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Universal and local controls of avulsions in southeast Texas Rivers
Abstract
This study addresses the relative importance of universal and local factors in river avulsions, from the perspective of the configurational complexity of the fluvial system as it relates to the potential for avulsions. Because local factors cannot be addressed without a specific context, several rivers of the southeast Texas coastal plain (Brazos, Navasota, Trinity, Neches, and Sabine Rivers) are studied based on field observations and surveys, digital elevation models, and geographical information system (GIS) analysis. Avulsions are influenced by a combination of universal factors relevant to any alluvial river, and local factors at least partly contingent on the environmental setting and history of the study rivers. The universal controls are factors that create conditions under which avulsions can occur: channel aggradation, banktop discharge, and cross-valley slope advantages. A rate of channel aggradation greater than that of floodplain accretion is an important setup factor, but superelevation (channel bed elevation greater than or equal to floodplain elevation behind the natural levee) is not required. Slope advantages are necessary, but not sufficient, for avulsions to occur. Local factors include abandoned channels on the floodplain, and basins or depressions associated with higher-discharge Pleistocene conditions. The local controls also include deflection factors that divert flow from the main channel during high discharges, and levee weaknesses. Relationships among the universal and local factors were represented as directed graphs, and the configurational complexity determined using connectance entropy. Results show that the relative importance of universal and local factors varies with the scale or scope of analysis. In the broadest context, universal factors contribute more than 75% of the connectance entropy, signifying the importance of identifying the key setup factors. Within a given aggradation or valley-filling context, however, local factors account for more than 80% of the connectance entropy, indicating that knowledge of the trigger factors and local floodplain morphology is the key to understanding and predicting avulsions. This study suggests a three-stage process for predicting avulsions. First, the universal factors can be used to identify reaches with the potential to avulse. Second, superelevation (a sufficient but not necessary condition) can be used to identify specific locations where avulsions are imminent, and (as a necessary but not sufficient condition) the absence of cross-valley slope advantages can be used to identify locations where avulsions cannot occur. The final stage—predicting avulsions at other locations within systems or reaches where the setup factors are present—requires specific consideration of local factors.
... Because avulsions occur when rivers seek other paths, most conditions for setup describe an instability of the river. For example, setup conditions such as channel aggradation and superelevation, lateral slope advantage, and decrease in channel capacity all implicitly describe the instability of the current river path (Allen, 1965;Makaske, 1998Makaske, , 2001Mohrig et al., 2000;Törnqvist and Bridge, 2002;Slingerland and Smith, 2004;Aslan et al., 2005;Assine, 2005;Phillips, 2011). Once a river is setup for avulsion, a trigger is usually required to initiate the event. ...
... Once a river is setup for avulsion, a trigger is usually required to initiate the event. Potential trigger events for avulsion include channel reoccupation, extreme flow discharge, ice jams, levee weaknesses, neotectonics, subsidence, substrate composition, change in sinuosity, human activities, etc. (Miall, 1996;Schumm et al., 1996;Törnqvist and Bridge, 2002;Aslan et al., 2005;Assine, 2005;Syvitski and Saito, 2007;Phillips, 2011;Morón et al., 2017). Thus far, the relative importance of the above-mentioned factors and events has not yet been thoroughly understood. ...
... Thus far, the relative importance of the above-mentioned factors and events has not yet been thoroughly understood. For example, gradient advantage has been argued to be necessary but not sufficient condition for avulsions (Aslan et al., 2005;Phillips, 2011). Phillips (2011) suggested that superelevation could be used to identify specific locations where avulsions are imminent and the absence of cross-valley slope advantages be used to identify locations where avulsions cannot occur. ...
The avulsion time scale of channels on the Yellow River delta (YRD) is about a decade due to the large sediment load, and rapid channel aggradation and progradation. Nevertheless, the Qingshuigou channel has been maintained for about four decades since 1976. This channel provides an ideal opportunity to study channel evolution following avulsion and to examine different avulsion criteria. In this study, we analyzed the geomorphic adjustment of the lower Qingshuigou channel during 1976–2015, and calculated normalized gradient advantage and superelevation at the channel to estimate how close the channel was to avulsion. Results showed that channel evolution processes may be divided into four phases: I (1976–1980) rapid aggradation, II (1980–1985) channel widening and enlargement, III (1985–1996) main channel aggradation and shrinkage, and IV (1996–2015) main channel incision and deepening. Evolution phases I, II and III are similar to the avulsion cycle observed in natural and experimental fluvial systems. The calculated values of normalized gradient advantage and superelevation in early 1990s exceeded the critical values suggested by previous studies, implying
that the channel was prone to avulsion. Nevertheless, avulsion was prevented mainly due to limited overbank flows, constriction from artificial dikes, and slowed channel extension as a result of reduced sediment load. The evolution of the Qingshuigou channel confirms previous arguments that superelevation and gradient advantage are not sufficient for avulsion, and multiple factors should be considered, including flood frequency, lateral
mobility, sediment diameter, and human interruptions.
... Aggradation and the presence of a potential cross-valley slope advantage provide the necessary setup conditions for a potential avulsion (Slingerland and Smith, 2004), and superelevation (channel bed aggraded to an elevation greater than that of the floodplain areas behind the natural levee) makes an avulsion virtually inevitable (Jerolmack and Mohrig, 2007). These events are thus an example of a tipping point where the potential occurrence is readily predictable, though exactly when and where an avulsion will occur is highly contingent (Slingerland and Smith, 2004;Phillips, 2011b). The focus here is on full avulsions (all discharge is ultimately diverted to a new channel), though partial avulsions also occur and are crucial in creating and maintaining anastomosing patterns. ...
... In most cases, however, avulsions are driven by interactions intrinsic to the system, as reviewed for the general case by Slingerland and Smith (2004) and Kleinhans et al. (2012), and for the study area by Phillips (2009;2011b;. The key general factors that comprise the setup conditions for avulsions in any alluvial river are high (near or above bank top) discharge, channel aggradation, and cross-valley slope advantages (gradients of alternative flow paths within the valley bottom). ...
... Flow deflection and levee weakness triggers are local factors specific to the fluvial setting. In southeast Texas rivers, the former include logjams and growth of sand bars, while levee weaknesses may be associated with uprooting of levee trees, animal or vehicle tracks across the levees, and surface or seepage erosion of local microtopographic lows (Phillips, 2011b). In addition, within the unfilled incised valleys, reoccupation of formerly abandoned channels is an important avulsion mechanism, as is flow to depressions associated with the large paleomeanders discussed earlier (Aslan and Blum, 1999;Blum and Aslan, 2006;Phillips, 2014). ...
Anticipating geomorphic tipping points requires that we learn from the past. Major geomorphic changes in coastal plain rivers of Texas resulting in river metamorphosis or regime shifts were identified and the major driving factors determined. Eleven such transformations--possible tipping points--were identified from contemporary observations, historical records, and Quaternary reconstructions. Two of the tipping points (between general aggrading and degrading valley states) are associated with reversals in a fundamental system control (sea-level). One (stable or aggrading vs. degrading channels) is associated with an abrupt change in sediment supply due to dam construction, and two others (changes from meandering to anastomosing channel patterns, and different anastomosis styles) are similarly related to changes in sediment supply and/or transport capacity, but with additional elements of historical contingency. Three tipping points are related to avulsions. One, from a regime dominated by reoccupation of former channels to one dominated by progradation into flood basins, is driven by progressive long term filling of incised valleys. Another, nodal avulsions, is triggered by disturbances associated with tectonic uplift or listric faults. The third, avulsions and related valley metamorphosis in unfilled incised valleys, is due to fundamental dynamical instabilities within the fluvial system. This synthesis and analysis suggests that geomorphic tipping points are sometimes associated with general extrinsic or intrinsic (to the fluvial system) environmental change, independent of any disturbances or instabilities. Others are associated with natural (e.g., tectonic) or human (dams) disturbances, and still others with intrinsic geomorphic instabilities. This suggests future tipping points will be equally diverse with respect to their drivers and dynamics.
... Relatively little work, by contrast, focuses on the types of channel shifts, the fate of abandoned channels, and potential geomorphic controls of avulsion regimes, though such studies have increased in recent years (e.g. Makaske et al., 2002;Aslan et al., 2005;Gouw, 2007;Stouthamer and Berendsen, 2007;Taha and Anderson, 2008;Phillips, 2009Phillips, , 2011b. ...
... Both internal or external conditions or forcings may promote or inhibit avulsions (see for example, Stouthamer and Berendsen, 2007;Taha and Anderson, 2008;Skorko et al., 2012). However, the specific location and timing of avulsions is not predictable, and is highly contingent on localized conditions and histories, and the timing of flood and other hydrogeomorphic events (Jones and Schumm, 1999;Makaske, 2001;Slingerland and Smith, 2004;Gouw, 2007;Phillips, 2009Phillips, , 2011b. ...
... Avulsions are best understood in a setup-and-trigger context. Setup factors make channel changes possible or likely, and include aggradation of channels and floodplains, low downvalley gradients, potential slope gradient advantages within the alluvial valley, and the presence of flood basins and/or paleochannels which may be re-occupied (Aslan and Blum, 1999;Makaske, 2001;Slingerland and Smith, 2004;Aslan et al., 2005;Stouthamer, 2005;Gouw, 2007;Phillips, 2009Phillips, , 2011b. Though extreme events such as earthquakes, landslides, and rare floods can cause channel shifts, in most cases an aggrading valley and the availability of potentially steeper flow paths outside the existing channel can be thought of as the necessary setup conditions for an avulsion. ...
The San Antonio River Delta (SARD), Texas, has experienced two major avulsions in the past 80 years, and a number of other historical and Holocene channel shifts. The causes and consequences of these avulsions – one of which is ongoing – were examined using a combination of fieldwork, geographic information system (GIS) analysis, and historical information to identify active, semi-active, and paleochannels and the sequence of shifting flow paths through the delta. The role of deposition patterns and antecedent morphology, large woody debris jams, and tectonic influences were given special attention. Sedimentation in the SARD is exacerbated by tectonic effects. Channel aggradation is ubiquitous, and superelevation of the channel bed above the level of backswamp areas on the floodplain is common. This creates ideal setup conditions for avulsions, and stable, cohesive fine-grained banks favor avulsions rather than lateral migration. Flood basins between the alluvial ridges associated with the aggraded channels exist, but avulsions occur by re-occupation of former channels found within or connected to the flood basins. Large woody debris and channel-blocking log-jams are common, and sometimes displace flow from the channel, triggering crevasses. However, a large, recurring log-jam at the site of the ongoing avulsion from the San Antonio River into Elm Bayou is not responsible for the channel shift. Rather, narrow, laterally stable channels resulting from flow splits lead to accumulation of wood. Some aspects of the SARD avulsion regime are typical of other deltas, while others are more novel. These includes avulsions involving tributaries and subchannels within the delta as well as from the dominant channel; tectonic influences on delta backstepping and on channel changes within the delta; avulsions as an indirect trigger for log-jam formation (as well as vice-versa); and maintenance of a multi-channel flow pattern distinct from classic anastamosing or distributary systems. Copyright © 2012 John Wiley & Sons, Ltd.
... Although we cannot order these controls with certainty, it is possible that the reduced mean stream power of flows within these channel reaches induces deposition of coarser particles but remains powerful enough to entrain finer bank materials [Powell, 1998]. Although this may account for lateral channel expansion, avulsions are a more complex form of adjustment whose occurrence cannot be characterized solely by variation in stream power [Jones and Schumm, 1999;Slingerland and Smith, 2004;Phillips, 2011]. The vast majority of avulsions noted in this study occurred within the channel (see Table 1 definitions). ...
... Slingerland and Smith, 2004]. If the imbalance is more significant, superelevation of the channel bed coupled with local variation in floodplain topography can instigate larger-scale avulsions [Jones and Schumm, 1999;T€ ornqvist and Bridge, 2002;Jerolmack and Mohrig, 2007;Phillips, 2011]. Reduction in stream power is often thought to induce deposition; however, responses can be more complex when aggradation induces channel avulsion and erosion [Reinfelds et al., 2004;Lea and Legleiter, 2016]. ...
Variability in channel function (behavior) can be assessed by characterizing different forms of adjustment over time. Here, historical channel adjustments in three tributary systems of the Lockyer Valley, Southeast Queensland (SEQ) are analyzed in order to evaluate the range of catchment- and reach-scale controls on channel behavior. Over 300 individual adjustments and 13 forms of adjustment were identified over a ∼130 year time span. We measured the width-to-depth ratio (W:D), mean stream power (ω), and basin area (A) at the location of all observed adjustments. The most common forms of adjustment were avulsions, lateral expansion of the channel, and bend adjustments. The tributary systems behave distinctly different from one another according to statistical comparisons between the W:D, ω, and A data for these forms of adjustment. We find that it is possible to develop process domains or typologies for forms of geomorphic adjustment found in the Lockyer Valley. These domains or typologies provide the foundations for synoptic comparisons between catchments and assessing the expectation of channel adjustment (forecasting), which should be included in process-based river management practice.
... Relevant to understanding floodplain evolution and channel behavior is assessing how channel avulsion at site 94 might be linked to changes in LSR flood regime, sediment load, and climate. Channel avulsion is associated with aggrading floodplains and tends to occur when channelized flow becomes super-elevated and unstable as a result of sedimentation (Bristow and Best, 1993; Bryant et al., 1995) or when channels become plugged and flow is diverted (Knighton, 1984:143)—processes that are not mutually exclusive (Phillips, 2011 ). Flooding facilitates channel avulsion either by removing barriers to lateral movement (e.g., penetration or erosion of vegetated levees through bank undercutting) or by rapid flood sedimentation that causes channelized flow to exceed a critical height and become diverted toward lower positions within the floodplain. ...
... The alluvial chronology at site 94 may be unique for the LSR but suggests that high energy, braided floodplains prone to channel avulsion cannot be ignored for potentially containing early human evidence in the Southwest. Although probabilistic models of channel avulsion have been developed (Phillips, 2011), deterministic prediction of the location and direction of braided channel shifts is not possible (Graf, 1988:212). This makes it difficult to reconstruct where such channel shifts occurred in the past and to forecast where buried early landscapes are likely to occur within the LSR floodplain. ...
... Several workers have pointed out that increase in S cv /S dv ratio is a necessary but not a sufficient condition for avulsion to occur (Slingerland and Smith, 2004;Aslan et al., 2005). As mentioned earlier, the Kosi River could have been close to the avulsion threshold since 2000 itself owing to 'universal' causes (Phillips, 2011) of in-channel sedimentation, but the continued planform changes, shifting of channel thalweg, development of seepage channel, and localised weakening of the eastern embankment provided the 'local' controls (Phillips, 2011). Some of these local controls are natural, but they were certainly accentuated by the human factors such as the construction of embankments and their poor maintenance. ...
... Several workers have pointed out that increase in S cv /S dv ratio is a necessary but not a sufficient condition for avulsion to occur (Slingerland and Smith, 2004;Aslan et al., 2005). As mentioned earlier, the Kosi River could have been close to the avulsion threshold since 2000 itself owing to 'universal' causes (Phillips, 2011) of in-channel sedimentation, but the continued planform changes, shifting of channel thalweg, development of seepage channel, and localised weakening of the eastern embankment provided the 'local' controls (Phillips, 2011). Some of these local controls are natural, but they were certainly accentuated by the human factors such as the construction of embankments and their poor maintenance. ...
... The dominant processes concept argues for adapting models to local conditions and needs, rather than attempts at "one size fits all" models (Grayson and Bloschl, 2000;Sivakumar, 2004Sivakumar, , 2008. Phillips (2011) generalized the dominant processes concept to a dominant controls concept in Earth and environmental sciences more broadly. The dominant controls concept holds that while many factors and processes can influence a given phenomenon (in this case landscape responses), in any given environmental system some will be irrelevant and others of comparatively negligible influence, leaving a few dominant controls to deal with. ...
Determining effects of climate change on landscapes involves numerous uncertainties. This paper presents and illustrates a protocol for climate attribution of landscape responses. The major steps are ascertaining potential climate-related responses, establishing plausibility for a climatic influence, identifying alternative or additional causes, testing possible climate and non-climate causes, and interpreting the role of climate and climate change in the landscape response. The protocol is based on existing practice in the historical and interpretive branches of Earth and ecological sciences, and explicitly considers negative (non-confirmatory) results for climate and other factors. The protocol is applied to the conversion of brackish marsh to open water in the upper Neuse River estuary, North Carolina. Conversion since at least the mid twentieth century can be attributed to relative sea-level rise, driven primarily by general climate warming, with no supporting evidence for any additional or alternative drivers. The only other factor with supporting evidence is human modification in the form of ditches, around which conversion was concentrated, though marsh loss also occurred in unditched portions. Rapid recent marsh loss is attributable to Hurricane Florence (2018), particularly the storm surge. Weak positive inferential support exists for a role of climate change in the storm, but aspects of the storm’s impact not linked to climate are more important for the marsh conversions. Overall, the landscape response can be linked to climate, exacerbated by direct human impacts of marsh ditching, and strongly influenced by local place factors and the specific storm track. Recent and ongoing climate change is a significant factor, but not paramount, in determining the landscape response. The Neuse River case study is not unusual—and is probably typical—in identifying a combination of climate and other factors strongly influencing landscape response.
... The time interval needed to form a deposit of unit thickness, i.e., equal to channel depth, may thus be regarded as a fundamental control on the time evolution of fluvial architecture (Bryant et al. 1995, Heller andPaola 1996;Jerolmack and Mohrig, 2007). Phillips (2011), Morόn et al. (2017b), and Nicholas et al. (2018) suggested that the normalized super-elevation at which avulsion is likely to occur decreases in the presence of episodic peak discharges as occurring in dryland fluvial systems. ...
An advection-diffusion model of fluvial processes was used to analyze the stratigraphic expression of avulsions in terminal river systems and understand their control on basin-fill architecture. The initial and boundary conditions of the model runs (i.e., catchment area, smoothed initial topographic surface, grain-size distribution and sediment supply rates) were extracted from the modern Rio Colorado dryland terminal river system in the Altiplano Basin (Bolivia). Water-discharge and sediment-load values were derived from global regression curves and the BQART equation, respectively. To evaluate the robustness of the simulations, the model was tested under increasing sediment-load scenarios ranging from 0.003 m³/sec to 0.095 m³/sec. Data-model comparison provided insights into the role of avulsions in the geomorphological evolution of terminal river systems. The observed stacking of sediments, as captured by geospatial and geochronological data from the Rio Colorado, is consistent with the high sediment-load scenarios, which start with a single-thread fluvial channel that in time radially expands over the floodplain by successive river avulsions on account of alluvial-ridge aggradation and channel-floor elevation above the surrounding floodplain. The model output shows a laterally extensive, convex-upwards lobate topography which is in agreement with the lateral and longitudinal geomorphology in the upper and lower coastal plain of the Rio Colorado. The simulated inter-avulsion period, which is the time period between two successive full (or stabilized) avulsions in the model, varies from 0.18 to 1.2 kyr and is consistent with the OSL-age determination in the Rio Colorado with inter-avulsion periods up to 1.28 ± 0.34 kyr.
... Studies with a variety of parameters have enabled the proposition of indexes (Sinha et al., 2014) and a hierarchy of the main conditions (Phillips, 2011) for the avulsion process. Although, with the breadth of factors involved, most researches have focused on limited parameters to advance understanding of avulsion susceptibility. ...
Meandering rivers are prone to dramatic changes with avulsion processes, which include a series of geomor-phological adjustments in the fluvial landscape. However, research into how the floodplain characteristics influence this process, as well as what are the geomorphological adjustments in the parent channel is still scarce. The lower course of the Peixe River, located in the southeastern region of Brazil, has a multi-thread reach with a ~ 14 km channel long formed by an avulsion in the 1970 s, which provides an excellent opportunity to evaluate avulsion aspects. We use remote sensing to evaluate morphological changes in the channels involved in the avulsion, and floodplain features as setups. We also investigated, with fieldwork measurements, potential adjustments in the channel affected by avulsion. The floodplain width variation regulated the avulsion extent. Planform analyses of the channels involved in the avulsion indicated distinct differences in sinuosity, and a development to an approximated equivalence in the current width of the two channels. The bank elevation in the parent channel showed a decrease with a strong adjustment in the downstream direction, and the absence of significant variation of this parameter in the reach with flow bifurcation. Sedimentary analyses in channel banks showed two deposits with significantly different sedimentometric characteristics and concentration of organic matter, described as bottom and top layers, the top being coarser than the bottom layer. The top layer has a significant variation of muddy sediments and organic matter in the avulsion reach, however, in the bottom layer this pattern was not observed. Setups in the Peixe River avulsion are associated with intrinsic factors from meandering dynamics, and evidence emerges from this study to test anthropogenic influences. Sedimentological parameters investigated in the parent channel appear more adjusted to the bifurcation flow than morphological parameters.
... Holocene sedimentation, dominated by avulsion processes and by the deposition of poorly extended alluvial units (Figs. 6 and 7). This aspect is particularly evident in the Bologna area (Fig. 7): in core 16, for example, paleosol PH was already buried around 8.2 kyr B.P., when paleosol H1 was still developing (see also Fig. 5); in the nearby core 15, at a distance of just 224 m, the burial of paleosol PH occurred only around 7 kyr B.P. The transition to an avulsive sedimentation pattern at the onset of the Holocene, recorded in several worldwide fluvial systems (Phillips, 2011;Stouthamer et al., 2011), is an indirect consequence of the landward migration of the coastline, which dramatically reduced the area available for sediment storing and lowered rivers longitudinal gradients. Decreased stream power, and capability to erode banks and create space for sediment storage, led to river aggradation, crevassing and avulsions (Blum et al., 2013). ...
The relationships between pedogenetic processes and fluvial-coastal dynamics in the Po Plain have been reconstructed through sedimentological analysis and correlation of ca. 170 core data chronologically constrained by 376 radiocarbon dates. Vertically stacked, weakly developed paleosols within Upper Pleistocene and Holocene mud-prone strata testify to intermittent pedogenesis, periodically interrupted by overbank sedimentation. Individual paleosols are laterally traceable for tens of km and exhibit A-Bk-Bw, A-Bk or A-Bw profiles. Stratigraphically ordered ¹⁴C calibrated ages from A organo-mineral horizons testify to slow aggradation during 4–6 thousand years-long exposure periods. Burial ages, with an error of few centuries, are provided by plant debris at the top of A horizons.
Millennial-scale climate oscillations and glacio-eustasy are the main drivers of the pedo-sedimentary evolution of the area during the last 50 kyr. Upper Pleistocene paleosols (P1-P3) developed in well-drained floodplain environments, during relatively warm periods. Paleosol burial occurred during colder phases. High-sediment-supply during the Last Glacial Maximum hindered pedogenesis and led to the accumulation of 3–10 m-thick overbank strata. Widespread soil development (paleosol PH) occurred at the end of Last Glacial Maximum, following the retreat of Alpine glaciers and the afforestation of Apennine drainage basins. At distal locations, paleosol PH was progressively buried under estuarine sediments during the Holocene phases of post-glacial sea-level rise. Beyond the area of marine influence, burial ages of paleosol PH change from a place to another without specific spatial trends and reflect upstream fluvial sedimentation dominated by avulsions and deposition of spatially restricted alluvial units. Holocene (H1−H2) paleosols show a poor correlation potential and laterally variable degree of maturity that reflect avulsive sedimentation patterns and crevassing. This paper provides insights on the timing and mechanisms of formation and burial of weakly-developed paleosols. The outcomes of this research are applicable to similar Quaternary alluvial systems, and may help interpreting ancient paleosol-bearing successions.
... This argues for adapting models to local conditions and needs, rather than attempting to construct "one size fits all" models designed to handle any watershed, anytime, anywhere (Grayson and Bloschl, 2000;Sivakumar, 2004Sivakumar, , 2008. In a study of river avulsions (Phillips, 2011b), I argued that the dominant processes concept can be generalized to a dominant controls concept (DCC) in geomorphology, and Earth and environmental sciences more generally. The DCC implies that, while there may exist a very large number of factors and processes that can influence a given phenomenon (in that case, avulsions) in any given geomorphic system some will be irrelevant and others of comparatively negligible influence, leaving a few dominant controls to deal with. ...
Landscape evolution often occurs over long-time scales that do not allow for direct observation and measurement. This chapter reviews approaches for observing, inferring, and reconstructing evolutionary trajectories. These include direct observation and monitoring (e.g., observatories), simulation models, and historical reconstruction. The latter encompasses documentary evidence, dating techniques, paleoenvironmental indicators, and inferential methods such as space-for-time substitutions and contemporary inferential indicators.
... In rivers such as the Sabine, Neches, and Trinity in southeast Texas, avulsions often occur by reoccupation of former channels, and cross-valley slopes are strongly affected by the presence and location of large Pleistocene palaeomeander depressions reflecting greater discharges that in the Holocene (Aslan and Blum, 1999;Blum and Aslan, 2006;(Phillips, 2009(Phillips, , 2011d(Phillips, , 2014). Fig. 7.7 shows the interactions of the various factors involved, distinguishing those applicable to any alluvial river (universal factors) as opposed to those applicable to the study rivers but not all alluvial rivers. ...
Thresholds are ubiquitous in Earth surface systems and fundamental to landscape evolution. They occur at the level of process mechanics, and at the broader level of landscape system states, and may be fuzzy or crisp in their occurrence and/or the ability to measure or define them. Five main types of thresholds occur: force vs. resistance, storage capacities, relative rates of linked processes, saturation and depletion effects, and limiting factors. Tipping points and regime shifts are types of thresholds that occur at the landscape level (or broader) and are abrupt. As virtually all landscape systems are strongly influenced by thresholds, and many are threshold-dominated, landscapes and ESS are nonlinear, opening up possibilities for complex phenomena that do not occur in linear systems. One of these is dynamical instability, which can be both a consequence (via nonlinearity) and a cause of thresholds. Instability is a cause of thresholds in the case of system-level meta-thresholds. These are shifts in the positive or negative effects, or in the relative magnitudes, of interactions within the system. They can result in switches between dynamically stable and unstable modes, often manifested as convergent or divergent evolution.
... During an overbank flood, at least suspended sediment from a river can be transported to the floodplain through these pathways and become deposited or resuspended, resulting in floodplain erosion/deposition depending on the flow conditions and supplied sediment (Zwoliński, 1992). Overall, a combination of overbank transport processes driven by water or wind, including turbulent diffusion and advection, can transport sediment in suspension and as bedload, leading to erosion and deposition and the formation of complex floodplain deposits and associated topography (Allen, 1965;Nanson, 1986;Phillips, 2011;Pizzuto et al., 2008). Floodplain surfaces built by periodic erosion and deposition consist of a range of grain sizes and typically fine with increasing distance from the main channel (Marriott, 1992;Nanson & Croke, 1992;Nicholas & Walling, 1996), however, floodplain topography can complicate this general pattern (Moody et al., 1999;Pizzuto et al., 2008). ...
Meandering river floodplains often contain intermittently flooded complex channel networks. Many questions remain as to the pervasiveness, function, and evolution of these floodplain channels. In this present work, we analyzed size‐specific sediment transport potential and assessed whether the channelized floodplain of the meandering East Fork White River near Seymour, Indiana is on a net erosional or depositional trajectory. We applied a two‐dimensional hydrodynamic model and used simulated model results to estimate the largest sediment size that can be moved in suspension and as bedload at various flows for grain size classes between 4 µm and 64 mm. We developed a probabilistic method that integrates the largest sediment size that can be moved at various flows to compute an effective grain size, which we compared to measured field data. Results show that the river is capable of supplying sand to the floodplain and these floodplain channels can transport sand in suspension and gravel as bedload. This suggests that sediment supplied from the river could be transported as bedload in floodplain channels. These floodplain channels are supply limited under the current hydrologic regime and the grain size distribution of the bed surface is set by the flow conditions; thus, these floodplain channels are net erosional. Finally, our proposed method of probabilistically integrating the largest sediment size that can be moved at various flows can be used to predict the upper end of the grain size distribution in suspension and in bed material, which is applicable to floodplains as well as coastal areas.
... River and disaster management lessons from other avulsing systems will not be sufficient because local knowledge is crucial for understanding specific avulsing systems and their appropriate management (Phillips, 2011). To begin with, the current literature on palaeochannels/avulsions in the Chiang Mai Basin needs to be reviewed. ...
... The locations of ancient river branches are after Bietak (1975); Butzer (2002); the extent of maximum transgression is as given by Stanley and Warne (1993a). Selected archaeological sites are also shown. in turn would have led to widespread crevassing, frequent avulsion, and high rates of floodplain aggradation (Aslan et al., 2005;Jerolmack, 2009;Kraus, 1996;Kraus and Aslan, 1993;Phillips, 2011;Mohrig et al., 2000;Slingerland and Smith, 2004). Rapid floodplain aggradation then inhibited soil formation and resulted in the development of a wetland landscape (Smith and P erez-Arlucea, 1994;Smith et al., 1989;Willis and Behrensmeyer, 1994). ...
The evolution of the Nile Delta, the largest delta system in the Mediterranean Sea, has both high palaeoenvironmental and archaeological significance. A dynamic model of the landscape evolution of this delta system is presented for the period c.8000–4500 cal BP. Analysis of sedimentary data and chronostratigraphic information contained within 1640 borehole records has allowed for a redefinition of the internal stratigraphy of the Holocene delta, and the construction of a four-dimensional landscape model for the delta's evolution through time. The mid-Holocene environmental evolution is characterised by a transition from an earlier set of spatially varied landscapes dominated by swampy marshland, to better-drained, more uniform floodplain environments. Archaeologically important Pleistocene inliers in the form of sandy hills protruding above the delta plain surface (known as “turtlebacks”), also became smaller as the delta plain continued to aggrade, while the shoreline and coastal zone prograded north. These changes were forced by a decrease in the rate of relative sea-level rise under high rates of sediment-supply. This dynamic environmental evolution needs to be integrated within any discussion of the contemporary developments in the social sphere, which culminated in the emergence of the Ancient Egyptian State c.5050 cal BP.
... Avulsion is the shift of a river course to a new position on a floodplain or delta and is a key process by which many rivers form new channels and adjust laterally (Smith et al., 1989;Slingerland and Smith, 2004;Stouthamer and Berendsen, 2007;Phillips, 2011). Following avulsion, the original channel may be abandoned or it may continue to operate alongside the new channel as a distributary or anabranch. ...
... Flow escapes its channel and carves a new path (or reoccupies an abandoned path) on the adjacent floodplain, altering the sediment delivery to coastal regions and the local rates of sediment accumulation, and strongly influencing the floodplain topography and alluvial architecture (Slingerland & Smith, 2004;Aslan et al., 2005;Kleinhans et al., 2013). Previous studies have been devoted to documenting historical avulsions at deltas (Coleman, 1988;Jones & Harper, 1998;Donselaar et al., 2013), investigating the mechanism of and reasons for avulsions (Bryant et al., 1995;Aslan et al., 2005;Assine, 2005;Phillips, 2011;Chatanantavet et al., 2012;Hajek & Wolinsky, 2012;Gupta et al., 2014;Ollivier et al., 2015) and developing methods for modeling avulsions (Jerolmack, 2009;Chatanantavet et al., 2012;Hajek & Wolinsky, 2012;Gupta et al., 2014). ...
The evolution of the Yellow River delta is characterized by heavy sediment load, rapid seaward migration, frequent avulsions, and intense anthropogenic disturbances. Evolution of the delta channel following avulsions is very complex and has not yet been thoroughly understood. In the research presented by this paper, we conducted comprehensive analyses of the changes in the water stages, slopes, longitudinal profiles, and the erosion and deposition in the Yellow River delta channels during a time period of over five decades. Results showed that, following each avulsion, channels migrated seaward at decaying rates and the slopes at the downstream of the avulsion point decreased exponentially with time and completed its major adjustment within about four to five years. A generalized geometric model was proposed to describe the changes in the longitudinal profiles of the delta channels. A calculation method to determine the characteristic water stages at the delta was proposed based on the geometric model and the delayed response model for the morphological responses of fluvial rivers to perturbations. Water stages corresponding to a discharge of 3,000 m³/s at Lijin and Xihekou during 1954 through 2012 were calculated by using the proposed method. The proposed method may be used to predict the evolution of the delta channels in response to artificial avulsions at the Yellow River delta in the future.
... Essa variação da fase de abandono do meandro e, outros fatores como a altitude do lago na planície de inundação, por sua vez são importantes no controle da conectividade hidrológica na planície de inundação. Por outro lado a distância lateral destes lagos com o canal possui pouca infl uência (Phillips, 2011;Hudson et al. 2012). Tais indicações sobre o controle na conectividade destas unidades geomórfi cas possibilitam compreender a variação de energia do fl uxo durante as cheias, o que implica no tipo de depósitos de cada unidade geomórfi ca. ...
Este estudo teve como objetivo identificar e caracterizar as formas e os processos fluviais dominantes associados ao padrão de canal meandrante no vale aluvial do baixo curso do rio do Peixe, estado de São Paulo. As interpretações geomorfológicas foram baseadas em imagens orbitais, fotografias de sobrevoo, dados hidrológicos e levantamentos em campo. No vale aluvial do baixo curso do rio do Peixe foram caracterizadas unidades de terraços, planície de inundação e canais, ordenados como 1º táxon. Nessas unidades encontram-se sub-unidades denominadas de unidades geomórficas, como por exemplo, na planície de inundação ocorrem paleocanais, lagos em ferradura e bacias de inundação, estas últimas, portanto ordenadas como 2 º táxon. Também foram descritos como de 2 º táxon a faixa de meandros, leque de espraiamento e leque aluvial. Na unidade do canal destacam-se as barras fluviais, principalmente, as barras em pontal, características dos rios meandrantes, além de barras centrais e laterais. Essas informações integradas em uma perspectiva hierárquica permitiram associar algumas morfologias como indicadoras de estágios evolutivos desse sistema fluvial. A dinâmica fluvial do padrão meandrante do rio do Peixe construiu formas, resultando em um mosaico de áreas úmidas de distinta morfogênese. O estudo demonstra que tanto fatores naturais como antrópicos exercem influência nas morfologias do vale aluvial do baixo curso do rio do Peixe.
... Geomorphic and sedimentary responses of alluvial rivers in relation to tectonics have been studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . The avulsion properties of rivers have also been wellstudied [15][16][17][18] . Some attempts have been made to understand the fluvial process-response system of the Brahmaputra River and its tributaries 2,[19][20][21][22][23][24][25] . ...
The Lohit River is a south bank tributary of the Brahmaputra River. Till 1987, the Lohit River used to meet the Brahmaputra at a place near Bairagi Chapari (27.77°N, 95.44°E). By 1995, the confluence point had shifted about 20 km downstream. One small channel of the Lohit River captured the Dangori River during the 1988 flood. Gradually the Lohit River started flowing along the captured channel. By 1995, it became the trunk channel of the Lohit River and Dibru Saikhowa region became an island. Banklines of Brahmaputra and Lohit rivers have undergone significant changes near their confluence point within the last few decades. By 1987, the south bank of the Brahmaputra near Rohmoria (27.55°N, 95.15°E) shifted about 1.6 km southward from its position in 1973. Interestingly, within the period 1988-90 the south bank shifted about 4.1 km south. This major shifting was the result of capturing of the Dangori River by the Lohit River. However, migration of the rivers towards the south has stopped after 1995. Analysis of SRTM DEM reveals that topographic elevation has played a major role in changing the course of the Lohit River.
... Mapping abandoned river channels offers possible interpretations of previous fluvial behavior, such as the paleoflow direction and morphology transformations (Mantelli et al., 2009;Hayakawa et al., 2010;Morais et al., 2012;Zani et al., 2012). Additionally, sedimentological perspectives contribute to understanding fluvial deposits through analysis of age and material characteristics, which results in the determination of morphogenetic stages in floodplains (Stevaux and Souza, 2004;Rowland et al., 2005;Piégay et al., 2008;Rossetti and Góes, 2008;Assine and Silva, 2009;Phillips, 2011;Tonen et al., 2012;Salvador and Berger, 2014). ...
... Figure 8c portrays a similar type of bedrock control where the channel is entrenched and deeply incised into bedrock without abutting a cliff. One way this can occur is when the channel reoccupies preexisting valley lows or paleochannels (Brakenridge, 1984;Harden, 1990;Aslan & Blum, 1999;Mohrig et al., 2000;Phillips, 2011). ...
Although Paleoindian sites in Indiana, USA, are commonly located on late Wisconsin (Last Glacial Maximum) outwash terraces, drainage basin development since deglaciation often obscures the visibility of such sites on flood plains by either burying them under alluvium or destroying them through erosion. Significant clusters of Paleoindian and Early Archaic sites, however, have been identified proximal to the modern White River channel in central Indiana on what is mapped as "floodplain." These site cluster locations are patterned. They typically occur within bedrock-controlled river reaches but are rare along unconfined meandering reaches. Subsurface reconnaissance and chronology indicate that despite the fact that they often flood, portions of the so-called flood plains within bedrock-confined reaches are actually terraces constructed of late Wisconsin outwash with minimal overbank sedimentation. Terrace preservation in these settings is a result of bedrock structure that protects older sediments from lateral erosion and differentially preserves archaeological sites near the modern channel in bedrock-controlled reaches. Comparisons of archaeological sites within bedrock-controlled segments of the White River to those in unconfined meandering segments suggests that significant numbers of Paleoindian and Early Archaic sites may be missing from river settings across the midcontinent. These findings demonstrate that bedrock channel controls are important to recognize when assessing prehistoric settlement distributions.
... The river terminus in Salar de Uyuni experiences a high frequency of avulsions, which is similar to other aggrading river systems either in the semi-arid Okavango fan in Botswana (McCarthy et al., 1992), humid subtropical river systems of the southeast Texas coastal plain (Phillips, 2011) or an experimental model (Ashworth et al., 2007). However, avulsions are triggered by sporadic peak floods in the non-vegetated river terminal (Fig. 6), which is different from other river systems, where high vegetation cover and the floating plant debris are regarded as avulsion triggers (e.g. ...
In this remote sensing-based study, we present the analysis of the geomorphological development at the low-gradient terminus of the modern Río Colorado dryland river system in the endorheic Altiplano Basin (Bolivia). Changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays, formation of new crevasse splays and in river path avulsion. Episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from ASTER global DEM (GDEM) remote sensing data. A time series of Landsat imagery for the period 1975–2001 was then used to analyze the river morphology changes after major peak discharge events. Compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system.
... These include floodplain depressions and paleomeander scars, fault reactivation, and stream capture (Phillips, 2009). Features inherited from earlier Quaternary and Holocene fluvial changes have also been shown to exert important influences over other aspects of alluvial and deltaic landforms and process regimes in the three rivers (Rodriguez et al., 2005;Anderson and Rodriguez, 2008;Phillips, 2008Phillips, , 2010Phillips, , 2011Phillips and Slattery, 2008). This suggests the possibilities that sub-channels or anabranches both within and between the Neches and adjacent fluvial systems may have multiple causes, and may be strongly influenced by antecedent alluvial morphology. ...
Active and semi-active anastomosing Holocene channels upstream of the delta in the lower valley of the meandering Neches River in southeast Texas represent several morphologically distinct and hydrologically independent channel systems. These appear to have a common origin as multi-thread crevasse channels strongly influenced by antecedent morphology. Levee breaching leads to steeper cross-valley flows toward floodplain basins associated with Pleistocene meander scars, creating multi-thread channels that persist due to additional tributary contributions and ground water inputs. Results are consistent with the notion of plural systems where main channels, tributaries, and sub-channels may have different morphologies and hydrogeomorphic functions. The adjacent Trinity and Sabine Rivers have similar environmental controls, yet the Trinity lacks evidence of extensive anastomosing channels on its floodplain, and those of the Sabine appear to be of different origin. The paper highlights the effects of geographical and historical contingency and hydrological idiosyncrasy. This article is protected by copyright. All rights reserved.
... Avulsions are considered to be features of aggrading river systems [159], and avulsion frequency shows a positive correlation with aggradation rate [160][161][162][163]. However, superelevation of the channel is a necessary but not sufficient condition for avulsions to occur [164]. Factors favoring floodplain incision are also necessary [165]. ...
The majority of the world's floodplains are dammed. Although some implications of dams for riverine ecology and for river channel morphology are well understood, there is less research on the impacts of dams on floodplain geomorphology. We review studies from dammed and undammed rivers and include influences on vertical and lateral accretion, meander migration and cutoff formation, avulsion, and interactions with floodplain vegetation. The results are synthesized into a conceptual model of the effects of dams on the major geomorphic influences on floodplain development. This model is used to assess the likely consequences of eight dam and flow regulation scenarios for floodplain geomorphology. Sediment starvation downstream of dams has perhaps the greatest potential to impact on floodplain development. Such effects will persist further downstream where tributary sediment inputs are relatively low and there is minimal buffering by alluvial sediment stores. We can identify several ways in which floodplains might potentially be affected by dams, with varying degrees of confidence, including a distinction between passive impacts (floodplain disconnection) and active impacts (changes in geomorphological processes and functioning). These active processes are likely to have more serious implications for floodplain function and emphasize both the need for future research and the need for an "environmental sediment regime" to operate alongside environmental flows.
... Geomorphically, rapid channel aggradation and increasing slope advantage relative to the flood basin to the east were aggravated by unstable bank conditions caused by coarse-textured substrates (Chien, 1961) that facilitate channel scouring, bank failure, and the development of crevasse splays (Aslan et al., 2005;Phillips, 2011). The pre-Han channel of the Yellow River had been relatively stable for millennia, and the Sanyangzhuang record indicates increasing flood frequency following the late Neolithic, suggesting the river had reached and begun to cross the geomorphic threshold that constrained it in this western channel alignment (Schumm, 2005). ...
The Sanyangzhuang site, Henan Province, China, has a 12-m-deep stratigraphic sequence with remains from the Tang (A.D. 618–907), late Western Han (ca. 140 B.C.–A.D. 23), Warring States (475–221 B.C.), Late Neolithic or Early Bronze Age (ca. 5000–1500 B.C.), Middle Holocene, and Early Holocene times. All of the paleosols are deeply buried. We investigate four issues relevant to the archaeology of the lower Yellow River Valley. First, we confirm that the Yellow River flowed north toward Bohai Bay throughout most of the Holocene. Second, we expand understanding of Holocene paleoenvironments. Long episodes of landscape stability punctuated by brief periods of Yellow River flooding represent the dominant environmental pattern. Third, we investigate how the complex relationships between climate, culture, and the environment affect Yellow River flooding, which in turn shapes Chinese civilization and history. Flooding in late Western Han times affected a vast area of north-central China; this catastrophe contributed to the downfall of the late Western Han Dynasty. Finally, this research sheds light on the role of Yellow River alluviation in site burial and preservation. Rapid alluviation in the region has buried many archaeological sites. Settlement pattern research needs to take seriously the limitations placed on site visibility in quickly aggrading floodplains. However, gentle alluviation has also preserved settlements and entire landscapes providing unparalleled opportunities to explore the archaeological and historical record of the lower Yellow River Valley.
... The signicance of this phenomenon for coastal evolution, sequence stratigraphy, and paleoenvironmental reconstructions is discussed by Blum Ongoing and accelerating Holocene sea level rise, coupled with subsidence in some coastal areas, would be expected to increase fragmentation due to drowning of conuences. Sea level rise also typically enhances set-up conditions for avulsions by promoting aggradation (Aslan & Blum, 1999; Phillips, 2009 Phillips, , 2011a). In the lower Sabine River, for instance, two tributaries enter the river 2.5 and 6 km upstream of Sabine Lake, with their thalwegs below sea level and the marsh areas surrounding the conuences at approximately sea level. ...
This study explores the origin of 15 small coastal watersheds (SCWs) conned entirely to the lower Coastal Plain, which lie between the watersheds of the major rivers owing across the Texas Coastal Plain. The relationship between SCWs and larger rivers was examined to determine whether the SCWs developed indepen-dently of the larger rivers, or became separated from them due to drowning of conuences by sea level rise or watershed fragmentation avulsions. None of the SCWs show evidence of developing independently of larger drainages. In 11 cases, the SCW main streams were connected to each other and/or the larger rivers at lower sea level stands. Ten of these former conuences occur under current estua-rine systems, indicating that these rivers were connected within the Holocene. In one case, the former junction is on the current continental shelf, where mapping of the shelf shows that at sea level lowstands associated with the last glacial maximum, the current eight large river basins and 15 SCWs were aggregated into four large watersheds. Watershed fragmentation avulsions in the lower Rio Grande, Colorado, and Brazos (2) Rivers are responsible for the other four SCWs. Avulsions are generally common in the study area, but for a channel shift to result in watershed fragmentation three conditions must occur. First, a successful avulsion (i.e., the new channel becomes dominant) must occur, involving the entire reach from the avulsion site to the river mouth. Second, hydraulic connection between the newer and older channels must be lost. Third, the abandoned channel must receive sufcient tributary input or upland run-off to maintain its path to the coast and avoid inlling.
... Other workers have ascribed avulsiongenerated alluvial stratigraphies to a mix of autogenic and allogenic factors depending on the particular situation (e.g. Stouthamer & Berendsen, 2007; Phillips, 2011 ). Channel avulsion is principally related to super-elevation of the channel above its floodplain because of differential deposition (e.g. ...
Little is known about controls on river avulsion at geological time scales longer than 10^4 years, primarily because it is difficult to link observed changes in alluvial architecture to well-defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milanko-vitch-sale orbital forcing in alluvial stratigraphy would provide unprece-dented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithologi-cal cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super-elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7Á1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21Á6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large-scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alterna-tions were probably paced by allogenic, astronomically forced climate change.
... Relevant to understanding floodplain evolution and channel behavior is assessing how channel avulsion at site 94 might be linked to changes in LSR flood regime, sediment load, and climate. Channel avulsion is associated with aggrading floodplains and tends to occur when channelized flow becomes super-elevated and unstable as a result of sedimentation (Bristow and Best, 1993; Bryant et al., 1995) or when channels become plugged and flow is diverted (Knighton, 1984:143)—processes that are not mutually exclusive (Phillips, 2011 ). Flooding facilitates channel avulsion either by removing barriers to lateral movement (e.g., penetration or erosion of vegetated levees through bank undercutting) or by rapid flood sedimentation that causes channelized flow to exceed a critical height and become diverted toward lower positions within the floodplain. ...
Recent archaeological excavations along the lower Salt River, Arizona resulted in the unexpected discovery of buried late Pleistocene soils and cultural features dating 5800–7100 cal YBP (Early Archaic), the latter representing the earliest evidence of human activity in the lower Salt River floodplain thus far identified. Because the lower Salt River floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures, our understanding of the river's geological history is limited. Here we present a late Quaternary alluvial chronology for a segment of the lower Salt River based on 19 accelerator mass spectrometry 14C and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations. Deposits are organized into allostratigraphic units and reveal a buried late Pleistocene terrace inset into middle-to-late Pleistocene terrace deposits. Holocene terrace fill deposits unconformably cap the late Pleistocene terrace tread in the site area, and the lower portion of this fill contains the Early Archaic archaeological features. Channel entrenchment and widening ~ 900 cal YBP eroded much of the older terrace deposits, leaving only a remnant of fill containing the buried latest Pleistocene and middle-to-late Holocene deposits preserved in the site area. Subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments. Our study confirms that the lower Salt River is a complex mosaic of late Quaternary alluvium formed through vertical and lateral accretion, with isolated patches of buried soils preserved through channel avulsion. Although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages, intrinsic geomorphic and local base level controls limit direct correlations of lower Salt River stratigraphy to other large rivers in the North American Southwest.
... For fluvial channels, avulsion is defined as a main flow path diverted from one channel to another, leading to the abandonment of a previous flow path (Field, 2001;Taha and Anderson, 2008;Phillips, 2011). The submarine channels/canyons are often compared to fluvial channels for their similar planform shape. ...
This paper discusses submarine morphology as a means of understanding the avulsion of the Fangliao canyon. Multichannel seismic profiles and bathymetric data were used to perform a morphological analysis. In response to tectonic uplifting, the Fangliao canyon was divided into two separate segments and two distinct trends. Initially, in the shelf edge–upper slope region the canyon extends downslope in a N–S direction. Then the canyon course bends to the SW at a depth of about 630 m because of a mud diapir uplift and then returns to and maintains a N–S course with the canyon mouth at a depth of 900 m, which is its present-day course. The Fangliao ridge, a mud diapiric ridge, lies east of the lower Fangliao canyon and extends upslope into the upper Fangliao canyon with a generally straight course and a nearly N–S trend. Because of the uplift of the Fangliao ridge a major step of the convex-upward profile, ~ 150 m above the canyon floor, resulted in the east Fangliao ridge canyon. The gravity flows are unable to rise up against the high gradient and, thus, have had to abandon the segment of paleocanyon once flowing along the trend of the Fangliao ridge. The internal seismic characteristics of the terraces located along the eastern wall of the Fangliao canyon suggest deposition of terrace mainly occurs in the inner bend of the Fangliao canyon. The sedimentary sequences of the terrace are divided into two units: (1) the lower sedimentary unit is characterized by oblique, high-amplitude reflections and is considered as coarse-grained canyon fills resulting from aggradation related to the canyon avulsion events; and (2) the upper sedimentary unit consists mainly of low-amplitude reflections that are interpreted as inner levee resulting from spillover deposition. Numerical simulations were performed to discuss avulsion of the Fangliao canyon. Initially, the course of the Fangliao canyon had a N–S trend and followed the regional slope (i.e., the direction gravity currents would flow). Subsequently, the mud diapir uplifted the segment of the paleocanyon of the Fangliao canyon once flowing along the present-day Fangliao ridge and diverted its course to the present-day Fangliao canyon. The model suggests that the uplift rate exceeds the erosion rate in the east Fangliao ridge canyon, and hence the sediment flows in the canyon may be unable to pass through the 150-m major step. This new model suggests that the avulsion of the Fangliao canyon is marked by a significant bend at the boundary between the upper and lower canyons.
... Other workers have ascribed avulsiongenerated alluvial stratigraphies to a mix of autogenic and allogenic factors depending on the particular situation (e.g. Stouthamer & Berendsen, 2007; Phillips, 2011 ). Channel avulsion is principally related to super-elevation of the channel above its floodplain because of differential deposition (e.g. ...
The Willwood Formation of the Bighorn Basin (Wyoming, USA) is a thick succession of upper Paleocene and lower Eocene fluvial-floodplain sandstones and mudstones. Reddish paleosols, formed on the floodplain mudstones, alternate rhythmically on various scales with heterolithic intervals of small-channel sandstones and mudstones showing weak pedogenesis. Spectral analysis of redness in the Willwood successions at Polecat Bench and Red Butte reveals significant spectral peaks corresponding to cycle thicknesses of similar to 8 and similar to 3 m. The similar to 8 m cycle reflects distinct clusters of 3-5 paleosols. Age constraints show that the period of this cycle closely matches the similar to 21 k.y. climatic precession cycle. The similar to 3 m cycle corresponds to individual paleosols, with a period of 7-8 k.y. This period is similar to millennial-scale sub-Milankovitch cycles found in marine and lacustrine successions of Pliocene-Pleistocene age. Precession and millennial-scale climate variations probably affected paleosol development through cyclic changes from predominantly overbank to predominantly channel-avulsion deposition, with the latter periodically halting soil formation because of high sediment accumulation. A new age model was developed for the Paleocene-Eocene carbon isotope excursion (CIE) at Polecat Bench, based on the precessional origin of paleosol clusters. The main body of the CIE spans similar to 5.5 precession cycles, or similar to 115 k.y., and the recovery tail of the CIE spans 2 precession cycles, or similar to 42 k.y. This outcome is consistent with, and independently confirms, recent estimates of CIE duration based on deep-sea cores.
Channel avulsion is a natural phenomenon that occurs abruptly on alluvial river deltas, which can affect the channel stability. The causes for avulsion could be generally categorized as topography- and flood-driven factors. However, previous studies on avulsion thresholds usually focused on topography-driven factors due to the centurial or millennial avulsion timescales of the world’s most deltas, but neglected the impacts of flood-driven factors. In the current study, a novel demarcation equation including the two driven factors was proposed, with the decadal timescale of avulsion being considered in the Yellow River Estuary (YRE). In order to quantify the contributions of different factors in each category, an entropy-based methodology was used to calculate the contributing weights of these factors. The factor with the highest weight in each category was then used to construct the demarcation equation, based on avulsion datasets associated with the YRE. An avulsion threshold was deduced according to the demarcation equation. This avulsion threshold was then applied to conduct the risk assessment of avulsion in the YRE. The results show that: two dominant factors cover respectively geomorphic coefficient representing the topography-driven factor and fluvial erosion intensity representing the flood-driven factor, which were thus employed to define a two dimensional mathematical space in which the demarcation equation can be obtained; the avulsion threshold derived from the equation was also applied in the risk assessment of avulsion; and the avulsion threshold proposed in this study is more accurate, as compared with the existing thresholds.
Megafans are partial cones of river sediment that reach unexpectedly large dimensions, with the largest on Earth being 700 km long. Due to recent developments in space-based observations, global mapping efforts have shown that modern megafan features cover vast landscapes on most continents. This book provides a new inventory of nearly 300 megafans across five continents. Chapters focus on regional studies of megafans from all continents barring North America and Antarctica. The major morphological attributes of megafans and multi-megafan landscapes are discussed, and the principal controls on megafan development are examined. The book also compares megafans with alluvial fans, deltas, floodplains and the recently recognised 'major avulsive fluvial system' (MAFS). The final part of the book discusses the application of megafan research to economic geology, aquifers and planetary geology including layered deposits on Mars. This is an invaluable reference for researchers in geomorphology, sedimentology and physical geography.
Pressure on large fluvial lowlands has increased tremendously during the past twenty years because of flood control, urbanization, and increased dependence upon floodplains and deltas for food production. This book examines human impacts on lowland rivers, and discusses how these changes affect different types of riverine environments and flood processes. Surveying a global range of large rivers, it provides a primary focus on the lower Rhine River in the Netherlands and the Lower Mississippi River in Louisiana. A particular focus of the book is on geo-engineering, which is described in a straight-forward writing style that is accessible to a broad audience of advanced students, researchers, and practitioners in global environmental change, fluvial geomorphology and sedimentology, and flood and water management.
Tributary rivers can contribute significantly to alluvial‐plain construction by supplying large volumes of clastic material. Their relation to the main axial river strongly influences sediment deposition and preservation. The Po Plain is fed by the Po River and a dense network of transverse tributaries draining the nearby Alpine and Apennine chains. Stratigraphic, sedimentological, petrographic and geochemical analyses on 38 cores permitted detailed differentiation of Po and Apennine sedimentary units. Po River deposits are vertically stacked channel‐belt sand bodies with high contents of quartz–feldspar and metamorphic rock fragments, combined with high chromium levels. These sand bodies, 20 to 30 km wide, are replaced southward by finer‐grained deposits that represent the distal Apennine tributary‐rivers system. Apennine sands, confined in narrow ribbons, show lower quartz‐feldspar contents, abundant sedimentary lithics and lower chromium levels. In the last 870 kyr, the boundary between the Po and the Apennine sediment delivery systems shifted along a north–south axis in response to distinct controlling factors. A 20 km northward shift of the Po channel belt, possibly related to a tectonic event, is recorded across a regional unconformity dating to the Marine Isotope Stage 12/11 transition. High sediment supply rates during glacial‐lowstand periods widened the Po channel belt southward towards the Apennine domain for a few kilometres. The Last Glacial Maximum channel‐belt sand body, 30 km wide and 40 m thick, progressively narrowed northward after the glacial culmination. During the Holocene, channel patterns became avulsive and distributive. Narrow channel belts (<3 km) formed along the Po River branches and abundant swamp and poorly drained‐floodplain muds were preserved in interfluvial areas. Activation and deactivation of the Po branches resulted in sharp narrowing and widening of the area available for Apennine‐rivers sedimentation. This work provides insights into tributary‐trunk river relations which control grain‐size distribution and compositional characters of subsurface deposits.
A declividade de canais possui um importante papel na morfologia, estrutura, dinâmica e evolução da paisagem fluvial, sendo uma variável chave para a interpretação de processos e formas de um sistema fluvial. A declividade pode ser obtida a partir de uma solução linear aplicada aos dados de cotas altimétricas, o que permite a obtenção de valores que correspondem aos trechos compreendidos entre as estações. Porém, frequentemente nota-se uma escassez de dados altimétricos ao longo do perfil longitudinal de um rio, o que limita as análises geomorfológicas detalhadas. Com base nesse panorama, o objetivo deste artigo foi discutir o uso de conceitos de cálculo diferencial para análise da declividade em perfis longitudinais. São apresentados os principais tipos de funções elementares, bem como são apresentadas soluções para obtenção da declividade utilizando-se algumas dessas funções. Observou-se que as funções polinomial e exponencial são úteis para representação de perfis longitudinais, assim como para obtenção da declividade a partir da derivada de primeira ordem. Adicionalmente, demonstrou-se que as derivadas podem ser calculadas para pontos específicos de interesse ou para um grande conjunto de pontos ao longo do perfil longitudinal. No segundo caso, tem-se como resultado que a declividade se torna uma variável contínua, podendo ser avaliada quantitativamente ao longo do perfil longitudinal. Esta pesquisa permite as seguintes conclusões: i) a partir de dados de altitude e distância horizontal de um perfil, é possível estabelecer funções que representam a forma de um perfil longitudinal; ii) a primeira derivada sobre essas funções equivale à declividade de canais fluviais; iii) a segunda derivada pode ser utilizada para obter informação sobre a concavidade do perfil e para encontrar seus pontos de inflexão; iv) finalmente, conclui-se que o Cálculo pode ser aplicado para o tratamento da variável declividade, tornando-o uma ferramenta a ser explorada em geomorfologia fluvial.
Delta channels are important landforms at the interface of sediment transfer from terrestrial to oceanic realms and affect large, and often vulnerable, human populations. Understanding these dynamics is pressing because delta processes are sensitive to climate change and human activity via adjustments in, for example, mean sea level, and water and sediment regime. Data collected over a 40‐year period along a 110km distributary channel of the Yellow River Delta offers an ideal opportunity to investigate morphological responses to changing water and sediment regimes and intensive human activity. Complementary data from the delta front provide an opportunity to explore the interaction between delta channel geomorphology and delta‐front erosion‐accretion patterns. Cross‐section dimensions and shape, longitudinal gradation and a sediment budget are used to quantify spatial and temporal morphological change along the Qingshuigou channel. Distinctive periods of channel change are identified, and analysis provides a detailed understanding of the temporal and spatial adjustments of the channel to specific human interventions, including two artificial channel diversions and changes in water and sediment supply driven by river management, and downstream delta‐front development. Adjustments to the diversions included a short‐lived period of erosion upstream and significant erosion in the newly activated channel, which progressed downstream. Channel geomorphology widened and deepened during periods when management increased water yield and decreased sediment supply, and narrowed and shallowed during periods when management reduced water yield and the sediment load. Changes along the channel are driven by both upstream and downstream forcing. Finally, there is some evidence that changing delta‐front erosion‐accretion patterns played an important role to the geomorphic evolution of the deltaic channel; an area that requires further investigation.
Examples of partial avulsion and river bifurcation around large stable islands make up a small but locally important portion of the total length of rivers in the Piedmont physiographic province of North Carolina (U.S.). Despite remaining ecologically significant, the partially abandoned (or “abandoning”) branches along such reaches have received less attention than the often dominant newly created branches. The existence of perennial flow through abandoning channels may be contingent upon their ability to rapidly adjust their morphologies to declining flows. In this paper we study the response of a partially abandoned channel of the upper Catawba River in North Carolina to major loss of flow following flood-induced partial avulsion in 1977. Human activities have strongly modified the floodplain in this area, assisting the avulsion and possibly influencing the natural erosion and sedimentation processes responsible for subsequent adjustments of the abandoning channel. Using a combination of downstream gaging data, flow estimation functions, channel mapping, and hydraulic geometry analyses, we assess the degree to which channel capacity reduction matches the amount of post-avulsion diminished discharge. Construction of fluvial sediment benches inset within the old single-thread channel has occurred along 83% of the channel's length, and most of these have crest elevations in the 1–2 yr flood stage range. Hydraulic geometry reconstructions indicate that length-weighted mean channel width, depth, and capacity deviate from predicted values by 36%, 18%, and 61%. Thus, channel cross-section dimensions for the abandoning channel appear substantially larger than predicted, a condition that is potentially discordant with a hypothetical necessity of rapid width adjustment for maintaining an open channel, depending on what constitutes a rapid rate of adjustment. Variables such as the morphology of the bifurcation node and load properties also influence the longevity of abandoning channels, and rates of adjustment or infilling can vary widely. Stratigraphic, pedogenic, and vegetative evidence together suggest that channel narrowing via inset bench formation has occurred gradually over at least two or more decades following initial flow loss. Comparisons with channel dimensions predicted from existing Piedmont regional curves show greater agreement, supporting the contention that Piedmont valley characteristics exercise at least as strong an influence on channel morphology as does the higher relief watershed topography and associated flow regime. The presence of perennial flow and active geomorphological processes in the abandoning channel indicates that it remains part of a dynamic and ecologically significant river reach, despite being currently designated as land in county land parcel records.
The science of geomorphology is increasingly used to inform river management efforts; however, the complexity of fluvial systems makes predictions of future channel adjustment difficult at best. The geomorphic concepts of landform sensitivity and sediment connectivity are well suited to aid river managers in assessing the probability and variability of river channel responses. This can be especially helpful in planning for impacts of future climate change or changes in management activity. River sensitivity and sediment connectivity datasets provide a necessary reach-in-catchment perspective to inform geomorphic interpretations of river behavior. Such interpretations constitute a first step towards providing the contextual geomorphic understanding necessary to establish expectations of future channel behavior. In this paper, we use geomorphic interpretations based on river sensitivity and sediment connectivity datasets to describe the historical trajectory and future adjustment possibilities for four channel reaches in the Lockyer Valley, southeast Queensland (SEQ). We apply our interpretations to three different scenarios of future climate and river management conditions to constrain what forms of geomorphic adjustment are possible for resilient-disconnected, resilient-connected, sensitive-connected, and sensitive-disconnected channels. Using scenario-building exercises to forecast possibilities of river adjustment can aid river managers by establishing expectations of future channel behavior. This information can then be fed into the decision-making process regarding where to prioritize management actions as part of catchment action planning that works with, not against, the natural behavior of riverine environments.
During the mid-Holocene, some of the world's first large-scale complex societies came into being within the lower and middle reaches of a number of large river systems. Around this time, as global sea-level stabilised, the hosting fluvial environments of Lower Mesopotamia, the Nile Delta and the North China Plain were evolving from spatially varied landscapes dominated by swampy marshland, to better-drained, more uniform floodplain environments. It is necessary to consider whether such environmental changes could have guided aspects of sociocultural evolution in these settings.
In the Nile Delta, the setting for which most data are available, these palaeolandscape changes are comprehensively mapped through the construction of a four-dimensional aggradation model of the Holocene alluvial plain. Development of this model takes place within the context of a full reinterpretation of the Upper Quaternary stratigraphy of the Nile Delta, which is itself further informed by substantial programmes of fieldwork in the western delta. The environmental changes were forced by a decrease in the rate of relative sea-level rise within the context of decreased discharge and sediment-supply due to regional climate change.
A geoarchaeological model links these changes in the landscape to sociocultural developments taking place in Egypt between 5500 and 2500 BC. Increased adoption of agricultural practices in the delta was stimulated by a decrease in the primary productivity of the landscape, which then led to population growth and shifts in settlement styles. The emergence of the first Egyptian capital of Memphis at the delta apex can also be seen as having been facilitated by changes in the palaeogeography of the fluvio-deltaic environment. Such linkages between the changing deltaic landscapes and social change are crucial in understanding the formation of the Ancient Egyptian State (c. 3100 BC), which involved increased involvement of regional elites using the delta as both an agricultural resource and trade route.
While geoarchaeology as a practice within archaeology grew out of many historical roots, a major role has been the explication of site formation processes and site-level contextual analysis. In recent years, geoarchaeological research has branched out to encompass larger geographic scales, and to play a greater role in environmental archaeological investigations. This paper argues that geoarchaeology has a great deal to contribute to the understanding of human history and to archaeological theory through the application of multiscalar approaches that place human behavior in a physical, environmental and ecological context and by creating linkages between physical processes and human responses. We use geoarchaeological data from the Yellow River valley to show that drainage/irrigation canal and bank/levee building had commenced in the lower reaches by ca. 2900–2700 cal B.P. The emphasis on flood plain flood control infrastructure was a result of long-term increases in sedimentation caused by large populations farming with increasingly efficient technologies in the fragile environments of the Loess Plateau. Ever increasing sedimentation set in motion a cycle of further investment in flood control works eventually leading to a massive flood catastrophe in the first 20 years of the first millennium A.D. as the Yellow River exceeded natural and human geomorphic thresholds that constrained it in its previous course. These floods arguably triggered the social and political events that brought down the Western Han Dynasty but the root causes are clearly more complex. Geoarchaeology thus contributes to an understanding of the multiple causes and consequences of large-scale social and political collapse.
The state of an Earth surface system (ESS) is determined by three sets of factors: Laws, place, and history. Laws (L = L1, L2, . . . , Ln) are the n general principles applicable to any such system at any time. Place factors (P = P1, P2, . . . , Pm) are the m relevant characteristics of the local or regional environment. History factors (H = H1 , H2, . . . , Hq) include the previous evolutionary pathway of the ESS, its stage of development, past disturbance, and initial conditions. Geoscience investigation may focus on laws, place, or history, but ultimately all three are necessary to understand and explain ESS. The LPH triad is useful as a pedagogical device, illustrated here via application to explaining the world's longest cave (Mammoth Cave, KY). Beyond providing a useful checklist, the LPH framework provides analytical traction to some difficult research problems. For example, studies of the avulsions of three southeast Texas rivers showed substantial differences in avulsion regimes and resulting alluvial morphology, despite the proximity and superficial similarity of the systems. Avulsions are governed by the same laws in all cases [L(A) = L(B) = L
Rivers have different patterns of plan forms. Straight and meandering rivers have single channel but braided, anabranching and anastomosing rivers have multi channels. Gamasiab River in Kermanshah province has different plan forms of channels. In this study, historical method is used and is investigated different plan form and river's changes with using of aerial photography of 1955, 1969, 2003 and IRS satellite imagery 2010. Aerial photographs are ortho photo in Arc Map software based on 1:25000 topographic maps. Channels and bars were digitized and river is divided to 12 reaches. Braided index and sinuosity for different reaches were calculated. Results show that river channels have remarkable changes during the past 56 years. Evolution occurrence on the upstream section has changed from meandering to anabranching. Channel avulsion is defined as the rapid lateral relocation of a river course across parts of its floodplain due to formation of new channel and makes a condition for creating an branching plan form. In the intermediate reach of stream section changes from meandering to braided river. But in the downstream reaches section have a meandering plan form and these reaches do not have changes in the plan form during in the past 56 years.
DefinitionAvulsion is the abandonment of a part or the whole of a channel belt in favor of a new course (Allen 1965). An avulsion channel is the new channel that results from avulsion.SynonymsBifurcation channelDescriptionAvulsion is a key process in the evolution of subaerial fans, river floodplains, and deltas. Avulsion occurs if sediment load and sediment-carrying capacity of two bifurcated channels are not in proportion. Erosion or deposition will occur in one or both channels thereby changing their discharges, slopes, and/or cross sections and consequently changing their capacities. This will continue until either sediment-carrying capacities of both channels equal their loads or until one channel is abandoned. If avulsion takes place, it depends on the nature of sediment partitioning at the bifurcation of both channels and the ability of both channels to change their sediment-carrying capacity (Slingerland and Smith 2004; Kleinhans et al. 2008).For a review on the avulsion proces ...
Although archaeological analysis emphasizes the importance of climatic events as a driver of historical processes, we use a variety of environmental and archaeological data to show that human modification of the environment was a significant factor in shaping the early history of the Yellow River region of North China. Humans began to modify site-specific and local-level environments in the Early Holocene (~11,500–7000 BP). By the Mid-Holocene (~7000–5000 BP), the effects of humans on the environment become much larger and are witnessed at regional and tributary river basin scales. Land clearance and agriculture, as well as related land use, are dominant determinants of these changes. By the Late Neolithic to Early Bronze Age (~5000–3500 BP), population growth and intensification of agricultural production expanded the human footprint across the Yellow River region. By the Mid to Late Bronze Age (~3600–2200 BP), larger populations armed with better technology and propelled by more centralized governments were altering lands throughout the Yellow River region, gradually bringing the environment under human control. By the Early Dynastic period (221 bc–ad 220), the Yellow River region was an increasingly anthropogenic environment wherein human land management practices were, in many instances, as consequential as natural forces. Throughout the Holocene history of the Middle and Lower Yellow River, anthropogenic, climatic, and natural environmental processes were acting to shape human history and behavior, making it difficult, if not impossible, to say whether human or climate processes were more consequential. There is a complex relationship in China’s early history between natural and human forcing much like there is today. The Early Anthropocene concept is useful here because it recognizes that when natural and cultural forces become so intertwined, it no longer makes sense to separate the two.
Geomorphologists have long studied the impacts of extreme floods, yet the association between the magnitude of flow parameters (discharge, velocity, shear stress, or stream power) and resulting geomorphic effectiveness remains vague and non-deterministic. Attempts have been made to include flow duration and total expenditure of stream power, in combination with peak unit stream power, as important variables, but there has been minimal exploration of this hydraulic combination. Taking advantage of Tropical Storm Irene's rapid track through eastern Vermont (USA) in late summer 2011, this paper presents the array of geomorphic responses to a short duration (time to peak of < 8 h) but high magnitude flood that was the twentieth century flood of record for numerous watersheds. We present herein the geomorphic imprint of Tropical Storm Irene flooding within a larger context of fluvial theory concerning the role of, and trade-off between, the magnitude of energy expenditure during a flood and its duration. Focusing on a detailed field effort within the 187-km2 Saxtons River basin in southeastern VT, augmented by select sites along the adjacent lower gradient Williams River (291-km2), we elucidate (1) the geomorphic effects of a short duration flood in a humid, well-vegetated landscape; (2) the relationship between geomorphic response and (a) peak stream power, (b) total stream power, and (c) flow duration of stream power above a critical threshold; and (3) the spatial variation of geomorphic effects relative to reach-scale geologic and geomorphic controls. Flooding associated with Tropical Storm Irene ranged from the 1000 year recurrence interval (RI) flood (based on Weibull flood frequency analysis) to the 300 year RI flood (log Pearson Type III). Discharges spawned a peak unit stream power of 712 W/m2 (Saxtons River) and 361 W/m2 (Williams River), with total energy expenditure throughout the event of ~ 16,000 × 103 and 15,000 × 103 J, respectively. For the Saxtons River, channel widening was spatially infrequent and limited in magnitude; however, other geomorphic effects were profound (1) the entrainment, transport, and deposition of extremely coarse material; (2) stripping of floodplain surfaces; (3) channel avulsions and incision into Pleistocene-aged material; and (4) deposition of coarse material across floodplains. Based on our extensive field data and hydrologic/hydraulic analyses, we contend that short duration, high energy flows can have profound sedimentological effects but have limited erosive, channel widening impacts. Gravel entrainment and deposition of a catastrophic nature can certainly occur under these flow regimes, but the impacts of these extreme flows on channel geometry may have limited expression.
Early Cretaceous, retro-foreland basin fluvial deposits throughout Wyoming record interactions between orogenesis, subsidence, sediment accumulation, basin physiography, and syndepositional structural deformation associated with the early stages of the Sevier Orogeny. Quantitative paleochannel reconstructions presented here are important for understanding these interactions, evaluating controls on alluvial architecture, and can be applied to basin-modeling studies. Most paleochannel sandstones and conglomerates represent point bars associated with meandering rivers, although some rivers may have been braided. Paleoflow of earliest Cretaceous Cloverly A-interval paleochannels (forebulge depozone, central WY) was generally to the north, northeast, and east, which suggests that most are deposits of basin-axial rivers. Discharges of overlying B-interval paleochannels are less than most of those of the A interval, possibly reflecting a temporal decrease in water supply related to the eastward expansion through time of an orographic rain shadow caused by progressive rising of the Sevier Orogen to the west. The Bechler (western WY), Cloverly B (central WY), and Lakota L2 (eastern WY) intervals are correlative and record deposition throughout the basin in the foredeep, forebulge, and backbulge depozones, respectively. Paleocurrents suggest that Bechler paleochannels are deposits of basin-transverse rivers that flowed to the east, whereas B and L2 paleochannels are deposits of basin-axial rivers that flowed dominantly to the north and northeast. The scales and discharges of most L2 paleochannels are much greater than those of the Bechler and B-interval. This eastward increase in discharge may reflect an eastward increase in precipitation related to the spatially decreasing effects of an orographic rain shadow caused by the Sevier Orogen to the west. Additionally, or alternatively, the higher discharges of most L2 rivers may indicate that they represent a more distal part of a tributary fluvial system than B-interval rivers (consistent with some lower slopes of L2 paleochannels).
The alluvial architecture of thick foredeep deposits contrasts markedly with that of stratigraphically equivalent, much thinner deposits farther east that were associated with the forebulge and backbulge depozones. Foredeep deposits are dominated by overbank and lacustrine mudstones, and channel deposits tend to be isolated with limited lateral extents typically on the order of 10's of meters. Forebulge and backbulge channel deposits tend to be laterally and vertically connected forming sandstones and conglomerates with lateral extents on the order of 10's of km to >100 km. Long-term compacted sediment accumulation rates for the foredeep (generally 10−2 mm year−1) are an order of magnitude greater than those for the forebulge and backbulge depozones (10−3 mm year−1). Quantitative simulations of channel-deposit proportions indicate that basin-wide differences in alluvial architecture are attributable to differences in sediment accumulation rates, which, in turn, reflect variable subsidence rates of the different depozones. Additionally, in some areas of the fore- and backbulge depozones, alluvial architecture was controlled by local syndepositional structures. However, the alluvial architecture in areas influenced by syndepositional structures is broadly similar to that in areas where such structures were absent, both reflecting the same general tectonic setting that experienced limited regional subsidence. Hence, the two cases are not easily distinguished solely on the basis of alluvial architecture.
Modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals, including channel avulsion. Simplified modeling approaches, such as cellular models and process-analogue experiments, are particularly useful for efficiently exploring alternative hypotheses and evaluating first-order controls on river avulsion and alluvial architecture. Here we review the history and current state of the art in simplified avulsion and alluvial architecture models, with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies, and vice versa. Simplified avulsion and alluvial architecture models have proliferated in the past decade, providing a wide variety of models to serve as a basis for future coupled field-modeling studies. We compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data. Outstanding questions highlighted by recent modeling efforts include 1) What thresholds control avulsion initiation in different systems? 2) How do floodplain processes and topography influence avulsion dynamics and alluvial architecture? 3) What factors determine where avulsion channels stabilize? Answering these questions will require targeted modeling efforts coupled to data from ancient systems. Hence our model comparison emphasizes features that can be used to choose or design fit-for-purpose models, and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits. Matching model goals with targeted questions, and model parameters and predictions with quantitative field data, will help tighten communication between field- and model-oriented sedimentary geologists, facilitating advances in our understanding of river avulsion and alluvial architecture.
Earth surface systems (ESS) are characterized by various degrees of historical contingency, which complicates efforts to relate observed features and phenomena to environmental controls. This article provides a conceptual framework for understanding and assessing historical contingency in ESS that is based on algebraic graph theory. ESS are conceptualized as consisting of components (e.g., climate, topography, and lithology) observed or inferred at time periods. Each component at each time period represents a node of a network or graph, and interactions among components constitute the links or edges. Four indexes are applied: the S-metric, which indicates the extent to which observations of part of the network (e.g., topographic changes between two time periods) are likely to represent the dynamics of the network as a whole; spectral radius, which measures coherence and potential amplification of changes or disturbances; Laplacian spectral radius, an index of the relationship between network stability and time steps and an indication of path dependence; and algebraic connectivity, which measures the inferential synchronizability. For each of these, an index on a 0–1 scale is developed, which represents high and minimum levels of historical contingency for a given n, q. These are applied to several archetypal graph structures that represent various forms of historical contingency in the geosciences and to two specific case studies involving Quaternary evolution of fluvial systems in Texas and Kentucky.
This article explores some of the philosophical and practical issues concerning the geomorphological exploration and study of landscapes. It argues that reductionist process geomorphology cannot account for emergent structures at the largest scales, and it discusses the ways in which geomorphology might develop in the future. Geography
Much geomorphological enquiry has been devoted to the understanding of landscapes via the construction of models based on the relationships between process and form. This paper examines the philosophical, theoretical and practical problems involved in bridging the gap between studies of geomorphological processes and explanations of landscape development. It argues that process geomorphology is essentially reductionist and discusses the practical and logical limitations of such an approach to science. It suggests that landscapes are emergent phenomena and, by drawing from the philosophical and practical lessons derived from the physics of non-linear systems, demonstrates that they are not amenable to reductionist explanations.
The historical development of our pedogenetic model is reviewed, and trends or directions toward the future are discussed. Pedogenetic models are characterized with respect to relative degree of computation, complexity, and level of organization. These three characteristics are used as a framework for classification. Early qualitative models have well served the purpose of soil survey to describe the distribution of soils in landscapes. Further development of qualitative models will contribute to the understanding of soil genesis in areas where soil surveys are not completed. In the developed countries, however, the scientific questions have changed dramatically. Greater interest lies in understanding the physical and chemical processes of soil formation acting within relatively short time frames and assessing the interactions between natural processes and environmental change or anthropogenic impacts. Quantification of pedogenetic processes is a life-line to other environmental disciplines, and a mechanistic understanding would, in an ideal situation, describe and predict the behavior of a system for a limited number of years under changing environmental conditions.A proposed approach to mathematical simulation of the dynamic pedogenetic processes is to integrate soil physical, chemical, and other ecological system models into a quantitative pedogenetic model. Models may be developed as research tools in data-intensive studies at the pedon or horizon level, or for the purpose of simulating a soil system at the catena or soil region level. Specific kinds of pedogenetic models have specific implications with respect to the availability and spatial variability of the data needed for development and testing.
Fluvial geomorphology has witnessed a continuing reduction in the time- and space-scales of research, with increasing emphasis on the dynamics of small site-specific river reaches. This shift can be regarded as part of a trend towards the understanding and explanation rather than description of how rivers change, which raises important questions regarding the relevance of such short time-scale and small space-scale research to understanding longer-term aspects of landform behaviour. The methodological challenges that arise from such intensive case study research are illustrated here using a detailed investigation of a river reach. Morphological changes within this reach are shown to be driven by: (i) catchment- scale processes associated with the interaction of discharge and sediment supply waves; and (ii) modification of these processes through morphological controls on erosion and deposition patterns and hence net channel change. The 'morphological conditioning' of channel response reflects the configurational aspects of channel change, and the importance of local characteristics in the understanding of system behaviour. Sensitivity to local conditions implies that short time-scale and small space-scale processes may be critical to channel behaviour, particularly if the system is interpreted in non-linear terms. Although it may be possible to identify statistically averaged stable states, non-linear system behaviour implies that system trajectories are sensitively dependent upon instantaneous system states. Thus, changes between average states can only be understood through an understanding of the sequence of configurational states through which the system evolves. © 1997 by John Wiley & Sons, Ltd.
It is often taken for granted that rivers organize transport into a single active channel. In some net-depositional environments, however, flow of water and sediment is distributed in several stable channels. Such branching rivers may be confined in valleys (anabranching or anastomosed) or unconfined on deltas (distributaries), and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes. Current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion, i.e., the rapid abandonment of a channel in favor of a new path at lower elevation. The requisite conditions for avulsion have been the subject of ongoing debate. Here we resolve the conditions leading to channel avulsion, and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion. A compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth. From the relative rates of bank erosion and channel sedimentation, we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur. Results may be directly applied to modeling landscape evolution over human and geologic time scales, and for inverting formative environmental conditions from channel deposits on Earth and other planetary surfaces.
Channel migration and meander-bend morphology are examined for the lower
Mississippi River between 1877 and 1924, prior to channel cutoffs,
revetments, and change in sediment regime. The spatial pattern of
meander-bend migration coincides with differences in flood-plain
deposits. Migration of meander bends averaged 45.2 m/yr in the upper
alluvial valley, where there are numerous clay plugs, but increased to
59.1 m/yr in the lower alluvial valley, where there are fewer clay plugs
in contact with the channel. The highest migration rates occurred with
meander bends having a curvature,
r m/W m
(ratio between meander-bend radius to channel width) between 1.0 and
2.0, which is a departure from previous models. Results from this study
suggest that rivers with complex flood-plain deposits exhibit patterns
and relationships that deviate from models derived in homogeneous
flood-plain deposits.
The spatial and temporal complexity of earth surface processes and landforms and the presence of deterministic chaos in many fundamental physical processes provide reasons to suspect chaos in geomorphic systems. A method is presented to assess the likelihood of chaotic behavior in a geomorphic system. The method requires identification of the fundamental system components, their positive, negative, or negligible influences on each other, and the relative strength or magnitudes of these links. Based on this information, the method can classify geomorphic systems as stable and nonchaotic, unstable and potentially chaotic, or unstable and generally chaotic. Positive, self-enhancing feedback is a key diagnostic of the likelihood of chaotic behavior. A sample application of the method to the problem of coastal marsh response to sea level rise is provided, which shows the marsh to be unstable. If changes in vegetation cover are partly dependent on vegetation density, the system is generally chaotic if marsh vegetation exhibits self-enhancing feedback (for example, seed source effects) and potentially chaotic if vegetation exhibits self-limiting feedback (competitive effects). The attractors controlling the chaotic dynamics represent states of pronounced erosion/drowning or accretion/expansion.
The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.
There is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river. Here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation. It is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river. Because the water entering the crevasse channel is derived from relatively high in the main flow, it contains low concentrations of suspended solids. Consequently, the crevasse flow is under capacity and the entrance is eroded. Deepening of the entrance increases the crevasse-channel discharge, and the concentration of suspended solids supplied, because more sediment-laden waters are tapped from the deeper flow in the main channel. The crevasse entrance is predicted to deepen until its sediment-carrying capacity is satisfied by the suspended solids entering from the main channel. Whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady-state hydraulic and sediment-transport conditions for crevasse channel depths that are equal to or less than the main-channel depth. This conceptual model is quantified by writing the unsteady, gradually varied, one-dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel. Sediment transport is computed as the integral of a cross-sectional mean velocity and a Rouse concentration profile. Solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main-channel slope.
▪ Abstract Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.
FIG. 1.—Geologic map of the southern Lower Mississippi Valley (LMV) showing the location of the Old River study areas (box). Regional geology is modified from Saucier and Snead (1989). Inset map shows the location of the Mississippi and Atchafalaya rivers in Louisiana. Locations of Figures 5 and 10 are shown. Solid triangles show locations of gradient calculations in Table 2. ABSTRACT: The emphasis on gradient advantages in studies of avul-sion is misleading. While gradient advantages are necessary for an avulsion to occur, the late Holocene avulsion history of the Mississippi River in Louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important. Cross-valley to down-valley slope ratios of the modern floodplain range from 16 to 110 and are typically 30. The slope ratio is 35 at the location of the Mississippi–Atchafalaya diversion (Old River) yet slope ratios are 83 to 110 immediately upvalley of Old River. All values of Mississippi River floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models. Shallow floodplain cores, 14 C dating of organic remains, and geologic mapping show that the Mississippi River has avulsed only four times over the past 5 ky in the southern Lower Mississippi Valley (LMV). Gradient advantages are widespread, yet avulsions are rare. These observations indicate that factors other than gradient advantage control Mississippi River avul-sion. Several examples of Mississippi and Red River avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion. Incipient Mississippi River avulsion and development of the Atchafalaya River involved re-occupation of abandoned Mississippi River channels and a Red River crevasse-splay complex. The modern Atchafalaya River also incises buried Mississippi River channel-belt sands. Abandoned channel belts and crevasse-splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the Mis-sissippi River. Abandoned channels provide ready-made conduits for Mississippi River flow that can efficiently develop into avulsive chan-nels. Multi-storied sheet sandstones in ancient fluvial deposits may pro-vide additional support for the idea that erodible substrates and flood-plain channel distributions are critical influences on avulsion. These features record episodic reoccupation of channel belts, which at least in some cases, may simply reflect successive avulsions rather than ma-jor changes in aggradation rate or extrabasinal factors such as climate.
Seismic data and sediment cores from the Galveston estuary complex were used
to reconstruct the evolution of the estuary during the Holocene. These data show that
the estuary complex has had a history of rapid and dramatic change in response to
(1) sea-level rise across the irregular topography of the ancestral Trinity River valley
and (2) changes in climate, which regulated sediment supply to the estuary. In general,
the valley morphology consists of a deep incision near the center and broad, terraced
flanks. As sea level rose during the Holocene and flooded the valley, the shape
of the estuary changed from narrow and deep to wide and rounded. As sea level rose
to the elevation of the relatively flat fluvial terraces, these areas were flooded rapidly,
resulting in expansions in bay area and dramatic reorganization of bay environments.
The most notable changes were up-valley shifts in the bayhead delta of tens of kilometers
in a few centuries. Radiocarbon ages indicate that these events took place
ca. 9600, ca. 8500, and between ca. 7700 and 7400 yr B.P. The early flooding events
occurred when sea level was rising rapidly (average 5.0 mm/yr; Milliken et al., this
volume, Chapter 1), perhaps episodically. The ca. 8500 yr B.P. flooding surface corresponds
to a prominent terrace at ca. 14 m. The ca. 7700–7400 yr B.P. flooding episode
was the most dramatic in terms of its impact on the estuary setting. Following this
event, the area of the estuary increased by ~30%. This flooding event occurred as the
rate of sea-level rise was starting to decrease. The level of this flooding surface also
corresponds to a terrace at ~10 m, but the magnitude of flooding is too large to be
explained entirely by flooding of this terrace. At the same time, Matagorda Bay to the
west and Sabine Lake to the east experienced similar dramatic flooding events. This event occurred when the climate of east-central Texas was in transition from cool
and moist to warm and dry, and the vegetation cover of the region was undergoing
a reduction in forest and an increase in grasslands. Hence, the ca. 7700–7400 event
was likely amplified by a reduction in sediment supply to the estuary triggered by this
regional climatic change coupled with an increase in sediment accommodation space
caused by flooding of a terrace. Following the ca. 7700–7400 yr B.P. flooding event,
the estuary setting changed relatively little as the rate of sea-level rise decreased to
less than 2.0 mm/yr. By ca. 2600 cal yr B.P., the modern Trinity bayhead delta had
begun to form. Circa 1600 cal yr B.P., the delta experienced a phase of rapid growth.
This more recent episode of delta growth may have resulted from an increase in the
rate of sediment supply, perhaps associated with human occupation and agriculture
in the drainage basin. More recent anthropogenic changes include accelerated subsidence due to
groundwater and hydrocarbon extraction. These changes are occurring at rates that
approach those that occurred during prior flooding events of the Holocene. Thus, the
Galveston estuary complex could be on the verge of another flooding event that would
mainly impact the Trinity bayhead delta and low-lying areas around the delta.
River avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges, and the associated increase in cross-valley slope relative to either the down-channel slope or the down-valley slope (the latter is termed the slope ratio in the present paper). Therefore, spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion-dominated models of alluvial architecture. Detailed evidence on centennial- to millennial-scale floodplain deposition has, to date, been largely unavailable. New data on such long-term overbank aggradation rates from the Rhine–Meuse and Mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto, and can be similar to observations for single overbank floods. This leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios, potentially resulting in increased avulsion frequencies. A revised input parameter for overbank aggradation rate was used in a three-dimensional model of alluvial architecture to study its effect on avulsion frequency. Realistic patterns of avulsion and interavulsion periods (≈1000 years) were simulated with input data from the Holocene Rhine River, with avulsions occurring when the slope ratio is in the range 3–5. However, caution should be practised with respect to uncritical use of these numbers in different settings. Evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value, irrespective of whether critical slope ratios are used, as in the present study, or superelevation as has been proposed by other investigators.
The “perfect storm” metaphor describes the improbable coincidence of several different forces or factors to produce an unusual outcome. The perfect landscape is conceptualized as a result of the combined, interacting effects of multiple environmental controls and forcings to produce an outcome that is highly improbable, in the sense of the likelihood of duplication at any other place or time. Geomorphic systems have multiple environmental controls and forcings, and degrees of freedom in responding to them. This allows for many possible landscapes and system states. Further, some controls and forcings are causally contingent. These contingencies are specific to time and place. Dynamical instability in many geomorphic systems creates and enhances some of this contingency by causing the effects of minor initial variations and small disturbances to persist, and grow disproportionately large, over time. The joint probability of any particular set of global controls is low, as the individual probabilities are < 1, and the probability of any set of local, contingent controls is even lower. Thus, the probability of existence of any landscape or earth surface system state at a particular place and time is negligibly small: all landscapes are perfect. Recognition of the perfection of landscapes leads away from a worldview holding that landforms and landscapes are the inevitable outcomes of deterministic laws, such that only one outcome is possible for a given set of laws and initial conditions. A perfect landscape perspective leads toward a worldview that landforms and landscapes are circumstantial, contingent results of deterministic laws operating in a specific environmental context, such that multiple outcomes are possible.
We propose a new model of river avulsion that emphasizes simplicity, self-organization, and unprogrammed behavior rather than detailed simulation. The model runs on a fixed cellular grid and tracks two elevations in each cell, a high elevation representing the channel (levee) top and a low one representing the channel bottom. The channel aggrades in place until a superelevation threshold for avulsion is met. After an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation. Flow path depends sensitively on floodplain topography, particularly the presence of former abandoned channels. Several behavioral characteristics emerge consistently from this simple model: (1) a tendency of the active flow to switch among a small number of channel paths, which we term the active channel set, over extended periods, leading to clustering and formation of multistory sand bodies in the resulting deposits; (2) a tendency for avulsed channels to return to their previous paths, so that new channel length tends to be generated in relatively short segments; and (3) avulsion-related sediment storage and release, leading to pulsed sediment output even for constant input. Each of these behaviors is consistent with observations from depositional river systems. A single-valued threshold produces a wide variety of avulsion sizes and styles. Larger “nodal” avulsions are rarer because pre-existing floodplain topography acts to steer flow back to the active channel. Channel stacking pattern is very sensitive to floodplain deposition. This work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution.
A holon is any stable sub-whole in a hierarchy. It is a self-creating, open system, governed by a set of laws which regulate its coherence, stability, structure and functioning. It also possesses the potential of adaptation to the challenge of environmental change. It examines interactions between levels in the landscape 'holarchy'. These are evaluated in terms of scale, communication, stability and evolution in a fluctuating energy stream. -from Author
Fluvial geomorphology has witnessed a continuing reduction in the time- and space-scales of research, with increasing emphasis on the dynamics of small site-specific river reaches. This shift can be regarded as part of a trend towards the understanding and explanation rather than description of how rivers change, which raises important questions regarding the relevance of such short time-scale and small space-scale research to understanding longer-term aspects of landform behaviour. The methodological challenges that arise from such intensive case study research are illustrated here using a detailed investigation of a river reach. Morphological changes within this reach are shown to be driven by: (i) catchment-scale processes associated with the interaction of discharge and sediment supply waves: and (ii) modification of these processes through morphological controls on erosion and deposition patterns and hence net channel change, The 'morphological conditioning' of channel response reflects the configurational aspects of channel change, and the importance of local characteristics in the understanding of system behaviour, Sensitivity to local conditions implies that short time-scale and small space-scale processes may be critical to channel behaviour, particularly if the system is interpreted in non-linear terms. Although it may be possible to identify statistically averaged stable states, non-linear system behaviour implies that system trajectories are sensitively dependent upon instantaneous system states. Thus, changes between average states can only be understood through an understanding of the sequence of configurational states through which the system evolves.
A three-dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution, proportion, and connectedness of coarse-grained channel-belt deposits in alluvial strata as a function of channel-belt width, floodplain width, bankfull channel depth, channel-belt and overbank sedimentation rates, avulsion location and period, compaction, and tectonism (tilting and faulting). In this model, a floodplain surface of variable width and length is occupied by a single channel belt. Changes in floodplain topography are produced by spatial and temporal variation of channel-belt and floodplain deposition rates and by compaction and local or regional tectonism. The location and timing of avulsions are determined by local changes in floodplain slope relative to channel-b lt slope and by flood magnitude and frequency. The diverted channel belt follows the locus of maximum floodplain slope. At the end of each simulation, architectural parameters are calculated, including channel-deposit proportion and connectedness and the dimensions of channel-belt sandstone bodies. Three-dimensional perspective diagrams, mesh surfaces, and two-dimensional stratigraphic sections can be plotted to illustrate depositional surfaces (time planes) and the location and geometry of coarse-grained channel-belt deposits within finer-grained overbank deposits. The model predicts that channel-belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross-section orientation. Upstream from avulsion points, sandstone bodies have low width/thickness ratios because of aggradation in a fixed channel belt. Immediately downstream from avulsion points, channel belts tend to be connected, resulting in sandstone bodies with high width/thickness ratios. Avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period. Such sequences may produce successions in which channel-belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years. Down-valley increases in aggradation rate or down-valley decreases in floodplain slope (for example, associated with a rise in base level) may result in an increase in channel-belt proportion and connectedness because of high avulsion frequencies in down-valley regions of the floodplain. Down-valley decreases in aggradation rate (as in alluvial fans, foreland basins, and during base-level fall) may result in high avulsion frequencies in up-valley parts of the floodplain. Tectonic tilting and faulting locally increase avulsion probabilities, and channel belts generally shift toward areas of maximum subsidence. Under certain conditions, however, depositional topography may cause channels to shift away from areas of maximum subsidence. Channel-deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain, but distribution (clustering) of channel belts may not be a reliable indicator of fault geometry or displacement. Models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified. The three-dimensional model presented here predicts many of the features of channel behavior observed in modern rivers, but there is a pressing need for better models and adequate natural data to test them.
A simple computer-simulation algorithm for the transport of sand by wind
produces forms resembling barchan, crescentic ridge, linear, and star
natural dune classes. Sand is moved as slabs composed of many grains
that are picked up at random, transported in a specified direction, and
deposited (1) with a probability that depends on the local presence or
absence of sand or (2) in shadow zones in the lee of dunes. The
simulated dune fields are interpreted as complex systems, with sand-dune
classes being dynamical attractors of these systems. The evolution of
dunes once formed becomes decoupled from the details of eolian sand
transport.
The extent to which river processes and morphology are dependent on climate is an old question. When looking at large time scales climatic cyclicity seems to coincide with alternating events in fluvial development. However, the shorter the time scales at which fluvial development is considered, the more it is difficult to find a close correspondence with climate evolution. Ultimately, it appears that the response time of river processes to climatic change may restrict the sensitivity of the fluvial systems to adapt to short climatic oscillations. The impact of climate on fluvial systems is largely transferred to these systems by the climate-determined soil cohesion and peak discharges. The latter factors are especially related to vegetation cover and permafrost conditions. Finally, a number of basin characteristics are discussed that demonstrate that fluvial systems are not completely controlled by climate.
Conceptual models of river-estuary interaction are typically based on a notion of systematic downstream change in the intensity of fluvial processes. Low slopes, backwater effects, and effects of antecedent topography and landforms may complicate downstream tends in water and sediment flux in coastal plain rivers. An analysis of the lower Trinity River, Texas shows no consistent downstream pattern of increases or decreases in the discharge, stream power, or water surface slope. Flows may decrease downstream due to coastal backwater effects in the lowermost reaches, and due to diversion of flow into valley-bottom depressions during high flows in both the fluvial and fluvial-estuarine transition reaches. In general, however, stream power and slope decrease in the lower reaches, consistent with earlier findings of limited fluvial sediment delivery to the coastal zone. Some tributaries may become distributaries at high but sub-bankfull flows, as backwater effects reverse flows into depressions associated with paleomeanders. The paleomeanders, and possibly the locations of these “reversible” channels, are related to antecedent topography associated with aggradation/degradation cycles over the past 100 Ka. Low-gradient coastal plain rivers may not function as simple conduits from land to sea. Further, the transition from fluvial to coastal dominance may be variable along the river, with the variability controlled not just by the relative magnitude of river and tidal or backwater forcing, but also by valley topography controlled in part by antecedent landforms.
Numerical models can be useful for explaining poorly understood phenomena or for reliable quantitative predictions. When modeling a multi-scale system, a `top-down' approach---basing models on emergent variables and interactions, rather than explicitly on the much faster and smaller scale processes that give rise to them---facilitates both goals. Parameterizations representing emergent interactions range from highly simplified and abstracted to more quantitatively accurate. Empirically based large-scale parameterizations lead more reliably to accurate large-scale behavior than do parameterizations of much smaller scale processes. Conversely, purposefully simplified representations of model interactions can enhance a model's utility for explanation, clarifying the key feedbacks leading to an enigmatic behavior. For such potential insights to be relevant, the interactions in the model need to correspond to those in the `real' system in some straightforward way. Such a correspondence usually holds for models constructed for predictive purposes, although this is not a requirement. The goals motivating a modeling endeavor help determine the most appropriate modeling strategies, as well as the most appropriate criteria for judging model usefulness.
The Sabine-Neches fluvial-estuary system is composed of deposits that represent fluvial, deltaic, central-basin, tidal inlet/delta, and chenier depositional systems. The Holocene deposits and associated environmental changes preserved in the drowned Sabine-Neches alluvial valley provide a valuable analog for present and future environmental changes. These deposits are bounded by flooding surfaces that record episodes of dramatic environmental reorganization during the Holocene. The most dramatic environmental changes are manifested as stratigraphic back stepping in which central-basin sediments overlie deltaic sediments. The magnitude of flooding varies from a few tens of kilometers to less pronounced back stepping followed by rapid progradation. Initial flooding of the onshore Sabine-Neches incised valley occurred around 9800 cal yr B.P., and by ca. 8900 cal yr B.P., a vast bayhead delta occupied the southern half of the valley. This delta backstepped up the valley during the relatively rapid sea-level rise of the early to middle Holocene, and by ca. 7100 cal yr B.P., it occupied the entire valley. After ca. 7100 cal yr B.P., the bayhead delta shifted up the valley again, and a central-basin setting existed in the lower half of the valley. The middle basin expanded episodically between ca. 5500 cal yr B.P. and 1700 cal yr B.P., and a brief period of delta growth occurred ∼300 yr ago. Controlling mechanisms for flooding surface formation include sea-level rise, changes in the antecedent topography of the incised valley, and sediment supply variations. Antecedent topography was influential in controlling estuarine evolution between ca. 7800 and 7500 cal yr B.P., when an extensive fluvial terrace was inundated. The fact that some flooding surfaces appear to be synchronous, within a few centuries, in several estuaries across the northern Gulf of Mexico suggests a eustatic rather than local control. Flooding events at ca. 8900 cal yr B.P. and ca. 8400-8000. © 2008 The Geological Society of America. All rights reserved. cal yr B.P. were likely caused by rapid, sub-meter-scale sea-level rise events. Sediment supply variations controlled by climatic forcing appear to have been the main cause of other flooding events. Unfortunately, the Holocene climate record for the east Texas-west Louisiana coastal region is poorly documented, and a direct relationship to central and western Texas climate records may be complex. So the exact nature of climate control on sediment flux to the estuary system remains elusive.
Instream flow science and management requires identification of characteristic hydrological, ecological, and geomorphological attributes of stream reaches. This study approaches this problem by identifying geomorphic transition zones along the lower Sabine River, Texas and Louisiana. Boundaries were delineated along the lower Sabine River valley based on surficial geology, valley width, valley confinement, network characteristics (divergent versus convergent), sinuousity, slope, paleomeanders, and point bars. The coincidence of multiple boundaries reveals five key transition zones separating six reaches of distinct hydrological and geomorphological characteristics. Geologic controls and gross valley morphology play a major role as geomorphic controls, as does an upstream-to-downstream gradient in the importance of pulsed dam releases, and a down-to-upstream gradient in coastal backwater effects. Geomorphic history, both in the sense of the legacy of Quaternary sea level changes, and the effects of specific events such as avulsions and captures, are also critical. The transition zones delineate reaches with distinct hydrological characteristics in terms of the relative importance of dam releases and coastal backwater effects, single versus multi-channel flow patterns, frequency of overbank flow, and channel-floodplain connectivity. The transitional areas also represent sensitive zones which can be expected to be bellwethers in terms of responses to future environmental changes. Copyright © 2007 John Wiley & Sons, Ltd.
Much geomorphological enquiry has been devoted to the understanding of landscapes via the construction of models based on the relationships between process and form. This paper examines the philosophical, theoretical and practical problems involved in bridging the gap between studies of geomorphological processes and explanations of landscape development. It argues that process geomorphology is essentially reductionist and discusses the practical and logical limitations of such an approach to science. It suggests that landscapes are emergent phenomena and, by drawing from the philosophical and practical lessons derived from the physics of non-linear systems, demonstrates that they are not amenable to reductionist explanations.
Hurricane Rita, a category three hurricane which struck the US Gulf Coast near the Louisiana/Texas border in 2005, did not cause extensive river flooding. However, the storm did result in extensive forest damage and tree blowdown. High-resolution post-storm aerial photography allowed an inventory of river bank trees blown into the channel along the lower Neches and Sabine Rivers of southeast Texas and southwest Louisiana. Blowdowns directly into the channel averaged 9·3 per kilometer in the lower Neches and 13·4 in the lower Sabine River, but individual reaches 10 to 20 km in length had rates of 20 to 44 blowdowns per kilometer. Though large woody debris (LWD) from Hurricane Rita was widely perceived to reduce the capacity of channels to convey flow, no strong evidence exists of increased flooding or significant reductions in channel conveyance capacity due to LWD from the storm. The Rita blowdown inventory also allowed an assessment of whether similar blowdown events could account for major logjams and rafts on Red, Atchafalaya, and Colorado Rivers on the Gulf Coast, which blocked navigation from tens to hundreds of kilometers in the 1800s. Results from Hurricane Rita suggest that blowdown into channels alone – not withstanding blowdown elsewhere in the river valleys or along tributaries which could deliver LWD to the river – is sufficient to completely block channels, thus providing a plausible mechanism for initiating such (pre)historic log rafts. Copyright © 2009 John Wiley & Sons, Ltd.
Avulsion, i.e. the relatively sudden displacement of a river channel, has an important effect on sediment distribution and on architecture of fluvial deposits because avulsion is a primary control on channel location on a floodplain. Most avulsions occur when a triggering event, commonly a flood, forces a river across a stability threshold. The closer the river is to the threshold, the smaller is the flood discharge needed to initiate an avulsion.Avulsions can be categorized by the processes or events that decrease stability and move the river toward the avulsion threshold, and/or serve as avulsion triggers. These processes or events produce one or more of the following:1 an increase in the ratio of avulsion course slope to existing channel slope caused by a decrease in gradient of the existing channel;2 an increase in the ratio of avulsion course slope to existing channel slope caused by an increase in gradient away from the existing channel;3 a non-slope-related reduction in the capacity of the existing channel to carry all the water and sediment delivered to it.In most cases several of these causes will combine to bring a river close to a threshold, after which the next triggering event of sufficient magnitude will push the system across the threshold and avulsion will occur.Avulsion frequency is controlled by the interaction between the rate at which various processes combine to move a river toward the avulsion threshold (instability) and the frequency of triggering events. If the combined processes that lead to instability proceed rapidly relative to triggering events, then the frequency of the triggering events will control avulsion frequency. In such settings, avulsion frequency may be a predictable function of flood frequency. If triggering events occur frequently relative to the rate at which the river becomes unstable, however, then the rate at which a combination of processes leads to instability will control avulsion frequency.
Channel cross-sectional changes since construction of Livingston Dam and Lake Livingston in 1968 were studied in the lower Trinity River, Texas, to test theoretical models of channel adjustment, and to determine controls on the spatial extent of channel response. High and average flows were not significantly modified by the dam, but sediment transport is greatly reduced. The study is treated as an opportunistic experiment to examine the effects of a reduction in sediment supply when discharge regime is unchanged. Channel scour is evident for about 60 km downstream, and the general phenomena of incision, widening, coarsening of channel sediment and a decrease in channel slope are successfully predicted, in a qualitative sense, by standard models of channel response. However, there is no consistent channel response within this reach, as various qualitatively different combinations of increases, decreases or no change in width, depth, slope and roughness occur. These multiple modes of adjustment are predicted by the unstable hydraulic geometry model. Between about 60 km and the Trinity delta 175 km downstream of the dam, no morphological response to the dam is observed. Rather than a diminution of the dam's effects on fluvial processes, this is due to a fundamental change in controls of the fluvial system. The downstream end of the scour zone corresponds to the upstream extent of channel response to Holocene sea level rise. Beyond 60 km downstream, the Trinity River is characterized by extensive sediment storage and reduced conveyance capacity, so that even after dam construction sediment supply still exceeds transport capacity. The channel bed of much of this reach is near or below sea level, so that sea level rise and backwater effects from the estuary are more important controls on the fluvial system than upstream inputs. Copyright © 2005 John Wiley & Sons, Ltd.
ABSTRACTA study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre-existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.
Examination of outcrops, satellite imagery and shallow (< 25 m long) floodplain cores shows that rivers of the Texas Gulf Coastal Plain undergo two distinct avulsion styles: (i) avulsion by channel reoccupation and (ii) avulsion by diversion into flood basins. Holocene avulsion histories of rivers with large sediment supplies, such as the Colorado, and rivers with small sediment supplies, such as the Trinity and Nueces rivers, further suggest that different styles of avulsion occur during different stages of incised-valley filling.
Downstream geomorphic responses of stream channels to dams are complex, variable, and difficult to predict, apparently because the effects of local geological, hydrological, and operational details confound and complicate efforts to apply models and generalizations to individual streams. This sort of complex geomorphic response characterizes the Sabine River, along the Texas and Louisiana border, downstream of the Toledo Bend dam and reservoir. Toledo Bend controls the flow of water and essentially prevents the flux of sediment from three-quarters of the drainage basin to the lower Sabine River. Although the channel is scoured immediately downstream of the dam, further downstream there is little evidence of major changes in sediment transport or deposition, sand supply, or channel morphology attributable to the impoundment. Channels are actively shifting, banks are eroding, and sandbars are migrating, but not in any discernibly different way than before the dam was constructed. The Sabine River continues to transport sand downstream, and alluvial floodplains continue to accrete. The relatively small geomorphic response can be attributed to several factors. While dam releases are unnaturally flashy and abrupt on a day-to-day basis, the long-term pattern of releases combined with some downstream smoothing creates a flow regime in the lower basin which mimics the pre-dam regime, at least at monthly and annual time scales. Sediment production within the lower Sabine basin is sufficient to satisfy the river's sediment transport capacity and maintain pre-dam alluvial sedimentation regimes. Toledo Bend reservoir has a capacity: annual inflow ratio of 1.2 and impounds 74% of the Sabine drainage basin, yet there has been minimal geomorphic response in the lower river, which may seem counterintuitive. However, the complex linked geomorphic processes of discharge, sediment transport and loads, tributary inputs, and channel erosion include interactions which might increase as well as decrease sediment loads. Furthermore, if a stream is transport-limited before impoundment, the reduced sediment supply after damming may have limited impact. Copyright © 2003 John Wiley & Sons, Ltd.
An important aspect of river science and management is the identification of key boundaries, transition zones and hinge points. Such critical areas are likely to be important foci or indicators of the effects of environmental change on river systems. While geological controls on such features are widely recognized in upland streams, inherited or antecedent forms may also be important controls of key transitional points or reaches in alluvial coastal plain rivers. The critical zone of the lower Trinity River, Texas marks an important transition in river channel and valley forms, dominant processes and resulting geomorphological, hydrological and ecological characteristics. Its location is not a transient result of upstream or downstream propagation of effects. Rather, the location marks the contemporary upstream extent of the effects of Holocene sea-level rise, which in turn coincides with the point at which the Pleistocene upper Deweyville alluvial terrace surface is encountered. A more rapid rate of change and relatively sudden upstream displacement of this zone is likely when the upper Deweyville surface is flooded. Antecedent fluvial and alluvial topography inherited from previous aggradation, degradation and lateral migration episodes is likely to be an important control over modern fluvial forms and processes in other alluvial coastal plain rivers as well. Identification and mapping of such features may be extremely useful in pinpointing critical transition zones for water resource managers. Copyright © 2008 John Wiley & Sons, Ltd.
Avulsions – relatively sudden changes in course, or establishment of new anabranches – are an important process in alluvial rivers. Their key role in floodplain construction and alluvial architecture, and the general conditions favouring avulsions, are well known. However, avulsion processes and evolution, and the factors controlling avulsion regimes, are poorly understood. In the southeast Texas coastal plain, where avulsions are common features of the river valleys, avulsions were studied on the lower Brazos, Navasota, Trinity, Neches and Sabine rivers using a combination of aerial imagery, digital elevation models and field surveys. Avulsions have important influences on the surface morphology and contemporary processes in all five rivers. Features associated with avulsions are active and distinct throughout the study area, and all the rivers have experienced geologically (if not historically) recent avulsions. However, no two of the study rivers have the same contemporary avulsion regime. First-order differences in avulsion style are controlled by the stage of valley filling, and within the three rivers characterized by an unfilled incised valley, antecedent morphology associated with late Quaternary and Holocene coastal and fluvial-deltaic processes accounts for the major differences. In the Navasota (27 avulsions in 185 km) and Neches (21 in 340 km) rivers, subchannels associated with avulsions exist in all stages of development from active to infilled, and some have occurred in recent decades. The other rivers have fewer avulsions, but both the Sabine and Trinity have experienced historic channel shifts. Only the Brazos River has experienced no avulsions within the past c. 300 years. Results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes. Copyright © 2008 John Wiley & Sons, Ltd.
Technological and methodological advances have facilitated tremendous growth in hydrology during the last century; however,
there are also concerns that these advances indirectly contribute to additional problems in our research. An insight into
hydrologic literature reveals our tendency to develop more complex models than perhaps needed, and our increasing emphasis
on individual mathematical techniques rather than general hydrologic issues. Some recent studies of diverse forms have suggested
that simplification in modeling and development of a common framework may help alleviate these problems. The present study
is intended to bring such studies together towards a more coherent approach to research in catchment hydrology. This is done
by highlighting the need for model simplification and generalization and proposing some potential directions for achieving
such. Through a discussion of difficulties in data measurements, the need for moving beyond the notion of “modeling everything”
to the notion of “capturing the essential features” is explained; the concept of dominant processes in model simplification
and the utility of integration of concepts for modeling improvement are discussed. Formulation of a catchment classification
framework is advocated as a possible means for a common framework in hydrology, and the role of dominant processes in this
formulation is presented; the problems due to adoption of different modeling terminologies are highlighted and potential ways
to overcome such are also discussed.
The Holocene avulsion history of the lower Brazos alluvial valley of east Texas, USA, was studied using 10 drill cores, 26 radiocarbon dates, aerial photos, and a digital elevation model. This study shows that two long-term processes, aggradation and localized valley tilting (along a normal listric fault), are responsible for generating two styles of avulsion. The first process precedes avulsion-by-progradation, while the second process precedes avulsion-by-annexation. As valley aggradation migrated updip over the last 7.5 ka, three regional backstepping avulsions occurred along the lower 140 km of the valley and each generated sizable deposits. A pattern emerges of landward stepping progradational avulsions tracking the locus of valley aggradation and of valley aggradation migrating inland even after the rate of sea level rise diminishes. At the same time, several local nodal avulsions occurred between 50 and 55 km updip of the current highstand shoreline but generated no observable deposits. Geomorphic evidence indicates that, since the late Pleistocene, active movement along a previously undocumented normal listric fault has occurred at the location of the nodal avulsion. These two long-term processes do not operate mutually exclusive of each other to promote avulsions; rather, they operate concurrently. Only aggradation promotes avulsions that affect floodplain alluviation, although the total volume of these deposits comprises a small portion of the valley fill.
A shallow coring and geophysical logging program has recorded the sedimentary fill of the Brazos River valley in the Texas Gulf Coastal Plain. Thermoluminescence dates together with new and recalibrated published radiocarbon dates show the valley fill to include extensive, sandy, buried falling stage and lowstand Oxygen Isotope Stage (OIS) 3 and 2 deposits. These alluvial deposits are punctuated by numerous paleosoil horizons that record alternating periods of cutting, bypass and accumulation. Maximum valley incision and two periods of terrace formation preceded marine lowstand conditions, suggesting significant discordance between preserved fluvial and classical marine system tracts. The latest Pleistocene incision and fill history appears related to cycles of increased discharge and incision, followed by system equilibration and terrace formation. Analysis of the Brazos River incised valley and its contained paleochannels indicates that latest Pleistocene mean annual discharge was as much as four times greater than that of today. This magnitude of discharge in the Brazos would require a two-fold increase in precipitation across the drainage basin. Such an increase is comparable to the present day measured positive El Niño winter precipitation anomaly across the region. Paleochannel geometries and the stratigraphic and sedimentologic data from this investigation support the hypothesis that periods of high-amplitude, El Niño-like climatic perturbations characterized the late Quaternary climate of the south-central and southwestern U.S. This period of high discharge coincides, at least in part, with late OIS 3 progradation of the Brazos delta to the shelf margin, OIS 3 and 2 valley incision across the Texas shelf, and concomitant sand bypass to intraslope basins beyond the shelf edge.
The passive margin Texas Gulf of Mexico Coastal Plain consists of coalescing late Pleistocene to Holocene alluvial–deltaic plains constructed by a series of medium to large fluvial systems. Alluvial–deltaic plains consist of the Pleistocene Beaumont Formation, and post-Beaumont coastal plain incised valleys. A variety of mapping, outcrop, core, and geochronological data from the extrabasinal Colorado River and the basin-fringe Trinity River show that Beaumont and post-Beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio-eustatic cycles.
The concept of diversity and its measurement is becoming increasingly attractive to soil scientists. This theme is explored using a terrace chronosequence in the Henares River valley, NE Madrid. Pedodiversity was computed at different hierarchical levels (Great Group, Subgroup and family of Soil Taxonomy) using appropriate indices including abundance and the Shannon diversity index. Taxonomic pedodiversity, i.e., the diversity of soil types, increased with time (from low to high terraces) at high hierarchical levels but no clear relationship was found at family level. Richness–area analysis showed that a logarithmic function fitted data from the low and middle terraces, while the high terrace data could be fitted with a power model. The geostatistical analysis revealed a decrease in the variability of soil properties from young to old deposits. This study showed that, at the Henares site, diversity and spatial variability of soil properties are not synonymous concepts. Indeed, the terrace with the highest taxonomic pedodiversity at Great Group and Subgroup levels, showed a low spatial variability of soil properties. Also, the same terrace exhibited the lowest connectance values. A non-linear dynamical system approach can be useful to explain these apparently contradictory results.