STORAGE AND TRANSPORT OF FINE SEDIMENT ALONG A REGULATED LOWLAND RIVER AND IMPLICATIONS FOR WATER QUALITY

The fate, transport, and storage of fine sediment along lowland rivers affects water quality and can also be used to better understand mobility and storage of contaminants. Predicting mobility of sand and fine sediment through a river system remains difficult. Resolving this issue is important where regulated flow releases are used to flush fines from gravel beds or reduce the loading of sediment and other particulates to downstream receiving waters. We conducted a cataraft-based bedload transport field campaign during the winters of Water Year 2022 and 2023 on the San Joaquin River, California downstream of Friant Dam during moderate flows and flood flow releases to quantify the transport rates of sand and fine sediment and to map changes in sand storage. Preliminary results show that transport of sand and fine sediment is negligible at flow releases of 1500 cubic feet per second (cfs) but transport occurs at flows of 6700 cfs. High flow releases are capable of mobilizing fine sediment stored in banks, floodplains, and the streambed, but are insufficient at flushing fines from a gravel bed during moderate flows. The highest sediment transport rates along the reach studied did not occur where flow velocity was highest. These results raise questions about the efficacy of moderate flows in flushing the sand supplied to the channel, how flow regulations can reduce in-channel sand storage, and how sediment transport affects water and aquatic habitat quality. Direct field measurements across a range of flows aid in improving predictions of the rates and conditions under which transport occurs.