Sedimentation in the Delta of the Sacramento and San Joaquin Rivers
(abstract from poster presented at the conference, State of the Estuary, 1999, San Francisco)
Randal L. Dinehart and David H. Schoellhamer
U.S. Geological Survey, Sacramento, California
Streamflow from the Central Valley of California has long delivered sandy sediment and fine silts and clays to the delta at the eastern entrance to San Francisco Bay. The U.S. Geological Survey (USGS) is measuring the transport of sands by volumetric methods; the flux of fine sediments from the tributary rivers is monitored by optical backscatterance sensors and calculated using hydraulic data.
The USGS has established a network of 13 sites in the delta and bay at which suspended-solids concentration is monitored. Optical backscatterance sensors are deployed at each site and sampled every 15 minutes. Water samples are collected periodically and analyzed for suspended-solids concentrations, which are used to calibrate the sensors. The transport of fine sediments by river discharge and tidal fluctuation can be tracked past the various river and delta sites to San Francisco Bay. For example, the first high river discharge of the rainy season is accompanied by the highest annual suspended-solids concentrations at Mallard Island because erodible sediments accumulate during low-flow periods in the Central Valley. During this first flush of the rainy season, sediment transport rates from the delta to the bay are maximized by the coincidence of high discharges and suspended-solids concentrations. During non-winter months, delta water discharge is lower and has little relation to suspended-solids concentration.
Riverbed profiling was chosen to obtain volumetric measurements of sand transport instead of resorting to traditional bedload-sampling programs or shear-based transport formulas. Riverbed profiling also documents the dynamism of riverbed channels through repeated surveys of bedforms and long bars and shoals. Bedform profiles have been measured several times per year in the Sacramento River from Garcia Bend to Walnut Grove, in the lower San Joaquin River, and in Three Mile Slough. A Global Positioning System receiver is used to guide the boat over mapped gridlines while a digital sonar device records depth every second in conjunction with the recorded position. Points are separated by 1 meter or less, with depth resolution of 1 to 2 centimeters.
Riverbed profiles show that sand waves are ubiquitous, that the riverbeds are subject to several meters of scour and fill in response to storm flows, and that bedload in tidally-affected reaches can be estimated from bedform movement. Initial calculations of bedform movement along the Sacramento River show migration rates around 2 meters per day, with low-flow transport rates on the order of 100 to 200 tons per day. Bedform profiles of Three Mile Slough show that the largest bedforms migrate south toward the San Joaquin River (right to left in Figure), although minor bedforms move northward in response to tidal flows.
Grids were surveyed in the lower San Joaquin above Three Mile Slough. The location of large bars and the orientation of sand dunes indicate that sediment may move up the San Joaquin River some distance during tidal cycles after delivery from Three Mile Slough.
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