Impacts of hydrograph shape on sediment transport in a gravel-bedded stream

Natural and managed rivers throughout the world display a wide range of hydrographs that vary in shape from steep and flashy to gradual and consistent, thus influencing the volume and timing of water available for aquatic ecosystems. Although hydrograph shape is an inherent feature of river systems, relatively few investigations have focused on the influence of flow variability on channel grain sizes and bedload transport, and many uncertainties remain with regard to accurately predicting sediment movement during high-flow events. We explored the effects of hydrograph shape on sediment transport volume and grain size distribution in a wadable gravel-bedded stream in California, USA. We tracked the movement of tracer grains marked with passive integrated transponder radio frequency identification tags and collected bedload samples through small and moderate storm events during 2015, a climatically dry year. Bedload flux during each storm event exhibited patterns of clockwise hysteresis such that bedload transport rates on the rising limb of the hydrograph were higher than transport rates on the falling limb. The grain size distribution of the bedload throughout each event also varied, not only with changes in bed shear stress as expected, but with timing within the event, suggesting other factors such as flow accelerations or sediment availability may be contributing to movement. While the larger storm moved a greater percentage of available tracer grains a larger distance downstream, the grain size distribution of transported tracers did not vary significantly between storms suggesting conditions of equal mobility throughout the study reach. The results from this study contribute towards a larger concurrent project that seeks a mechanistic understanding of the influence of hydrograph shape on temporal variations in bedload transport, grain size distributions, and armor persistence.