David H. Schoellhamer and John C. Warner
U.S. Geological Survey,
Placer Hall, 6000 J Street, Sacramento, CA 95819
Suspended sediment affects the ecosystem in Suisun Bay by limiting light availability and photosynthesis, carrying food to benthic filter feeders such as Potamocorbula amurensis, and providing a transport pathway for contaminants. One objective of freshwater flow management (i.e., the X2 standard) is to maintain a turbidity maximum, a zone where suspended sediment accumulates (locally known as the entrapment zone), adjacent to the broad, shallow waters of Suisun Bay. Thus, suspended-sediment dynamics is an essential component of the U.S. Geological Survey Suisun Bay study.
To better understand processes affecting suspended-sediment concentration (SSC), continuous measurements of SSC, hydrodynamics, and/or meteorology were made at 19 sites in Suisun Bay from August 1999 to June 2000. The sites were concentrated in the shallow waters of Grizzly Bay and in the adjacent deep tidal channel.
Many factors affect the spatial and temporal variability of SSC in Suisun Bay, including bathymetry, salinity, spring/neap tidal cycle, wind, erodible sediment supply, freshwater flow (Knowles, this volume), watershed disturbance (Jaffe, this volume), and the semidiurnal tidal cycle. When salinity is present, a sill in the channel adjacent to Grizzly Bay is the terminus of a gravitational circulation cell (Burau, this volume) and the location of a turbidity maximum. In this channel, tidally averaged SSC is determined by the variation of tidal energy primarily by the spring/neap tidal cycle. Energetic spring tides resuspend bottom sediment and prevent vertical stratification of the water column, a process that promotes deposition. Thus, spring tides increase SSC, compared to weaker neap tides. In Grizzly Bay, however, SSC is determined by wind-wave resuspension and the quantity of erodible sediment on the bed. Tidal variability of SSC in the center of Grizzly Bay often distinguishes two water massesÑone that has moved into Grizzly Bay during flood tide from the turbidity maximum in the channel and one that has moved during ebb tide from shallower water in Grizzly Bay where wind-wave resuspension is greatest.