The influence of lee sediment behind large bed elements on bedload transport rates in supply-limited channels

A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic boulder-sized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements.     

Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo

The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.     

EQUILIBRIUM AND NONEQUILIBRIUM CONDITIONS IN DRYLAND RIVERS

Rivers in drylands typically are characterized by extreme flow variability, with long periods of little or no flow interspersed with occasional large, sometimes extreme, floods. Complete adjustment of river form and process is sometimes inhibited, resulting in a common assumption that equilibrium conditions may rarely, if ever, exist in dryland rivers, and that transient and unstable (nonequilibrium) behavior is the norm. Examples from the Channel Country and the Northern Plains in central Australia challenge that notion. Along the middle reaches of these intermediate and large, low-gradient rivers, where long duration floods generate moderate to low unit stream powers and boundary resistance is high as a result of indurated alluvial terraces, cohesive muds or riparian vegetation, there is evidence that: (1) channels have remained essentially stable despite large floods; (2) sediment transport discontinuities, while present at a catchment scale, are largely insignificant for channel form and process in individual reaches; (3) there are strong correlations between many channel form and process variables; and (4) many rivers appear to be adjusted to maximum sediment transport efficiency under conditions of low gradient, abundant within-channel vegetation and declining downstream discharge. In these middle reaches, rivers are characterized by equilibrium conditions. However, in the aggradational lower reaches of rivers on the Northern Plains, where upstream terraces are buried by younger sediments and channels are less confined, nonequilibrium conditions prevail. Here, channels sometimes undergo sudden and substantial changes in form during large floods, sediment transport discontinuities are readily apparent, and landforms such as splays remain out-of-balance with normal flows. Hence, dryland rivers can exhibit both equilibrium and nonequilibrium conditions, depending on factors such as catchment size, channel gradient, flood duration, unit stream power, channel confinement, sediment cohesion, and bank strength. [Key words: dryland rivers, floods, equilibrium, nonequilibrium, central Australia.]     

Annual sediment budget of a UK mountain torrent

Research into torrent erosion focuses heavily on bedload transport dynamics and debris flow propagation during specific events. As a result, there is limited understanding of the sediment budgets operating in torrent systems over longer timescales. The aim of this study is to construct a sediment budget of the main geomorphological processes operating in a mountain torrent sediment system over a full year.
The study site is Iron Crag which is a small torrent system (catchment area 2.4 ha) situated in the northern Lake District, UK. The site has the characteristic morphology of a torrent: multiple hillslope sediment sources, steep channel, gorges, and a basal alluvial fan. A measurement scheme was designed to monitor process activity, linking the sediment sources and sinks, from December 1998 to December 1999. Over this time period the sediment budget demonstrates that 184 tonnes of sediment was supplied to the alluvial fan (which acted primarily as a sediment sink). Channel (70%) and bank (25%) sources dominated the sediment supply, and surface processes and rockfall on the hillslopes (5%) made only a minor contribution.
Temporal variations in process activity are significant. Surface processes and rockfall display seasonal variations in yield, whilst channel and bank yields are influenced by individual storm events. Site–specific meteorological data are used to explain these observations and freeze–thaw activity and rainfall characteristics are shown to be important controlling factors.     

Sediment discharge variability in Arctic rivers: implications for a warmer future

A new model for predicting the sediment flux in ungauged river basins is applied to 46 Arctic to sub-Arctic rivers. The model predicts the preanthropogenic flux of sediment to within a factor of 2, across four orders of magnitude in basin area and run-off. The model explains for the first time why Arctic rivers carry so little sediment when compared at the global scale. Sensitive to drainage basin temperature, the model is used to examine the impact of a climate warming scenario on the loads of high latitude rivers. As the Arctic warms, rivers will carry increased sediment loads, similar to more temperate rivers. For every 2°C warming, the model predicts a 22% increase in the flux of sediment carried by rivers. For every 20% increase in water discharge there will be a 10% increase in sediment load. The model also aids the interpretation of palaeoclimate records obtained from Arctic continental margins.     

Human impact on land–ocean sediment transfer by the world's rivers

Land–ocean transfer of sediment by rivers is a key pathway for material transfer on Earth. Contemporary data on the sediment loads of rivers provide clear evidence of significant recent changes in the sediment fluxes of several rivers in response to human impact. The key drivers of increased sediment loads include land clearance for agriculture and other facets of land surface disturbance, including logging activity and mining. Although, programmes for soil conservation and sediment control can result in reduced sediment loads, the trapping of sediment by dams represents the dominant cause of reduced loads. This influence is currently assuming increasing importance at the global scale. Any attempt to link these drivers to changes in the global land–ocean sediment flux must take account of the aggregation and buffering effects that operate in larger basins, which can cause damping and even removal of signals of increasing flux within the upstream basin, and complicate the link between upstream and downstream response to human impact. Further work is required to provide a precise quantitative assessment of the human impact on global land–ocean sediment fluxes and the net effect of increasing and decreasing fluxes. Particular attention must be paid to the temporal perspective and the variation of impact trajectories in different areas of the globe and for river basins of different sizes.     

FIELD EVALUATION OF EMPIRICAL EQUATIONS IN STRAIGHT ALLUVIAL CHANNELS

Six experimentally derived formulae that predict the conditions for alternate bar formation and equilibrium bar dimensions are assessed using field data. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bar sediment size, and flow conditions over a two-year study period. Experimental flume studies suggest that alternate bars form in wide, straight, shallow streams, have wavelengths between 4 and 15 channel widths, and have heights that are roughly equal to the average depth of flow. Bar formation under unsteady flow conditions can be predicted accurately by the experimentally derived steady-flow formulae, but these formulae fail to predict bar dimensions sucessfully. These results suggest that the process of bar formation in artificially straightened, sand-bedded natural streams may occur outside the range of flow conditions predicted by existing empirical models. Further work should focus on attempting to isolate physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy flow regimes [Key words: alluvial channels, artificial channels, geomorphology, rivers, sedimentology].     

Sediment storage and release from Himalayan piggyback basins and implications for downstream river morphology and evolution

Piggyback basins developed at the mountain fronts of collisional orogens can act as important, and transient, sediment stores along major river systems. It is not clear, however, how the storage and release of sediment in piggyback basins affects the sediment flux and evolution of downstream river reaches. Here, we investigate the timing and volumes of sediment storage and release in the Dehra Dun, a piggyback basin developed along the Himalayan mountain front in northwestern India. Based on OSL dating, we show evidence for three major phases of aggradation in the dun, bracketed at ca. 41–33 ka, 34–21 ka and 23–10 ka, each accompanied by progradation of sediment fans into the dun. Each of these phases was followed by backfilling and (apparently) rapid fan‐head incision, leading to abandonment of the depositional unit and a basinward shift of the active depocentre. Excavation of dun sediment after the second and third phases of aggradation produced time‐averaged sediment discharges that were ca. 1–2% of the modern suspended‐sediment discharges of the Ganga and Yamuna rivers that traverse the margins of the dun; this sediment was derived from catchment areas that together comprise 1.5% of the drainage area of these rivers. Comparison of the timing of dun storage and release with upstream and downstream records of incision and aggradation in the Ganga show that sediment storage in the dun generally coincides with periods of widespread hinterland aggradation but that late stages of dun aggradation, and especially times of dun sediment excavation, coincide with major periods of sediment export to the Ganga Basin. The dun thus acts to amplify temporal variations in hinterland sediment supply or transport capacity. This conceptual model appears to explain morphological features of other major river systems along the Himalayan front, including the Gandak and Kosi Rivers, and may be important for understanding sediment flux variations in other collisional mountain belts.     

Relation between the erosion and sedimentation zones in the Yellow River, China

Sedimentation in the lower reaches of the Yellow River is a major problem requiring implementation of large-scale control measures in the upper and middle drainage basin. For maximum benefit, major sediment generation areas must be delimited. For this purpose, the upper and middle drainage basin of the Yellow River has been divided into four major sediment and water source areas. A series of databases of runoff and sediment yields from these source areas and for the corresponding quantities of sedimentation in the lower Yellow River channel have been established. On this basis, a set of multiple-regression equations has been established that define the relationships between sedimentation in the lower Yellow River and the yearly or event-based runoff and sediment yields from the four source areas or subsystems. Based on the regression equations obtained, the contribution of the four major source areas to the sedimentation in the lower Yellow River channel can be estimated. The results obtained indicate that, given other factors, for each ton of sediment reduced from the coarse-sediment producing area (CSA), the sediment deposited in the lower Yellow River would be reduced by 0.455 ton; for each ton of sediment reduced from the fine-sediment producing area (FSA), the sediment deposited in the lower Yellow River would be reduced by 0.154 ton only. Therefore, if limited erosion control measures are applied to the coarse-sediment producing area, the benefits for sedimentation reduction in the lower Yellow River will be much larger than if similar resources are applied to the fine-sediment producing area.     

Calculation method for sediment load in flood and non-flood seasons in the Inner Mongolia reach of the Yellow River

Based on an empirical sediment transport equation that reflects the characteristics of “more input, more output” for sediment-laden flow in rivers, a general sediment transport expression was developed, which can take into account the effects of upstream sediment input, previous cumulative sediment deposition, critical runoff for sediment initiation, and the differences in sediment particle sizes between the mainstream and tributaries. Then, sediment load equations for non-flood and flood seasons for the sub-reaches from Bayangaole to Sanhuhekou and from Sanhuhekou to Toudaoguai, as well as the whole Inner Mongolia reach from Bayangaole to Toudaoguai, were formulated based on data collected between 1952 and 2010. The corresponding sediment deposition and the cumulative values at each river reach were calculated using the proposed sediment transport equations for the period 1952 to 2010 according to the principle of sediment conservation. Comparisons between the calculated and measured values using the proposed sediment load equations for the sub-reaches and the entire reach showed that the calculated sediment load and sediment deposition and the corresponding cumulative values in the flood and non-flood seasons were in good agreement with the measured values. These results indicated that the proposed methods can be applied to calculate the sediment load and the associated sediment deposition in the flood and non-flood seasons for long-term trend analysis of sediment deposition in the Inner Mongolia reach of the Yellow River.     

Sediment concentration changes in runoff pathways from a forest road network and the resultant spatial pattern of catchment connectivity

Hydrological connectivity is a term often used to describe the internal linkages between runoff and sediment generation in upper parts of catchments and the receiving waters. In this paper, we identify two types of connectivity: direct connectivity via new channels or gullies, and diffuse connectivity as surface runoff reaches the stream network via overland flow pathways. Using a forest road network as an example of a landscape element with a high runoff source strength, we demonstrate the spatial distribution of these two types of linkages in a 57 km² catchment in southeastern Australia. Field surveys and empirical modelling indicate that direct connectivity occurs primarily due to gully development at road culverts, where the average sediment transport distance is 89 m below the road outlet. The majority of road outlets were characterised by dispersive flow pathways where the maximum potential sediment transport distance is measured as the available hillslope length below the road outlet. This length has a mean value of 120 m for this catchment. Reductions in sediment concentration in runoff plumes from both pathways are modelled using an exponential decay function and data derived from large rainfall simulator experiments in the catchment. The concept of the volume to breakthrough is used to model the potential delivery of runoff from dispersive pathways. Of the surveyed road drains (n=218), only 11 are predicted to deliver runoff to a stream and the greatest contributor of runoff occurs at a stream crossing where a road segment discharges directly into the stream. The methodology described here can be used to assess the spatial distribution and likely impact of dispersive and gullied pathways on in-stream water quality.     

Water Discharges Determined From Sediment Distributions: A Palaeohydrological Method

The article presents a simple sediment deposition model, based on current knowledge of the processes of transport and deposition of suspended material. It indicates possibilities of interpreting flow conditions from grain-size distributions at two localities, situated along the water flowline. The model reveals new information on ancient hydrological and sedimentological processes and events. Previously the model has been applied to forecast trap efficiency of reservoirs and predict reservoir sedimentation. In this study the model is used as a palaeohydrological method in studies of annual varves.
In varve geochronology a major attention has been paid to varve correlations and varve thickness variations, whereas other sedimentological data have been more or less disregarded. In this article new information on the spatial distribution of particle sizes demonstrate that further knowledge can be gained by careful studies of the patterns of grain-size distributions.
Annual variations in water discharges during late glacial time were studied in the Indalsälven and Dalälven valleys in Sweden. Water discharges during the deglaciation seem to have been of the same order of magnitude as in present-day rivers during normal years. During some years catastrophic events occurred, resulting in exceptional water discharges. The magnitudes of these discharges were estimated, and drainage directions were established. It appears from the studies that there are promising prospects of finding out both the magnitude and frequency of processes that until now have been described only in qualitative terms.     

泥沙解吸重金属污染物动力学模式研究

从吸附动力学方程和质量守恒出发,导得静态试验中泥沙解吸重金属污染时,单位重量泥沙剩余吸附量和水相浓度随时间变化的计算公式,进而导得单位重量泥沙剩余平衡吸附量和水相平衡浓度的计算公式。在此基础上,结合试验进一步研究了泥沙解吸重金属污染物的动力学模式。     

The timing and magnitude of coarse sediment transport events within an upland,temperate gravel-bed river

This paper describes the application of a new instrument to continuously measure bedload transport, an impact sensor, to a 72 km² test catchment in the Yorkshire Dales, northern England. Data from a network of impact sensors are linked to repeat surveys of channel morphological response, to get a better understanding of the conditions that lead to sediment generation and transfer. Results suggest certain areas of the catchment act as key sediment sources at the annual time scale, with material being quickly delivered to the lower parts of the catchment along the steep bedrock channel. Sediment transfer within the tributaries occurs in significantly smaller magnitudes than within the main channel; but it moves more frequently and at different times of the year, with transfer rates being strongly conditioned by larger-scale valley geomorphology. The lower 5.6 km reach sees a significant reduction in gradient and a widening of the valley. This permits significant accumulation within the channel, which has persisted for many years. This lower reach is very sensitive to changes in sediment supply and there is good agreement between changes in bedload transport data and the surveyed channel response. These observations have major implications for how river management projects should be developed in upland environments, especially those where large-scale geomorphological controls have a major impact upon the sediment transfer process. Evidence suggests that where river management restricts lateral movement of the channel and transfer of sediment into floodplain storage, changes in sediment supply can lead to areas of severe accumulation, acceleration of bank erosion and exacerbated flood risk.     

A field comparison of three pressure-difference bedload samplers

The movement of bedload over a cross-section is often sampled using a “pressure-difference bedload sampler”, such as the Helley–Smith. Whereas several types are in use, no one device has gained universal acceptance as the standard for use in all types of streams. Moreover, evidence suggests that similar devices may collect substantially different amounts of bedload because of only slight modifications in design. In this study, sample weights collected by three types of pressure-difference samplers are compared to determine whether differences are statistically significant or whether sampler performance is so irregular and overlapping that one might regard them as being the same. The results confirm that the weights of samples collected by the devices are significantly different. Generally, the US BLH 84 collected less material, the Sheetmetal Helley–Smith collected more material, and the Original Helley–Smith was intermediate; these tendencies were consistent at two sites where bedload was measured. The implication of these results is that measured transport rates will vary depending on the sampler used and, therefore, they are not directly comparable without some mode of calibration. To place this finding in a larger context, sediment rating curves, determined from weights of samples and measurements of flow, were integrated over available flow records and used to estimate annual yield. Three estimates of annual yield, one for each device, were then compared with measures of annual accumulation from a weir pond below one of the collection sites. The results indicate that despite differences between the devices, data obtained with pressure-difference samplers estimated annual accumulations of sediment reasonably well. Predicted accumulations were within 40–50% of the measured yield for two samplers whereas the third sampler predicted within 80%.     

黄河内蒙古河段非汛期和汛期冲淤量计算方法（英文）

Based on an empirical sediment transport equation that reflects the characteristics of "more input, more output" for sediment-laden flow in rivers, a general sediment transport expression was developed, which can take into account the effects of upstream sediment input, previous cumulative sediment deposition, critical runoff for sediment initiation, and the differences in sediment particle sizes between the mainstream and tributaries. Then, sediment load equations for non-flood and flood seasons for the sub-reaches from Bayangaole to Sanhuhekou and from Sanhuhekou to Toudaoguai, as well as the whole Inner Mongolia reach from Bayangaole to Toudaoguai, were formulated based on data collected between 1952 and 2010. The corresponding sediment deposition and the cumulative values at each river reach were calculated using the proposed sediment transport equations for the period 1952 to 2010 according to the principle of sediment conservation. Comparisons between the calculated and measured values using the proposed sediment load equations for the sub-reaches and the entire reach showed that the calculated sediment load and sediment deposition and the corresponding cumulative values in the flood and non-flood seasons were in good agreement with the measured values. These results indicated that the proposed methods can be applied to calculate the sediment load and the associated sediment deposition in the flood and non-flood seasons for long-term trend analysis of sediment deposition in the Inner Mongolia reach of the Yellow River.     

Predicting fine sediment dynamics along a pool-riffle mountain channel

Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches.Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of 7000 m³ of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations.The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field.A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment.     

Predicting sediment inputs to aquatic ecosystems across England and Wales under current environmental conditions

The awareness of water quality issues has never been higher. As part of its continuing strategic diffuse pollution policy support, ADAS recently undertook to identify catchments across England and Wales that could potentially fail recently proposed suspended sediment yield targets under current environmental conditions. The total suspended sediment loads (SSL) delivered to all rivers were assumed to comprise contributions from diffuse sources in the agricultural and urban sectors, as well as from eroding channel banks and point sources represented by sewage treatment works (STWs). Diffuse agricultural sediment loss to rivers was predicted using the PSYCHIC model. Corresponding inputs from diffuse urban sources were estimated on the basis of an Event Mean Concentration (EMC) methodology. Channel bank sediment inputs were calculated using a prototype national scale model, while point source sediment contributions were based on a register of consented effluent discharges. Modelled SSL were validated (r²=68%) against PARCOM data (1999–2003) for the delivery of sediment to different regions of the UK maritime area. The results of the validation were considered to be realistic for a national scale predictor. The modelling exercise suggested that those catchments currently at risk of exceeding proposed suspended sediment yield critical thresholds are largely confined to upland areas across Wales and northwest England and the chalklands of southern and eastern England.     

Sedimentological and Geomorphological Impacts of The Jökulhlaup (Glacial Outburst Flood) In January 2002 At Kverkfjöll, Northern Iceland

Jökulhlaups (glacial outburst floods) are common hazards in many glaciated environments. However, research on the controls on the sedimentological and geomorphological impact of jökulhlaups is rare. Developing a more comprehensive understanding of flood impacts may be useful for hazard identification, prediction and mitigation. This study determines the controls on the sedimentological and geomorphological impact of a jökulhlaup in January 2002 at Kverkfjöll, northern Iceland. This jökulhlaup, caused by geothermal activity, reached a peak discharge of 490 m³s^?1 as recorded at a permanent gauging station 40 km downstream from the glacier snout. However, reconstructed peak discharges in the proximal part of the jökulhlaup channel near the glacier snout indicate a peak discharge of 2590 m³s^?1. The jökulhlaup hydrograph was characterized by a rapid rising stage and a more gradual falling stage. As a result, sedimentary and geomorphological impacts included poorly sorted, structureless, matrix‐supported deposits; massive sand units; clast‐supported units; ice‐proximal cobbles, rip‐up clasts and kettle‐holes; and steep‐sided kettle‐holes. These features are proposed to be characteristic of rapid rising stage deposition. Additionally, large‐scale gravel bars and bedload sheets prograded and migrated during the rapid rising stage. The development of these bedforms was facilitated by high bedload transport rates, due to high discharge acceleration rates during the rapid rising stage. During the more prolonged falling stage, there was sufficient time for sediment incision and erosion to occur, exhuming cobbles, ice blocks and rip‐up clasts, and creating well‐defined terrace surfaces. This study provides a clearer understanding of hydrological and sedimentological processes and mechanisms operating during jökulhlaups, and helps to identify flood hazards more accurately, which is fundamental for hazard management and minimizing risk.     

Impacts of backwater hydrodynamics on fluvial–deltaic stratigraphy

The hydrodynamics of rivers approaching a receiving basin are influenced by the onset of backwater conditions that give rise to decelerating reach-average flow velocity and decreasing boundary shear stress. These changes occur across a spatial gradient over which decreasing sediment transport capacity triggers morphodynamic responses that include sediment deposition at the transition from uniform to nonuniform flow. As a consequence, the channel width-to-depth ratio and bed sediment grain size decrease downstream. While nonuniform flow and associated morphodynamic adjustments have been investigated in modern fluvial–deltaic systems, the impacts to fluvial–deltaic stratigraphy remain relatively unexplored. This represents an important unresolved gap: there are few contributions that link morphodynamic response to nonuniform flow, impacts on sediment deposition and influence on the rock record. This study uses a numerical model to explore how variable channel hydraulics influence long-term (1000s years) patterns of sediment deposition and development of stratigraphy. The model results indicate that: (a) nonuniform flow propagates upstream beyond the backwater transition that is traditionally estimated with a basic backwater length scale relationship. (b) Base-level fluctuations, especially rising, enhance the impact of nonuniform flow. (c) Sediment deposition shows large spatio-temporal variability, which ultimately contributes to unique stacking patterns of fluvial–deltaic stratigraphy. (d) Nonuniform flow imparts spatial variation in flow depth, channel bed slope and sediment grain size over the delta, and these signatures are potentially preserved and recognizable in the rock record.