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The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event. [ABSTRACT FROM AUTHOR]

Rivers are the most effective agent of erosion on earth, transporting massive amounts of detrital and dissolved matter into depositional basins. Therefore, the relationship between denudation of continents and the pathways of sediment transport by large rivers has been extensively examined (e.g., sediment flux gauging, basin-wide erosion rates, recognizing sediment source and mixing). However, due to the complex nature of sediment storage and transport dynamics in large-scale fluvial systems, our understanding of the seemingly simple relationship between erosion and deposition in large rivers is limited.In large-scale fluvial systems, where sediment is stored for extended periods, stable cosmogenic nuclides are a useful tool for quantifying the time sediment spends exposed during fluvial transport. We examined changes in cosmogenic Ne concentrations along an ~450 km section in the lower basin of the modern Colorado River. Sediments along the river show no clear trend between Ne concentration and distance down the river. Nevertheless, the variability in Ne concentrations can be translated to the maximal time the sediment was exposed during its transport down the lower Colorado River (~120 kyr).Applying these understandings to an ancient river, we collected Lower Cretaceous fluvial sediments from 5 locations along an ~400 km transect in the general downstream direction of the Lower Cretaceous Kurnub River. This river flowed across the passive margin of northern Gondwana into the Tethys Sea. Both variability and maximum duration of exposure (up to ~130 kyr) of sediments in the Kurnub River are similar to the Colorado River.These results, combined with the residence time of sediments in large modern rivers deduced from U-series isotopes, demonstrate that throughout the geologic record large rivers show a similarity in sediment dynamics expressed by large variability in sediment residence time ranging up to several hundreds of thousands of years. [ABSTRACT FROM AUTHOR]

The landscape development in the Kosi River basin in the central Himalaya is strongly influenced by large tectonic and climatic gradients. Dynamic behaviour of the Kosi River in the alluvial part has been well-documented and is strongly linked to high sediment supply from the upstream hinterland area. Hence, reach scale erosion mapping in the hinterland area is a major issue, which can be tackled by highlighting sediment generating areas within the Kosi River basin. Sediments generated through hinterland erosion are routed downstream and spatially distributed in lower reaches of the Kosi River. The spatial distribution of energy available for sediment transport is governed by stream power (function of discharge and slope). This sediment transport competency approach has been applied in this paper to unravel erosion at basin/reach scale using spatial distribution pattern of specific stream power (SSP). A physically-based semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT) has been used to simulate the reach scale stream power variability through estimation of channel slope and discharge. Stream power distribution pattern for different months was extracted at reach scale, through SWAT model by utilizing basic inputs such as precipitation, temperature, elevation, land use land cover and soil data. We observed that stream power is higher in channel reaches of the Sun Kosi and Arun. Downstream variation in SSP plot shows that mid-channel reaches in the Higher Himalaya have higher stream power in the Kosi River basin. The results also highlight a significant role of channel slope on stream power distribution pattern. Spatial distribution of slope data shows higher channel slope in western and main channel of the Kosi River that also results in higher stream power. This increased fluvial incision event is manifested in the spatial distribution of sediment yield. Further, since the estimation of SSP incorporates spatial variation in channel width, the peaks in specific stream power shift downstream in comparison to stream power distribution due to spatial variability in channel width. The reaches of high specific stream power and stream power therefore represent the major erosional zones, and therefore this approach can be used as a proxy for sediment erosion to understand sediment dynamics in the Kosi River basin. [ABSTRACT FROM AUTHOR]