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Suspended-Solids Concentrations in San Francisco Bay, California

David H. Schoellhamer, U.S. Geological Survey, Sacramento, California, USA

Figure 1 Suspended solids are an important component of San Francisco Bay, California (USA) because they transport adsorbed toxic substances, provide habitat for benthic organisms, limit light availability and photosynthesis, and deposit in ports and waterways which require dredging. The U.S. Geological Survey has established a network of eight sites in San Francisco Bay at which suspended-solids concentration (SSC) is monitored. Optical backscatterance sensors are deployed at two depths at each site and measurements are automatically collected every 15 minutes. Water samples are collected periodically and analyzed for SSC and the results of these analyses are used to calibrate the sensors. In addition to the continuous monitoring sites which are located in relatively deep water (greater than 10 meters), an instrument package that measures current velocity, water depth, SSC, and wave properties has been deployed in relatively shallow waters (about 3 meters) of the Bay for periods of several weeks to collect data on wind-wave and tidally generated sediment resuspension.

The data are used to determine the factors (such as wind waves, tidal currents, spring/neap cycle, and watershed runoff) that affect SSC in the Bay. Wind waves resuspend bottom sediments in shallow water, and these resuspended sediments are transported to deeper water by tidal currents, especially during spring tides. Stronger winds during spring and summer increase sediment resuspension in shallow water and thus increase SSC throughout the Bay. About one-half the variance of SSC is caused by the spring-neap cycle, and SSC lags the spring-neap cycle by about 2 days. Relatively short duration of slack water limits the duration of deposition of suspended solids and consolidation of newly deposited bed sediment during the tidal cycle, so suspended solids accumulate in the water column as a spring tide is approached and slowly deposit as a neap tide is approached. Winter is the rainy season during which increased watershed runoff from the Central Valley of California transports suspended sediments to the Bay and increases SSC. These results show that SSC is affected by process that occur at several different time scales.

For More Information:

Storm of 1995: Suspended sediment at Mallard Island, San Francisco Bay

Access USGS San Francisco Bay
Napa/Sonoma marsh: A joint study with the UC Davis Department of Civil and Environmental Engineering
Bibliography of continuous monitoring in San Francisico Bay
Summary of Findings About Circulation and the Estuarine Turbidity Maximum in Suisun Bay, California

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