significant coastal landforms that are inhabited by nearly 600 M people globally. In recent history, rapid socioeconomic development has dramatically changed many of the World's mega deltas, which have typically undergone agricultural intensification and expansion, land-use change, urbanization, water resources engineering and exploitation of natural resources. As a result, mega deltas have evolved into complex and potentially vulnerable socio-ecological systems with unique threats and coping capabilities. The goal of this research was to establish a holistic understanding of threats, resilience, and adaptation for four mega deltas of variable geography and levels of socioeconomic development, namely the Mekong, Yellow River, Yangtze, and Rhine deltas. Compiling this kind of information is critical for managing and developing these complex coastal areas sustainably but is typically hindered by a lack of consistent quantitative data across the ecological, social and economic sectors. To overcome this limitation, we adopted a qualitative approach, where delta characteristics across all sectors were assessed through systematic expert surveys. This approach enabled us to generate a comparative assessment of threats, resilience, and resilience-strengthening adaptation across the four deltas. Our assessment provides novel insights into the various components that dominate the overall risk situation in each delta and, for the first time, illustrates how each of these components differ across the four mega deltas. As such, our findings can guide a more detailed, sector specific, risk assessment or assist in better targeting the implementation of risk mitigation and adaptation strategies.
... Extensive and intensive research has been conducted on the longterm trends of river sediment of the Yangtze River to the sea (e.g. Zhang et al., 2006;Dai et al., 2008Dai et al., , 2016Gao and Wang, 2008;Yang et al., 2002Yang et al., , 2006Yang et al., , 2018. Furthermore, some work has been conducted on the interpretation of previous flood events based on sedimentary characteristics associated with subaqueous Yangtze delta and adjacent subaqueous sediment bodies Zhan et al., 2010;Wang et al., 2011). ...
Article
River sediment fluxes to the sea contribute to the morphological evolution, as well as environmental and biological conditions of estuaries and their adjacent coasts. Especially for river basins that are dominated by a monsoonal climate, extreme high discharges can occur due to intense rainfall during the flood season, which will considerably affect the amount of sediment that is transported to the sea. In this paper, 4 major floods of the Yangtze River (2 extreme floods, one affecting most of the basin in 1954, and one more pronounced in the upper reach in 1981; and 2 large floods, one impacting mainly the middle reach in 1983, and one affecting most of the basin in 1998) are analyzed as these provide a unique opportunity to investigate fluvial sediment characteristics to the sea affected by major flooding. Results reveal that more sediments were delivered to the sea during the flood season when regional-sized major floods occurred, and this was even more pronounced when major floods occurred in the upper reach of the Yangtze River. Sediment retention in the middle reach of the Yangtze River during the major floods significantly reduced the amount of sediments reaching the sea. As for the flood season of 1981, the sediment size distribution to the sea was not significant different than that of the perennial average. However, coarser-grained sediments were delivered to the sea during July 1983 due to the remarkably high water discharges at that period. These findings can provide enlightenment regarding the timing of wetland restoration and interpretation of paleo-flood deposits in the deltaic zones of the Yangtze River.
... The most populated area in South China is located at the upper reaches of the Pearl River estuary (Fig. 1). After a number of reservoirs and dams were built in the Pearl River basin in the 1990s, the sediment load and suspended particulate matter concentrations in the estuary decreased greatly (Dai et al., 2008a;Zhang et al., 2008). Due to constrained physical mixing and increased anthropogenic nutrient inputs from industrial and agricultural activities, phytoplankton production has become the major source of organic matter to this estuary (Chen et al., 2004;Guo et al., 2015;Zhang et al., 2014), and this biodegradable organic matter experiences extensive consumption and diagenetic alteration during transport in the estuary (He et al., 2010a,b;Li et al., 2018). ...
Article
To better understand the sources and behavior of estuarine labile organic matter, we measured stable carbon isotope patterns of individual amino acids in suspended particles and surface sediments from the Pearl River Estuary in China; samples were taken along a salinity transect in December, 2016. Here we demonstrate that carbon isotope values (δ¹³C) of individual amino acids in these samples gradually increase with salinity downstream, reflecting the increase in δ¹³C values of algal-derived organic carbon along the salinity gradient. The isotopic difference between amino acids and bulk organic carbon varies, most likely due to changes in the relative contributions of algal-derived organic matter and refractory terrestrial input. In addition, algal-derived organic matter can consist of labile and semi-labile organic matter in varied proportions depending on degradation state. This isotopic difference between amino acids and bulk organic carbon is much larger in surface sediments than in suspended particles, suggesting that labile organic carbon contributed more to suspended particles than to sediments. Using the relative abundances and δ¹³C ratios of amino acids and total organic carbon, a Lability Model was constructed to evaluate the relative contributions of three forms of estuarine organic carbon: labile algal material as amino acids, semi-labile algal material as lipids and acid-insoluble material, and refractory terrestrial organic material. The model suggests highly dynamic contributions of semi-labile algal-derived organic carbon and terrestrial organic carbon to estuarine particulate organic carbon. This evaluation of organic carbon sources illustrates the importance of decomposition in shaping the molecular composition and isotopic signature of particulate organic carbon in the estuary.
... Therefore, the assessment results of SPEI3 can better reflect the future drought conditions in the NWR. In addition, with population growth and rapid economic development, human activities may affect the dryness conditions in the basin (Chen and Sun, 2015), such as the overexploitation of groundwater in the SR, LR, HR and Huai (Chen et al., , 2019; the massive construction of reservoirs, dams and sluices in the YR, YZR, ZR, SER and SWR (Dai et al., 2008;Deng et al., 2018;Li et al., 2016); and the excessive urbanization in the HR, Huai, ZR, SER and the middle and lower YZR Xu et al., 2010). If these actions continue, they will seriously affect the drought management of the basin, and the basin management committee should formulate corresponding mitigation policies. ...
Article
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Global warming will exacerbate the damages and losses caused by drought and significantly affect human society and economic development. Future drought outlook is of vital importance for policy-making to combat drought risk and water crises over the long term. Here, 3- and 12-month standardized precipitation evapotranspiration indices, which can aid short- and long-term drought monitoring, respectively, are coupled with the SOM-Kmeans two-stage clustering technique to map the future drought conditions in China for the future period (2021-2050) relative to the baseline period (1976-2005). The calculation is based on downscaled outputs of 10 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) under two representative concentration pathways (RCPs). The temperature increases by 0.5-2.0 °C and 1.0-2.5 °C under the selected RCP4.5 and RCP8.5 scenarios, respectively. The results show that the differences between precipitation and potential evapotranspiration are projected to increase throughout China. Most parts of China are projected to experience more severe, frequent and longer duration droughts in the next three decades compared with the baseline period. Global warming is projected to have a serious impact on future droughts in the basins located in the northern part of China. The drought conditions in the northwest region of the “Hu Huanyong Line” in China are projected to be serious and complex in the future, while the opposite is true in the southeastern region. Drought conditions in northern China are more complex under high temperature changes (1.0-2.5 °C) than under low changes (0.5-2.0 °C). The results of this study provide insights on future drought planning and management in China and demonstrate that SOM-Kmeans can be used for the classification of drought conditions.
... In other basins located in the northern part of China (the ZR, HR, SR, LR and Huai) and some basins in the southern part of China (the SER, ZR and the middle and lower YZR), the population and GDP account for more than 90% of those of China overall Yang and He, 2017), and human activity may be an important factor affecting these basins' health (Chen and Sun, 2015;Zhang et al., 2017;Zhao et al., 2011). Most of the dams taller than 30 m in China are distributed in these basins (especially in the YZR and ZR, http://www.chincold.org.cn/), and there are numerous water gates in parts of the SER, ZR and lower YZR (Dai et al., 2008;Deng et al., 2018;Han et al., 2016). Although these water conservancy facilities have played an important role in flood control and water resource management, they could alter the hydrological and hydraulic conditions and cause adverse effects on the health of the basins (Nilsson et al., 2005;Steele and Heffernan, 2014;Xu et al., 2010). ...
Article
Increasing dryness conditions under global warming are posing severe threats to water resources management in China. Projecting river basin responses to dryness conditions is beneficial to effectively managing water resources. However, existing studies have seldom considered the impact of multiple dryness conditions on future river basin health under global warming. Therefore, we combine the 3- and 12-month standard precipitation evapotranspiration index (SPEI) and reliability-resilience-vulnerability framework (RRV) to map future river basin health based on the responses of basins across China to different dryness conditions from 2021 to 2050. The calculation is based on downscaled outputs of 10 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) for three future emission scenarios (i.e., RCP2.6, RCP4.5 and RCP8.5). The results show that water deficits are projected to occur in most areas of China and significantly increase in the basins located in the northern part of China in the next 30 years due to global warming effects. The conditions in parts of the basins located in the northern part of China (especially in the Northwest River basins and Yellow River basin) are projected to be unhealthy and deteriorate significantly in the future, while the basins located in the southern part of China are projected to be moderate. The health status is anticipated to be worse under the RCP8.5 scenario than the RCP2.6 and RCP4.5 scenarios. Integrated results from the three thresholds indicated that normal dryness is applicable to most areas of northeastern, northern and southern China, while abnormal dryness is applicable to the remaining areas. Our findings could help reduce the impact of future dryness conditions on water resources and provide insights into risk planning and management for river basins in China under global warming.
... Prior to widespread reservoir construction beginning in the 1980s, the aSL (measured at the Gaoyao, Shijiao and Boluo stations) of the Pearl River is about 80 Mt/yr (Dai et al., 2008). However, more than 14,000 reservoirs have been constructed within the Pearl River basin since 1980, especially after the construction of the Yantan (YT) and Longtan (LT) reservoirs ( Fig. 1 and Fig. 8a) in 1992 and 2006, trapping large amounts of sediment and causing a drastic decrease in sediment load of~30 Mt/yr (Wu et al., 2014(Wu et al., , 2016(Wu et al., , 2018. ...
... The SCS occupies a large deep basin with broad shallow shelves , and the Pearl River delivers about 75.3 Mt of sediments to the northern SCS annually (Dai et al., 2008). The Pearl River is comprised of three main tributaries, the West, East and North Rivers (Fig. 1a). ...
Article
Large rivers discharge great amounts of terrigenous organic carbon (OCterr) to the ocean, 90% of which are trapped in the coastal areas. The OCterr processing during transport to and within the coastal oceans are not well known. The Pearl River is the second largest river in China in terms of annual runoff. Here, we studied OCterr delivered from the river in the Pearl River Estuary (PRE) and the northern South China Sea (SCS) shelf using stable and radioactive carbon isotope compositions (δ¹³C and F¹⁴C) of OC in surface sediments to (1) constrain sources of OC, (2) explore the role of hydrodynamic processes for OCterr transport, and (3) better understand the burial and degradation processes of OCterr. A three end-member mixing model based on δ¹³C and 1/(C/N) values of bulk OC was used to calculate relative contributions of OCterr, OC derived from marine (OCmar) and riverine primary production (OCRpp). Results showed that the PRE and its western coastal inshore mud deposit are dominated by OCterr (70 ± 3% and 54 ± 10%, respectively), but other areas receive increasing contributions from OCmar: the slope (49 ± 10%), the eastern coast (58 ± 2%) and the outer shelf (68 ± 3%). OCRpp accounted for substantial proportions in the PRE (14 ± 6%) and rapidly decreased in the offshore area (3–5%), likely due to extensive aerobic respiration of organic matter (OM). Subsequently, average F¹⁴C values of OCterr were calculated ranging from 0.271 to 0.639 using a Monte-Carlo simulation strategy and based on the assumption that F¹⁴C values of OCRpp and OCmar varied within narrow ranges. Together with OCterr contents and grain sizes of sediments, F¹⁴C values of OCterr were used to distinguish two regions of distinctive sedimentological characteristics in the study area. Region I, including the PRE and inner shelf, showed a decrease of OCterr content along the trajectory of westward along-shelf transport, reflecting resuspension-dominated conditions for OCterr transport. Region II, including the outer shelf and one site on the inner shelf (E701), exhibited relatively old OCterr (9180 ± 730 yr BP), which is attributed to preferential accumulation of coarser sediments by bedload movement. Finally, a first-order degradation rate constant was calculated from contents and ¹⁴C ages of OCterr, yielding a slow OCterr degradation rate of (2.88 ± 0.61) × 10–⁴ yr–¹, indicating a more refractory nature and likely effective mineral protection of OCterr. The ΣCO2 efflux due to OCterr degradation was estimated to be 3.68 ± 1.39 Gg C yr–¹ for the entire surface sediments in the Pearl River derived mud belt, equivalent to only 0.7 ± 0.3% of the total particulate OC flux of the Pearl River. This, hence, suggests that the OCterr is more persistent and the preservation of OCterr in the marine system is better than previously thought, but experiences substantial degradation on centennial to millennial timescales.
... At all stations, the six-year running streamflow from 1997-2002 was the lowest for the 60-year period . Streamflow in rivers, e.g., the Mississippi [33], the Yangtze [34] and the Pearl River [35], normally increases downstream from the headwater to the river mouth. However, abnormal downstream decreases in streamflow were found in two reaches in the YR (i.e., the 1400 km reach between Lanzhou and Toudaoguai and the 800 km reach between Huayuankou and Lijin). ...
Article
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Human society and ecosystems worldwide are increasinAagly threatened by water shortages. Despite numerous studies of climatic impacts on water availability, little is known about the influences of socioeconomic development on streamflow and water sustainability. Here, we show that the streamflow from the Yellow River to the sea has decreased by more than 80% in total over the last 60 years due to increased water consumption by agricultural, industrial and urban developments (76% of the streamflow decrease, similarly hereinafter), decreased precipitation (13%), reservoir construction (6%) and revegetation (5%). We predict that if the past trends in streamflow will continue, year-round dry-up in the lower Yellow River will commence in the late 2020s or early 2030s, unless effective countermeasures such as water diversion from the Yangtze River are taken. These results suggest that streamflow in semiarid basins is highly vulnerable to human impacts and that streamflow decline would in turn hinder further socioeconomic development and endanger river-sea ecosystems.
... As the major sediment source to the northern SCS, the Pearl River with a modern average suspended sediment discharge of ~69 × 10 6 t/yr (Milliman and Farnsworth, 2011), originates from the Yunnan-Guizhou Plateau and connects to the SCS in Guangdong Province, with a total length of 2214 km. The region has a warm and humid climate, with an annual precipitation of 1470 mm and an average temperature of 18 • C (Dai et al., 2008), and the lithology of the eastern part is principally composed of Mesozoic-Cenozoic granites and Paleozoic sedimentary rocks. ...
Article
Pearl River sediments have been reported to have changed greatly in composition due to the impact of human activities on erosion and landscapes in southern China since 2500 years BP. Here, sediments from four cores (PRD-1, PRD-3, PRD-6 and PRD-7) from the Pearl River Delta (PRD) were collected for high-density clay mineral analyses to determine the vertical variations in clay mineral compositions since the middle Holocene. The results showed that the clay mineral compositions of these four cores remained constant. In the PRD, the clay mineral assemblage dominantly consists of illite and kaolinite (averaging more than 85%), with smaller amounts of chlorite (averaging 10%) and scarce smectite (averaging 1%). The stable clay mineral assemblages in the PRD confirm that the clay mineral compositions of the sediments continuously supplied by the Pearl River have not changed. However, the smectite percentage in core PRD-7 increased by 7% downward from a depth of 12.4 m, which was correlated with the weathering of local volcanic materials and sediment transport in the east of the Pearl River under the influence of the Guangdong coastal current in winter. The stable clay mineral compositions in the core sediments indicate that the Pearl River sediments have accumulated naturally in the PRD instead of under the influence of human activities, which would have altered the compositions of the core sediments.
... These findings are compared to research on the sediment fluxes in the Pearl River, China, which decreased dramatically due to the quantity of sediment deposited in up-stream reservoirs located in the upstream of the river Dai et al. (2008). In contrast, dam removal was associated with an uncontrolled sediment release which lead to an increase in the supply of bed material to the Patapsco River (USA) (Collins et al., 2017). ...
Article
Evaluation of the spatial and temporal composition of floodplain sediments and soils is critical in the creation of soil management strategies for impacted riverine catchments. The objective of this study was to determine the distribution, and to identify the sources, of particulate trace elements and fallout radionuclides in the catchment of the River Avon (SW England), where sedimentary processes had been altered by reservoir construction in the 1950s. The catchment was compartmentalized into its main functional units namely, cultivated land, pasture, woodland, wet moorland, and channel bank. Surface soils were collected in each unit, along with four strategically-placed cores, all of which were analyzed for particle size, fallout radionuclides and elemental concentrations. Sediment particle sizes and sediment accumulation rates were affected by the construction of the reservoir, specifically the distributions of silt and clay. The concentrations of fertilizer constituent Cr and P were highly correlated in the mid-catchment but were unrelated downstream due to elevated concentrations of Cr from geological deposits. Copper, As, Pb and Sn had variable down-core distributions, with pulses in concentrations due to mining inputs. The contributions of the end-member sources of particulate elements in the sedimentary mixtures were evaluated, quantitatively, using a Bayesian Mixing Model and the cultivated land was identified as a significant contributor to the mixtures, independent of space and time. The results contribute to advances in soil quality and conservation measures as components of a catchment management plan for the Avon, an approach maybe applicable to other small catchments in the UK and internationally.
... Since the area of the Pearl River Basin area is 368 × 10 3 km 2 , the total thickness of the denuded area reached ∼2,900 m since the Middle Eocene. Moreover, modern observations (Dai et al., 2008) show that the annual mean sediment flux from the Pearl River is 75 Mt/yr. During the entire Cenozoic, even if the sediment flux from the Pearl River was much less than the present value (e.g., only half, or 37.5 Mt/yr), the height of the eroded rock adjusted for isostatic balance is ∼2,000 m. ...
Article
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Inconsistencies in the Eocene climates of East Asia have been revealed in both geological studies and simulations. Several earlier reconstructions showed an arid zonal band in mid‐latitude China, but others showed a humid climate in the same region. Moreover, previous Eocene modeling studies have demonstrated that climate models can simulate both scenarios in China. Therefore, it is essential to investigate the cause of this model spread. We conducted a series of experiments using Norwegian Earth System Model 1‐F and examined the impact of mountains in Southern China on the simulated Eocene climate. These mountains, including the Gangdese and Southeast Mountains, are located along the main path of water vapor transport to East Asia. Our results reveal that the Southeast Mountains play the dominant role in controlling the simulated precipitation in Eastern China during the Eocene. When the heights of the Southeast Mountains exceed ∼2,000 m, an arid zonal band appears in mid‐latitude China, whereas humid climates appear in Eastern China when the elevation of the Southeast Mountains is relatively low.
... Additionally, Lu et al. (2007) have measured annual channel cross-sections at Sanshui hydrological station in the lower North River, and pointed out that extensive sand mining in the lower North River has caused rapid channel incision and water level reduction. Dai et al. (2008) analyzed temporal variations in sediment load at Shijiao station in the North River and found that the sediment flux of the station has reduced. The reduction of sediment load is due to effective soil and water conservation projects, dam constructions and regular channel Stochastic Environmental Research and Risk Assessment dredging in upstream areas, and such human activities are likely to contributed to bed downcutting to some extent as well (Lu et al. 2007). ...
Article
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The Lowest navigable water level (LNWL) is an important indicator used for navigation design to balance the relationship between navigation safety and economic benefits of a waterway. However, it is a challenge of accurately estimating LNWLs due to the nonstationary characteristics of observed water level data series. In this study, a comprehensive framework was developed for handling this issue. In this framework, inter-annual variabilities in both the mean and variance of water level series were described by decomposing original series and were eliminated by composing new series. Intra-annual variability was determined by detecting indicators describing intra-annual water level distributions. Considerations of inter- and intra-annual variabilities were combined by designing annual water level processes for the past and current environments. Shipping risks during both annual and multi-annual periods were considered in the framework as well. The framework was demonstrated in estimating LNWLs at the Gaodao and Shijiao stations in the North River basin, southern China. The recommended LNWLs at the Gaodao station were 22.32 m for 95% guaranteed rate and 21.84 m for 98% guaranteed rate; LNWLs at the Shijiao station were 0.27 m for 95% guaranteed rate and 0.15 m for 98% guaranteed rate. The impact of variance variability on estimations of LNWLs was also evaluated. Results indicated that the recommended LNWLs would have errors of 0.11 ~ 0.48 m at the Gaodao station and 0.03 ~ 0.04 m at the Shijiao station if the variance variability was not considered. The proposed framework was then compared with Nonstationary synthetic duration curve (NSDC) method, and results illustrated that the duration curves plotted by NSDC method were unreasonable, leading to inaccurate design values. Overall, the developed framework is more reasonable and suitable for designing LNWLs of waterways where the variabilities of the water levels at different time scales are different or where the historical water level data contain various variations .
... Dams is known to modify aquatic ecology and river hydrology upstream and downstream, affecting water quality, quantity and breeding grounds [1]. Dams affect the water quality and physical changes, where downstream of the dam may change to pools during the dry seasons due to decreased water discharges and elevated temperature in daytime [2].Moreover, dams have a severe effects on fish, impoundment [2], basin erosion [3,4], material fluxes [5] and nutrient characteristics [6,7]. Although much of studies investigated the impact of dams on the rivers, there is almost no publications had studied the effect of dams on the chemical and isotopic characteristics of the rivers. ...
Research
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Impacts of constructed dams on Damietta branch of river Nile were investigated for their effect on the chemical and isotopic composition of the Nile water in Damietta branch. Environmental isotopes, namely 2 H and 18 O were employed along with major chemical ions to study the Damietta branch water characteristics. The three dams Delta, Zefta, and Faraskour, which are constructed on the branch segregate it into two main parts; fresh and marine character. Chemical and isotopic results show that Delta and Zefta dams affect evaporation, recharging and salination processes while Faraskour dam is totally isolate the last sector of the branch and make it with marine character. Isotopic techniques prove that the salination of the last sector was due to the intrusion of Mediterranean seawater rather than the up-effluent from saline groundwater.
... Channel migration is one of the significant characteristics that can change the morphology of the river buffer area, with major problems for human life (Güneralp et al. 2012). River geomorphological factors are important indicators to investigate environmental changes and are especially related to changes in river buffer areas, such as various LULC types in the river floodplains and drainage basin (Dai et al. 2008). These geomorphological changes are associated with changes in sediment load and river discharge variation (Cserkész-Nagy et al. 2010). ...
Chapter
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Dynamicity of the channel is the main characteristic of the Kaljani River in the Himalayan foothill. The present work intends to document the historical changes in the land use and land cover (LULC) pattern driven by channel migration during 1987–2020 at the Kaljani River adjacent village area. In this study, the sinuosity index, the radius of curvature, meander wavelength, amplitude, meander width, channel width, arc angle, direction angle, rate of channel migration, and direction of migration have been calculated for the years of 1987, 2004, and 2020. The historical positions of both bankline and dynamic channel width and meander width indicate that a large portion of the floodplain area depicts an erosion-accretion sequence with time. This work also investigated LULC changes in the Kaljani River adjacent village area using supervised image classification with an overall accuracy ranging between 85 and 89%. This research has demonstrated the application and capability of RS and GIS technology and generated a detailed evaluation of temporal and spatial changes in river channel processes and adjustment of LULC types. The LULC result revealed that their water bodies and dense forest are decreased and sandy area and built-up areas are increased. The LULC changes by the direct effect of bankline migration have a bad impact on the dwellers of the floodplain adjacent village area of the Kaljani River. The results of this study can represent an important indicator of the vulnerability of the Kaljani River adjacent village area and also provide information about geomorphological instabilities of the study area.
... The Pearl River is the third largest river of China, flowing through six provinces (Dai et al., 2008). The GDP of all the Pearl River Basin cities in 2012 was 9.2 trillion RMB, accounting for 17.1% of China's GDP, and approximately equivalent to that of Spain and half that of France. ...
Article
Cities are leading carbon mitigation but are heterogeneous in their mitigation policies due to different socioeconomic backgrounds. Given that cities are increasingly inextricably linked, formulating mitigation policies of different cities cannot be easily achieved without comprehensive carbon inventories, who taking the inter-city supply chains into account. The Pearl River Basin is one of the important economic zones in China, with huge disparity in its cities, but very limited information is available on their consumption-based CO2 emissions. To fill this gap, we compiled a consumption-based inventory of 47 cities in the Basin for 2012. We found that the total consumption-based emissions of 47 cities was 933.8 Mt, accounting for 13.1% of China's emissions. There were huge differences in the consumption-based emissions, ranging from 3.6 Mt (Heyuan City) to 153.1 Mt (Shenzhen City). The consumption-based emissions were highly concentrated in the largest seven cities, which accounted for 52.8% of the total emissions of the Basin. The consumption-based emissions per capita also varied greatly, from 1.2 to 14.5 tons per capita. Large scale infrastructure was the biggest driving force for most cities, resulting in 42.1% to 75.6% of the emissions. At sector-level, construction, heavy industry and services were leading in emissions, contributing more than 80% of emissions. The major inter-city carbon transfers occurred within upstream cities in the developing regions and downstream cities in the Pearl River Delta respectively, instead of the transfers between upstream and downstream cities. The findings highlight that the regional mitigation strategies could mainly focus on cities in intra-province boundary, rather than inter-province boundary, and also the city-level mitigation strategies should pay attention to the key emission sectors and drivers in respect of the heterogeneity of cities.
... 600~2 000 mm[5] ,流域面积 约 44 万 km 2 , 每年入海泥沙量约 8 000~8 500 万 t[6] 。 Fig. 1 Study area and sampling site HKUV16 1.2 沉积柱状样定年 沉积柱 HKUV16 的年代建立在 7 个贝壳的放射 性 14 C 定年基础上(图 2) 。放射性 14 C 由美国 Beta 分析实验室(Beta Analytic laboratory)完成。通过 对表层 50 cm 的沉积物进行 210 Pb 测年,确定表层 30 cm 为现代 100 a 内的沉积物, 表明表层沉积物没 有缺失。该沉积柱 10 m 处的年代为距今约 8 000 a, 平均沉积速率为 0.126 cm/a。从表层 2 个放射性碳 14 年龄和 210 Pb 活度随深度变化可以看出,该沉积Fig. 2 Radiocarbon 14 C dating and 210 Pb activity with depth of core HKUV16 ...
... Flow from the Yangtze River and East China Sea enters from the northeast side of the bay and flows out of the bay through its south side within one tidal cycle (Xie et al., 2013). Despite decreased sediment load from the Yangtze River from around 4 × 10 8 tons to less than 2 × 10 8 tons since the construction of the Three Gorges Dams (Dai et al., 2008;Dai et al., 2011;Zhou et al., 2017), the sediment accumulation in the Qiantang River estuary and Hangzhou Bay has increased due to coastal embankments and erosion from the outer Yangtze River Delta (Xie et al., 2017). In fact, there has been increased contribution from the Qiantang and Ou Rivers as well as relict materials from the Zhoushan islands to the Zhejiang coastal zone (Xu et al., 2016b). ...
Article
Two sediment cores were collected from the southern coastal area of Zhoushan Island, and their lignin-derived phenol, bulk elemental composition, stable carbon isotope, particle size and ²¹⁰Pb activities were determined. The objective of this study was to determine the compositional changes of the sedimentary organic matter along this coastal area in the past 80 years as a result of anthropogenic and oceanographic-related processes. The results of this study showed that the sources and distribution of sedimentary organic matter along this coastal area were affected by the accelerated development on these islands and mainland China. Moreover, the increased contribution of terrestrial organic matter from the rivers in the mainland from the 1930s to the 1980s was likely the result of human activities. The drastic increase in the vanillic acid to vanillin ratio, (Ad/Al)v, in the 1970s and 1980s coincided with the start of rapid development in Zhoushan, resulting in the increased erosion of more degraded terrestrial organic matter along the coast. Trends of increasing marine organic matter toward the present could be due to a combination of reduced river input and sea level rise. Overall, our results showed that the sedimentary organic matter along the southern coastal zone of Zhoushan main island was affected by riverine input from the mainland, and showed a tendency toward decreasing terrestrial organic matter and particle size. Thus, local input and increased phytoplankton production will play a more important role in the carbon dynamics in this coastal area in the future. (245 words).
... higher than its discharge to the sea (40 Mt yr −1 ) at a basin scale (Dai, Yang, & Cai, 2008). Therefore, the decline of suspended sediments will certainly deepen the euphotic layer and further promote biological productivity in summertime. ...
Article
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Long‐term patterns of dissolved oxygen (DO) in estuarine and coastal waters remain poorly understood. Here we summarized DO concentrations and analyzed the crucial drivers of hypoxia in northwestern and southern Hong Kong and Mirs Bay over the past three decades. Deoxygenation was weak in the bottom water in northwestern Hong Kong, although DO was consistently undersaturated, whereas the annual minimum DO in the bottom water exhibited a significant decrease in southern Hong Kong (−0.06 ± 0.01 mg L⁻¹ yr⁻¹) and Mirs Bay (−0.10 ± 0.02 mg L⁻¹ yr⁻¹). Seasonal hypoxia in the bottom water was accompanied by supersaturated DO and high Chl‐a in surface waters of southern Hong Kong, indicating a crucial role of local extensive productivity in the oxygen depletion of the bottom water. The rapid deoxygenation was also attributed to the water stratification preventing oxygen replenishment in southern Hong Kong and the predeoxygenation of bottom water retained in Mirs Bay. The dissolved inorganic nitrogen concentrations have increased from 1 to 3 mg N L⁻¹, which increased the primary productivity contributing to the decrease in water DO over the past three decades. Therefore, these results suggest that biological oxygen consumption and seasonal stratification are mainly driving the formation and maintenance of hypoxia in the Pearl River Estuary and adjacent areas.
... observed from the bottom to surface sediments, in agreement with significant older radiocarbon signature ( Figure 5A) with increased MGS in the study ( Figure 3F). The dam construction on Pearl rivers have greatly modified the transport of sediment since mid-1980s (Dai et al., 2008;Wei et al., 2020). The decrease of TOC ( Figure 3A) and F terr ( Figure 7C), as well as significant correlation between sediment load and F terr (R=0.64, ...
Article
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Estuaries have experienced significant changes due to global climate change and human perturbations since the last century. However, the climate and anthropogenic influence on the burial of sedimentary organic carbon (OC) in estuaries is still not understood well yet. Here, a 3-meter sediment core was taken from the Pearl River Estuary (PRE) in China. Depth profiles of both bulk OC and lignin biomarker data indicated three stages with different features of buried OC during the 130-year sediment deposition. The 1893-1957 stage showed 20% more burial of marine derived OC, which was mostly adsorbed on finer minerals compared to the years after 1957. The 1957-1980 period witnessed 4.6 times higher burial rate of petrogenic OC, which made the radiocarbon age of total organic carbon 42% older than before due to soil erosion and carbonate rock weathering. The 7-year running average variation of terrestrial OC input based on endmember mixing model was correlated with the Pacific Decadal Oscillation index before 1957, but correlated with the Atlantic Multidecadal Oscillation between 1957 and 1980 in the region. The reduction of land derived OC content after 1980s was mostly affected by human perturbations such as deforestation and dam construction which corresponded to the beginning of Economic Reform and Open Up in China. The overall increase of lignin content from bottom to surface sediment indicated increased vascular plant derived OC due to deforestation activities during the urbanization process. The study suggested different time periods when climate or human disturbance dominantly affected the OC burial in the PRE, which have significant indications for local and global carbon cycling and environmental ecology.
... Two regions in the northern South China Sea -the adjacent shelf of the Pearl River Estuary (PRE shelf, Figure 3b) and the southwest coast of Taiwan Island (Figure 3c) -are both characterized by point sources of terrigenous OC and established dispersal pathways, and thus serve as systems where aged OC in surface sediments is potentially caused by both terrigenous supply of old OC input and by lateral transport-induced aging Kao et al., 2014;Wei et al., 2020). The Pearl River comprises a major source of OC in the coastal area adjacent to the PRE (Dai et al., 2008), and fluvial materials are transported to and deposited on the outer shelf, during which are subject to hydrodynamic forcing (Wei et al., 2020). Applying our method, the b AgedOC values of coastal sediments (0.8321, R 2 = 0.98, Figure 3b) was distinctly higher than those of outer shelf sediments (0.7780, R 2 = 0.99, Figure 3b). ...
Article
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Aged organic carbon (OC) is widely observed in surficial sediments deposited on continental margins, resulting from several factors including old OC input and sedimentological processes. While the supply of old OC to marine sediments has been examined and quantified in a range of continent margin settings, there have been few attempts to constrain the extent to which hydrodynamic processes, specifically lateral transport, contribute to the old age of sedimentary OC. Here, we propose a mathematical index, bAgedOC, to differentiate the causes of sedimentary aged OC, and apply this approach to two typical sediment transport pathways that follow the supply and dispersal of terrestrial OC in the marine environment, and further extrapolate it to submarine canyon, Arctic and deep‐sea environments. The index, representing a modification of a widely used quantitative method, provides a novel approach to distinguish the contribution of lateral transport to the pre‐depositional aging of OC in the oceanic region.
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The peak in sediment transport in alluvial rivers generally lags behind the peak in discharge. It is thus not clear how the hysteresis in the sediment/discharge relationship may be impacted by damming, which can fundamentally alter the water and sediment regimes in the downstream reaches of the river. In this study, a total of 500 flood events in the Yichang‐Chenglingji Reach (YCR) of the Middle Yangtze River immediately downstream of the Three Gorges Dam (TGD) are analyzed to study the impacts of dam operations on the hysteresis of suspended sediment transport. Sediment rating curves, hysteresis patterns, as well as lag times, are investigated to determine the relationship between suspended sediment concentration (SSC) and flow discharge (Q) at different temporal scales, from inter‐annual to individual flood events, for the pre‐ and post‐TGD period from 1992‐2002 and 2003‐2017, respectively. The results showed that the TGD operation decreased the frequency and magnitude of floods. The decrease in peak flow and increase in base flow weakened the flood contribution to the annual discharge by nearly 20%. However, the relative suspended sediment load contribution during flood events was much higher than the discharge contribution, and was little impacted by the dam. At seasonal and monthly scales, more than 80% of the suspended sediment was transported by ~65% of the water discharge in the summer and early fall. The monthly SSC‐Q relationship changed from a figure‐eight to an anti‐clockwise pattern after the construction of the TGD. For single flood events, the TGD operations significantly modified the downstream SSC‐Q hysteresis patterns, increasing the frequency of anti‐clockwise loops and the lag time between peak Q and peak SSC. These adjustments were mainly caused by differences in the propagation velocities of flood and sediment waves and the sediment “storage‐mobilization‐depletion” process, whereas the influence of lateral diversions was small.
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One of the largest contiguous karst areas in the world is southwest China, where special geological conditions, shallow soil layers, the frequency of precipitation extremes, unreasonable land use, and dam construction have rendered the relationship between water discharge and sediment load increasingly complex. Few studies have been devoted to this region to clarify the potential influencing factors of the water–sediment relationship on a monthly basis. Correspondingly, the present study evaluated the temporal variations in sediment rating curves (SRCs: Qs=aQb, where Qs is the sediment concentration or load, Q is the water discharge, the coefficient a and index b are fitted parameters) and decoupled the effects of climate change and vegetation dynamics on SRC parameters using partial least squares-structural equation modeling (PLS-SEM). To these ends, 2003 to 2017 data on the monthly water discharge, sediment load, climatic and vegetation factors of six karst watersheds were collected and analyzed. No significant trends in interannual variations of SRC parameters occurred. The PLS-SEM showed that the combined effects of climate and vegetation explained 18% to 43% and 8% to 57% of the variations in coefficient a and index b, respectively. In most cases, however, no significant relationship was found among climate, vegetation-related factors, and the SRC parameters. Special hydrogeological conditions and dam construction in the area increasingly complicated the sediment transport process, thus potentially weakening the influence of climate and vegetation on the SRC parameters. This study can serve as guidance for regional ecological management and the sustainable development of soil and water resources.
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This study examined estuarine morphology and depositional processes in front of lateral river-dominated outlets in a tide-dominated estuary, such as the Lingding Bay in South China Sea. The estuary receives sediments from four deltaic distributary outlets of the Pearl River Delta, of which three outlets (in the western part of the estuary) are river-dominated and one at the head is a tide-dominated inlet. The results obtained from an analysis of historical bathymetric maps and numerical experiments of the sediment dynamics are consistent and indicate that tides predominate the morphological shaping of the lateral river mouth shoals. Accordingly, the eastward progradation of the shoal areas decreased from the upper to lower estuary. The resultant estuarine morphology could be characterised by exponentially decreasing cross-sectional bathymetric areas from the lower to upper estuary. This morphological evolution occurred despite intensive human disturbances. Reduced tidal influences may enhance the southeastwardly deposition of riverine sediments, while increased influences would enhance the erosion of the lateral river mouth shoals. Furthermore, increased tidal influences enhance sediment resuspension and their eastward transportation towards the west trough of the bay, such that lesser sediments are deposited at the river mouth shoals. Hence, it can be concluded that lateral river-dominated outlets in a tide-dominated estuary like the Lingding Bay induce unique estuarine geomorphic features and sedimentary processes that are strongly influenced by tidal action.
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Assessing the trend of sea surface wind is important for understanding the response of the marine environment to climate change. Analysis of wind data reveals that the summer wind direction in the Pearl River Estuary (PRE) shifts anticlockwise at a rate of −0.36°yr −1 over the past 42 years (1979-2020). The mean wind direction in July shifts from 183.6° (in 1979) to 169.3° (in 2020) and is predicted as 142.1° by 2100. How this long-term wind direction change affects the PRE hy-drodynamic circulation structure has not been examined yet. A fully calibrated high resolution 3D hydrodynamic model is used to evaluate the response of local hydrodynamics to wind direction shifting in this study. The model results indicate that both the cross-channel wind-driven transport and along-channel seaward flow are weakened as wind direction shifts. Consequently, the lateral circulation is slowed down significantly while the longitudinal exchange flow is weakened slightly. A remarkable increase in stratification occurs in the coastal sea adjacent to the Modaomen where hypoxia has been frequently reported. The residence time of Lingding Bay increases slightly. The Momentum budget indicates the wind direction shifting can cause major changes in the barotropic pressure term, which is mainly balanced by the baroclinic pressure term and diffusion term.
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Utilizing the Standardized Precipitation Evapotranspiration Index (SPEI) and the Normalized Difference Vegetation Index (NDVI), the variation trend of NDVI, SPEI and the response of vegetation productivity to drought events at different spatial–temporal scales were evaluated by correlation analysis. The results show that the annual change trend of vegetation coverage is obviously mainly distributed in the dense area of the river network, which indicates that the river network is dense and abundant in water, which is beneficial to promote vegetation growth. The vegetation coverage NDVI is related to the drought index SPEI for the same month, and the corresponding month has a strong correlation with the spatial distribution of river networks. The corresponding month appears in autumn in the coastal area, in summer in the middle region, and in spring in the western region. Elevation, distance from coast, and density of river network will affect the response of vegetation productivity to drought events.
The Lancang reservoirs play an essential role in China's national economy and life, and the study of reservoir siltation is of great significance to ensure its sustainable service for the nation and people. In this paper, reservoir sedimentation is quantified in stages by empirical models and theoretical methods using reservoir information and sediment data to reveal the latest status of siltation in the Lancang reservoirs. Results show that the storage capacity loss of the Manwan and Jinghong reservoirs reached 51.4% and 1.54% by 2019, which illustrates that the situation of siltation is serious. The theoretical trapping efficiency of Manwan reservoir was about 69% and the estimation result of the Brune method performed best with a value of 67.5% among the empirical methods. The Brune method was then modified with a correction coefficient and the revised Brune method can be used for the estimation of trapping efficiency in other reservoirs. Overall, this paper presents relatively accurate information for managers to understand the current state of siltation in the Lancang reservoirs, and can provide scientific guidance and data support for them to take measures to reduce sedimentation and ensure the sustainability of reservoirs. HIGHLIGHTS Phased calculation of sedimentation in the Manwan and Jinghong reservoirs is presented.; A detailed update on the sedimentation of Lancang reservoir is shown to support its sustainable service.; The Brune method performed best for estimating trapping efficiency and could be applied to other Lancang reservoirs for reference.;
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In order to examine the role of human drivers in shoreline advance of the Pearl River delta plain over the past 7500 years, this study reconstructed several palaeo-shorelines based on sedimentary records, archaeological findings, historical archives, family ancestral books and modern survey data. With these shorelines, this study quantified the land growth rate for each of seven time periods spanning from the Neolithic-Bronze Ages, through the agricultural period, to the modern industrial era. The results show that, since c. 7500 cal. a BP as the sea level ceased to function as the primary driver, fluvial discharge and tide became the main natural drivers. Between 5500 BCE and 200 BCE, as fluvial discharge was progressively reduced, the amount of sediment drained out to the sea by tidal action was also progressively reduced due to the decrease of the estuarine space. As a result, the land growth rate was kept around 0.36–0.38 km²/a for this period. During the agricultural stage between 200 BCE and 1950 CE, human activity has led to a continuous increase of land growth rate, from 0.78 km²/a caused by land clearance (200 BCE to 960 CE) to 2.69 km²/a triggered by early land reclamation (960–1370 CE) and further to 4.50 km²/a as more advanced techniques were used (1370–1950 CE). Finally, a better organization of human power in land reclamation and the use of machinery since 1950 CE has accelerated the land growth to an average of 10.56 km²/a. The river damming and soil preservation projects in the past three decades have greatly reduced sediment supply and caused a significant slowdown in the shoreline advance.
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The temperature proxies U37K′, LDI, TEX86H, and RI-OH are derived from lipid biomarkers, namely long-chain alkenones from coccolithophorids and long-chain diols ascribed tentatively to eustigmatophytes, as well as glycerol dialkyl glycerol tetraethers (GDGTs) and OH-GDGTs produced by Archaea. The applicability of these proxies in the South China Sea (SCS) has been investigated previously. However, in each study only one or two of the proxies were compared, and the recently updated calibrations or new calibrating methods such as BAYSPAR and BAYSPLINE were not applied. Here, we investigate four proxies in parallel in a set of surface sediment samples from the northern SCS shelf and relate them to local sea surface temperature (SST), which allows for us to compare and assess similarities and differences between them and also help improve regional multiproxy seawater temperature reconstructions. Our results indicate that U37K′ reflects annual mean SST with a slight bias toward the warm season. Terrestrial inputs appear to have a significant impact on LDI, TEX86H, and RI-OH proxies near the coast, leading to colder LDI- and TEX86H-derived temperatures but a warmer RI-OH temperature estimate. After excluding samples influenced by terrestrial materials, we find that LDI-derived temperature agrees well with annual SST, while TEX86H- and RI-OH-derived temperature estimates are close to SSTs in seasons dominated by the East Asian winter monsoon and summer monsoon, respectively. The different seasonal biases of these temperature proxies provide valuable tools to reconstruct regional SSTs under different monsoonal conditions.
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In recent decades, the surface sediments in the Pearl River Estuary (PRE), China, have been intensely disturbed by human activities. Data from 1975 to 2017 show that the mean surface sediment grain size in PRE is distributed between 5 and 8 φ, while the sorting coefficient of the sediments vary between 0.39 and 4.53, and the skewness is between −0.31 and 0.55. From 1975 to 1994, sandy sediment was mainly distributed near the outlets. However, sandy sediments have been present in the south of the West Shoal and Middle Shoal, and north of Lantau Island, and have had a patchy distribution since the 2000s. From 1975 to 2017, the average total grain size of the surface sediments in PRE changed from 6.4 to 5.3 φ, indicating a coarsening trend. This was mainly caused by the sharp reduction in upstream sediment load since the 1980s, as well as channel dredging and sand mining in PRE. On the one hand, the decrease in the sediment load caused an increase in the erosion area, which caused a large amount of fine-grained sediment to be eroded and transported to the open sea. On the other hand, channel dredging and sand mining resulted in coarse-grained sediments in the paleo-sedimentary layer being stirred to the surface, thus making a significant contribution to the coarsening of the average grain size. Channel dredging and sand mining are also the main reasons for the patchy distribution of sediments in PRE. In the future, with further reductions in the sediment load and the persistence of reclamation, channel dredging, and sand mining, the grain size of surface sediments in PRE will continue to show a coarsening trend.
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Hydropower dams produce huge impacts on renewable energy production, water resources, and economic development, particularly in the Global South, where accelerated dam construction has made it a global hotspot. We do not fully understand the multiple impacts that dams have in the nearby areas from a global perspective, including the spatial differentiations. In this study, we examined the impacts of hydropower dam construction in nearby areas. We first found that more than one-third of global gross domestic production (GDP) and almost one-third of global population fall within 50 km of the world’s 7,155 hydropower dams (<10% of the global land area sans the Antarctic). We further analyzed impacts of 631 hydropower dams (≥1-megawatt capacity) constructed since 2001 and commissioned before 2015 for their effects on economy, population, and environment in nearby areas and examined the results in five regions (i.e., Africa, Asia, Europe, North America, and South America) and by different dam sizes. We found that recently constructed dams were associated with increased GDP in North America and urban areas in Europe but with decreased GDP, urban land, and population in the Global South and greenness in Africa in nearby areas. Globally, these dams were linked with reduced economic production, population, and greenness of areas within 50 km of the dams. While large dams were related with reduced GDP and greenness significantly, small and medium dams were coupled with lowered population and urban land substantially, and large and medium dams were connected to diminished nighttime light noticeably in nearby areas.
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Precipitation extremes are recognized as one of the main factors that influence annual runoff and sediment yield in specific watersheds. However, it is not clear whether precipitation extremes account accurately for the variability in runoff and sediment yield for different karst watersheds, especially in climate-sensitive areas, such as the karst critical zone of southwest China. The objective of this study was to quantify the relative importance of the 11 precipitation extremes variables that explain the total variance in annual runoff and sediment yield with a boosted regression tree (BRT) model for 40 typical karst watersheds in southwest China. Results indicated that the most important precipitation extremes for annual runoff and sediment yield were heavy precipitation amount, consecutive dry days, heavy precipitation days, consecutive wet days, and rainstorm amount. The BRT model accounted for 76% of the variation in runoff, which demonstrated that precipitation extremes were crucial factors for generating runoff. Although precipitation extremes had significant effects on sediment yield, the BRT model only explained 32% of the variability in sediment yield. Hydrogeological conditions and dam construction in these karst watersheds may be responsible for the difference in total variance explained by the BRT model between annual runoff and sediment yield. Because of the inherent nonlinearity present in many hydrological systems, the BRT model provided a robust capability to generate data-driven, objective, insight into complex relationships. We recommend this model for the application of non-stationary and nonlinear hydrological processes.
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The relative contributions of decreased upstream sediment loads and local estuarine engineering activities to the estuarine channel geometry are poorly understood. In this study, we analyze the hydrological changes and identify the location, duration and intensity of the estuarine engineering activities based on the channel morphologic changes from 1965 to 2017 at the five stations in the Pearl River Estuary. Thereafter, the Mann-Kendall (M-K) statistical test, empirical orthogonal function (EOF) tests, and channel geometry reconstruction based on the hydrological coefficient were performed to quantitatively estimate the relative contributions from upstream dam construction and estuarine engineering activities. The results show that the geometric changes in the five transects over the last 50 years could be divided into three stages. Stage I extends over approximately 23–33 years at the different channel transects, during which the channel geometries were mainly influenced by natural factors, with a balance between erosion and deposition. Stage II occurred during the next 11–20 years and the changes in the cumulated water depth in comparison to the values in the previous adjacent years at this stage are approximately 5–25 times the values in stage I. The human activities (e.g., sand excavation) contribute to >70–90% of the extreme geometric changes. Stage III lasted for <3–11 years in the different transects with a slight depositional trend, and policies regulating sand excavation were implemented during this stage. The rapid increase in the channel area and water depth caused by sand excavation can cause the downcutting of the riverbed, a decrease in the water level, and redistribution of the water and sediment discharge. Therefore, the monitoring, simulation and analysis of the variation in the typical channel geometry over the long term provide important means to understand the human activities occurring and insights for future sustainable estuarine management.
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Typhoon Mangkhut, in 2018, was one of the worst typhoons in recent history that has made landfall in the Pearl River Estuary (PRE) China. It swept along the coast of Guangdong Province, causing severe damage to many areas and affecting more than one million people. To study the hydrodynamics and sediment dynamics in different sub-regions of the PRE during the typhoon, we implemented a 3D wave-current-sediment coupled ocean model based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). A series of numerical experiments were conducted to study the variation in suspended sediment concentration (SSC) under different external forcing mechanisms, including tides, waves, river discharges and strong winds. Moderate Resolution Imaging Spectroradiometer (MODIS) images, collected after the typhoon landed, were used to assess the simulation results. We found that in the area where the water depth was shallower than 20 m, the increased total suspended sediment concentration was dominated by the regional erosion of fine sediment which was caused by near-bottom wave orbital velocity and bottom shear stress, especially at the four western outlets and along the western coast of the PRE. For the deeper coastal zone, at depths between 20 and 30 m offshore of the western coast, the high total SSC resulted from the contribution of fine sediment transported from the upper and middle estuaries by southwestward advection. A small amount of sand eroded and migrated locally, owing to the corresponding increase in the bottom shear stress. The variation in SSC during the typhoon had a time lag compared with other dynamic conditions, resulting in a maximum one hour delayed response after the typhoon made landfall. Based on the results of the sensitivity experiments under the same typhoon conditions, we found that, as a result of wave-enhanced bottom shear stress, the SSC values associated with wave-current interaction were significantly higher, by as much as 1.5 and 1.3 times those of simulations without the waves at the surface and near-bottom layers, respectively. This indicated that the sediment resuspension induced by waves was important during typhoon landfall. However, the strong wind-induced current was weakened owing to the joint effect of wave radiation stress and enhanced bottom stress; thus, considering wave effects, the magnitude of the resulting sediment transport rate was smaller than that without considering wave effects, especially in shallow waters. During the approach, landfall and retreat of Typhoon Mangkhut, Lingding Bay experienced two transitions from a slight state of sediment loss to a relatively high state of sediment gain, followed by an even greater state of sediment loss. During an M2 tidal cycle after Typhoon Mangkhut made landfall, the net seaward suspended load sediment transport in Lingding Bay reached 0.37 megatonnes. For the typhoon period covering three M2 tidal cycles, the overall state of suspended load sediment transport in Lingding Bay was sediment loss, with the magnitude of sediment loss on the west side being larger than that of sediment gain on the east side. The magnitude of the overall sediment loss in Lingding Bay would be enlarged unrealistically by approximately 73% if the wave effects were excluded. In each period, the extent of sediment transport between the two sides of the bay was strengthened by the wave-current interaction under typhoon conditions. We determined that the net effect of wave-current interaction and strong wind-induced currents under typhoon conditions was a key factor for erosion, which resulted in SSC-influenced areas being approximately 8.7 and 4.4 times as large as those under normal weather conditions for the surface and near-bottom layers, respectively. The turbidity maximum moved from the West Shoal of Lingding Bay under normal weather to the Modaomen Estuary, which was the most affected area during the typhoon when wave effects were considered. We also showed that the relative location of the typhoon path with respect to the estuary had a more significant effect on erosion than the intensity of the typhoon.
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The dynamicity of the channel is the main characteristic of the Raidak-I River in the eastern Himalayan foothill. The present study evaluates riverbank migration in association with erosion-deposition changes along the river Raidak-I using DSAS models. The present work intends to evaluate the relationship between the riverbank erosion-deposition and geomorphological and tectonic adjustment. For the study, earth observatory data like MSS, TM, ETM+ and OLI datasets of 1972, 1979, 1987, 1995, 2003, 2011, and 2020 have been used to demarcate the bankline position. Temporal analysis reveals that the river has changed its bank position by extensive erosion-accretion processes and modified its floodplain area uses significantly. The historical positions of both banklines indicate that a large portion of the floodplain area depicts an erosion-accretion sequence with time. In the timeframe of the last 48 years, the Raidak-I River has an average erosion-deposition at -0.23 m/year on the right bank and 1.57 m/y on the left bank. A general observation from this research is that the most dynamic or migrant part of the river is zone A and zone B compared to zone C which is relatively stable. In this study, the river course in zone C (both banks) is the most dynamic part of this entire river. The changes by the direct effect of banking migration have a bad impact on the dwellers of the floodplain adjacent village area of the river. The results of this study can represent an important indicator of the vulnerability of the Raidak-I River buffer area and also provide information about the geomorphological instabilities of the study area.
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Anthropogenic changes can significantly alter estuarine environments and their fluvial-marine interactions. In this study, we investigate the evolution of wave impacts on the Modaomen Estuary (Pearl River Delta, China) in response to man-made changes by quantifying the relative strength of fluvial and wave forces and by analyzing morphological changes. River sediment input to the estuary has decreased significantly due to dam construction and decreased sediment diversion at junctions of the estuary due to sand excavation. The nearshore wave power and potential maximum alongshore sediment transport has increased substantially due to an intensive seaward extension of the coastline caused by estuarine regulation projects. Consequently, the Modaomen Estuary has shifted from a river-dominated environment to one alternately controlled by wave and river effects, with waves being dominant in the dry season. Morphological alterations due to anthropogenic modifications in the Modaomen Estuary are characterized by a rotation of shoals from perpendicular to the coast to parallel to the coast, a straightening of the shoreline, and development of an asymmetrical subaqueous delta with a strong littoral drift, indicating evident wave impacts on estuarine morphological evolution.
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The coastline of the Pearl River Estuary (PRE) experienced significant changes due to the increasing demand for land. Previous studies have focused on coastline changes of the mainland in China; thus changes of island coastlines have not been well investigated. In this study, the coastlines of the continent and islands of the Pearl River Estuary from 1978 to 2018 were extracted and analyzed by integrating Canny edge detection and Otsu methods based on Landsat data, and the coastline was classified into six types including biological, sandy, muddy, estuarine, rocky, and artificial via visual interpretation. The coastline length, coastline change rate and change area were calculated and analyzed for the 40 years period. The precision test showed that the coastline extraction methods presented in this paper meet the accuracy standard. The coastline length of the Pearl River Estuary was found to have increased rapidly from 789 km in 1978 to 979 km in 2018. During the past four decades, a large proportion of natural coastline converted into artificial one (66% in 2018), while 577 km² land increased in this area. The coastline of the western PRE demonstrates more rapid change rates than that of the eastern part, and more land reclamation has occurred in the western area. Between 1988 and 1997, the coastline length and area of reclamation experienced the most extensive changes. Both natural factors, including sediment transport, sea-level rise, wind, tide, as well as anthropogenic factors, such as land reclamation and dam construction, have impacted coastline variation of the Pearl River Estuary in the past four decades. In general, the western part of the study area showed more remarkable coastline change and experienced the enormous influence of land reclamation, dam construction and sediment transport, compared with the eastern part.
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Climate change is one of the biggest challenges facing humankind. Higher temperatures and changes in precipitation will lead to greater frequency of drought, wildfire, and intense storms that will threaten systems already vulnerable to climate and anthropogenic change. These threats will transform carbon delivery across the terrestrial–aquatic interface. This study investigated how anthropogenic activities and climate influence organic carbon delivery using Englebright Lake, a reservoir in California, as a model system. Organic carbon accumulation in three depositional settings (bottomset, foreset, topset) was analyzed using fatty acid, sterol and lignin biomarkers and compared to records of watershed events to determine responses to dam construction, mining impacts and flood events. Concentrations of long chain saturated fatty acids, plant sterols, epi-brassicasterol/brassicasterol and lignin biomarkers increased by an order of magnitude in foreset deposits in response to flood events (p < 0.007 for each biomarker). Hydraulic mining for gold was recorded as near-zero concentrations of terrigenous biomarkers in topset deposits, whereas decreases in diacids coincident with increases in aquatic sterols in bottomset deposits reflected the response to dam construction (p < 0.007 for each biomarker). Organic carbon accumulation was controlled by event magnitude and duration, and climate-driven event signals were up to an order of magnitude larger than anthropogenic-driven event signals. These data demonstrate the importance of understanding the depositional environment because the ability to characterize three different depositional settings in Englebright Lake enabled us to identify the smaller anthropogenic signals that would have been obscured by the much larger response to climate events.
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Draining at the southern part, the Pearl River is the third largest river in China. The Pearl River is the collective name of West River, North River and East River. Based on long series data (1955-2005) of the water and sediment discharge at the main hydrological stations and the precipitation in the drainage basin, this paper aimed to reveal the variation pattern of the sediment discharge and Lhe causes of the Pearl River. The results are: (1) sediment discharge at Shijiao (North River), Boluo (East River), Qianjiang and Gaoyao (West River) stations show a decreasing trend, while Liuzhou (at Liujiang River, a tributary of West River) and Nanning (at Yujiang River, a tributary of West River) an increasing trend in 1955-2005; (2) the average sediment discharge into the sea from the Pearl River (the sum of the North River, East River and West River) in 1955-2005 is 7529× 104 t/yr with a decreasing trend (not significant statistically). The main conclusions are: (1) the variation of precipitation contributed greatly to the yearly fluctuation of the sediment flux, but little to the decreasing trend of the sediment discharge into the sea; (2) the phased variation trend of sediment discharge into the sea is relevant to soil loss and water and soil conservation; (3) dam construction is the main cause of the decrease of the sediment discharge into the sea; and (4) a further decrease of the sediment discharge into the sea may occur in the future decades, which may cause serious environmental problems at the estuary such as slowdown of coastline progradation or even retreat. Further and in-depth study is urgently needed in view of the economic significance of the Pearl River Delta in China and the environmental issues in the estuary, including the coastal region of Hong Kong.
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Despite their relatively small drainage areas, European rivers reflect a wide variety of hydrologic regimes, although with very few exceptions they have been strongly affected by human activity. Scandinavian rivers (particularly those draining Iceland and western Norway) can have high runoff, and, except for those draining Iceland, all have very low suspended and dissolved sediment loads. Northern and western European rivers have somewhat lower runoff, among the lowest suspended sediment yields in the world, and anthropogenically enhanced dissolved solid loads. Annual discharge of many of these rivers appears to vary inversely with the North Atlantic Oscillation index. Rivers discharging from the southern Alps into the Mediterranean Sea have relatively high runoff, high suspended sediment yields (reflecting younger, more easily erodable rocks as well as generally smaller drainage basins), and high dissolved yields, although presumably with somewhat less human influence. European rivers and their estuaries tend to reflect the terrestrial environments of their drainage basins (i.e. climate, landscape geomorphology, geology), but they also display strong anthropogenic signatures. Sediment erosion increased dramatically in the last several millenia in response to deforestation, farming and mining. In the past 50 years, however, increased soil conservation and local reversion of agricultural land to forest, as well as river diversion and dam construction, have decreased the suspended sediment loads of many European rivers. Improved mining and manufacturing techniques, as well as more effective use of fertilizers and improved waste treatment, almost surely will result in lower dissolved solids and nutrient fluxes to the coastal environments, which presently are the highest in the world. The long-range effects of changed land use on estuarine and coastal environments remain to be seen, although decreased sediment loads in the past 20-40 years have already resulted in increased shoreline erosion. Decreased nutrient fluxes almost certainly will affect water quality in European coastal waters, and decreased silicate delivery by some dammed rivers may result in proliferation of new (and perhaps harmful) estuarine and coastal ecosystems. Everything points to further changes as European rivers and their drainage basins continue to change in the coming years.
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The 1951–2004 time series of annual sediment supply from the Yangtze were analyzed using the wavelet method. Coastal bathymetric data were processed using the Arc-GIS software. A typical intertidal flat was measured from May 2002 to August 2005 for comparison of bed levels before and after the Three Gorges Dam (TGD). There is a significant decreasing trend in riverine sediment supply since late 1960s, which is attributed mainly to the dam constructions. TGD reduced the sediment load to147 mt/yr in 2004, only 35% of the average. In response to this drastic decrease, the deltaic coast is turning from progradation to recession. We predict that the Yangtze sediment supply will further decrease and coastal erosion will be intensified in the coming decades, which poses a great challenge to coastal management.
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Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, riv
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1] On the basis of estimates of sediment accumulation in reservoirs, the impact of 50,000 dams on sediment supply and intertidal wetland response in the Yangtze River catchment is examined. The total storage capacity of reservoirs is 200 Â 10 9 m 3 , or 22% of the Yangtze annual runoff. The sediment accumulation rate in reservoirs has increased from $0 in 1950 to >850 Â 10 6 t/yr in 2003. Although sediment yield has increased with broader soil erosion in the river basin, the total riverine sediment discharge rate shows a strong decreasing trend from the late 1960s to 2003, likely due to dam construction. Consequently, the total growth rate of intertidal wetlands at the delta front has decreased dramatically. A significant relationship exists between intertidal wetland growth rate and riverine sediment supply that suggests riverine sediment supply is a governing factor in the interannual to interdecadal evolution of delta wetlands. Regression analysis of intertidal wetland growth rate and sediment supply shows that intertidal wetlands at the delta front degrades when the riverine sediment discharge rate reaches a threshold level of <263 Â 10 6 t/yr. Owing to the construction of the Three Gorges Dam and other new dams, the sediment discharge rate of the Yangtze River will most likely decrease to below 150 Â 10 6 t/yr in the coming decades. Therefore unless current management policies are adjusted, drastic recession of Yangtze River delta intertidal wetlands can be expected to occur. (2005), Impact of dams on Yangtze River sediment supply to the sea and delta intertidal wetland response, J. Geophys. Res., 110, F03006, doi:10.1029/2004JF000271.
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New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about 15 x 106 tons) contribute about 7 x 109 tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about 13.5 x 109 tons annually; bedload and flood discharges may account for an additional 1-2 x 109 tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.-Authors
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This study constructs sediment budgets for the delta of the Yellow River, one of the world’s largest muddy rivers, and, based on these budgets, discusses the characteristics and related mechanisms of sediment dispersal at the river mouth. The sediment budgets are derived from quantifying deposits in the deltaic lobe formed at the Diaokouhe mouth of the river. A detailed investigation of the dry bulk density of deposits in the delta, which yields models of dry bulk density of deposits in the main depositional settings of the delta, guarantees the reliability of the sediment budgets. Results show that deposits behind the delta front of the Diaokouhe lobe over the period of 1965 to 1974 account for 73.5% of the incoming sediment. The constructed sediment budgets reveal that the proportions of the deposits accumulated behind the delta front vary considerably over time. The main factor responsible for the variations is the change in sediment discharge of the river. There is no evidence for the anticipated positive relationships between the proportion of sediment escaping from the delta and water discharge, clay content of incoming sediment, or mouth channel efficiency. The characteristics of sediment dispersal on the delta are related to the rapid extinction of sediment-charged hyperpycnal underflows, which are dominant in the plumes off the Yellow River mouth, and sediment deposition in the overbank flow on the delta plain because both of these processes trap a higher proportion of sediment at a higher sediment discharge.
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The regularities of the Ebro River delta formation and the impact of hydraulic construction on the river sediment runoff are discussed. As shown, the drastic reduction of the sediment runoff after the construction of two large reservoirs in the lower river reaches in the 1960s slowed down the delta protrusion and intensified the delta coastline washout.
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It has become increasingly difficult to assess the impact of changes in the sediment flux to the coastal zone because of the conflicting impacts of humans. Globally, soil erosion is accelerating (e.g., deforestation, some agriculture practices), while at the same time sediment flux to the coastal zone is globally decelerating (e.g., water diversion schemes, dams). Reduced loads delivered to the coastal zone result in an accelerated coastal erosion and a decrease in habitat. The reduction of the seasonal flood waves also means that sediment is dispersed over smaller areas of the continental margin. One of the great challenges facing hydrologists, with regard to our understanding of anthropogenic versus climate-change impacts on the hydrological cycle, is the quality and duration coverage of hydrological data, particularly on pristine rivers. For many rivers, the length of hydrological coverage is measured in years rather than decades, with no rivers with reliable observations across centuries. In other words, if a river's discharge is monitored during periods where the impacts of humans or nature produce a long-term trend, then the return-interval concept for floods and droughts, for instance, becomes meaningless. In addition, river systems are conditioned by events that are largely prehistorical. Forensic studies of paleoriver systems provide us with some ability to assess and discern the impacts of humans versus climate.
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The transport of sediment by rivers to the oceans represents an important pathway in the global geochemical cycle, a key component of the global denudation system, and an important measure of land degradation and the associated reduction in the global soil resource. With the growth of interest in global environmental change, it is important to consider the extent to which this important index of the functioning of the earth system is changing. Evidence from longer-term sediment load records indicates that river sediment fluxes are sensitive to many influences, including reservoir construction, land clearance and land use change, other forms of land disturbance, including mining activity, soil and water conservation measures and sediment control programmes, and climate change. Some of these influences cause sediment loads to increase, whilst others, namely, soil and water conservation and sediment control programmes, and reservoir construction cause decreased sediment fluxes. In many cases, it is difficult to disentangle the influence of climate change from that of other changes in catchment condition. Although there is clear evidence that the sediment loads of some rivers are changing, others show little evidence of any significant temporal trend. This could reflect either lack of change in the controlling factors or the buffering of any change by the river basin. To provide a preliminary assessment of current trends in the sediment loads of the world's rivers, longer-term records of annual sediment load and runoff were assembled for 145 major rivers. Simple trend analysis of these data indicated that ca. 50% of the sediment load records showed evidence of statistically significant upward or downward trends, with the majority evidencing declining loads. In the case of the annual runoff series, far fewer rivers (i.e. ca. 30%) showed evidence of statistically significant trends. The evidence afforded by the sample of the world's rivers indicates that reservoir construction is probably the most important influence on land–ocean sediment fluxes, but the influence of other controls resulting in increasing sediment loads could also be detected. A larger database, however, is required to provide a more definitive assessment of current trends in land–ocean sediment transfer by the world's rivers.
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Land–ocean transfer of sediment by rivers is a key pathway for material transfer on Earth. Contemporary data on the sediment loads of rivers provide clear evidence of significant recent changes in the sediment fluxes of several rivers in response to human impact. The key drivers of increased sediment loads include land clearance for agriculture and other facets of land surface disturbance, including logging activity and mining. Although, programmes for soil conservation and sediment control can result in reduced sediment loads, the trapping of sediment by dams represents the dominant cause of reduced loads. This influence is currently assuming increasing importance at the global scale. Any attempt to link these drivers to changes in the global land–ocean sediment flux must take account of the aggregation and buffering effects that operate in larger basins, which can cause damping and even removal of signals of increasing flux within the upstream basin, and complicate the link between upstream and downstream response to human impact. Further work is required to provide a precise quantitative assessment of the human impact on global land–ocean sediment fluxes and the net effect of increasing and decreasing fluxes. Particular attention must be paid to the temporal perspective and the variation of impact trajectories in different areas of the globe and for river basins of different sizes.
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Data on sediment discharge and suspended sediment concentration (SSC) at two hydrological stations from 1951 to 2000 were utilized to study the variation in river sediment supply to the delta. Literature and documents on dam construction and water withdrawal were collected, and combined with field investigations on deforestation/afforestation and extraction of riverbed sediment, to examine the influence of human activities on river sediment load. From 1950's to 1960's, the river sediment discharge and SSC increased by 10 and 12%, respectively. Since 1960's, both values have shown a significant reduction, with sediment discharge 34% lower and SSC 38% lower in the 1990's than in the 1960's. These changes are found to be governed by the balance between two aspects of human activities: deforestation and dam construction. It was predicted that river sediment discharge and SSC in the future 100 years would probably be reduced to less than 50% of the present (the average of the past 50 years) due to human activities especially the Three Gorges Dam, which will be put into operation in 2003. The effect of this reduction on the deltaic coast is also briefly addressed.