Nathaniel Wysocki, Elizabeth A Hajek

The amount of silt and clay supplied to rivers can be a primary control on the form and dynamics of channel networks, and it affects the distribution and interconnectedness of buried fluvial reservoirs. Despite its importance, it is difficult to reconstruct how much fine sediment was supplied to ancient rivers. The presence of silt and clay accumulations in sandy river deposits is often interpreted as an indication of variability in flow conditions due to seasonal stagnation or tidal influence, but it has not been tested whether these deposits can be used to evaluate how much fine sediment was transported in ancient rivers. Here we report results from a series of experiments designed to evaluate how much clay and silt are preserved in sandy riverbed deposits under constant and variable discharge conditions. Our results demonstrate that 1) clay-silt deposits, including drapes and lenses, form under constant high-discharge conditions, 2) the amount of fine sediment recovered from bed-material deposits is higher when more fine sediment is supplied, and 3) the fraction of fine sediment trapped during bed aggradation is higher than what is retained during bypass conditions. These results confirm that fine-sediment accumulations are not unique indicators of variable flow conditions and that the net retention of clay and silt in sandy riverbed deposits may be more indicative of the overall amount of fine sediment supplied to a river.