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Summary of Findings About Circulation and the Estuarine Turbidity Maximum in Suisun Bay, California
United States Geological Survey
David H. Schoellhamer and Jon R. Burau

Existing conceptual model of gravitational circulation in Suisun Bay

Existing gravitational circulation conceptual model
Figure 2: Existing conceptual model of the entrapment
and nullzones, Suisun Bay (see next page for further
discussion).

The existing conceptual model of circulation and entrapment in Suisun Bay is based on the aforementioned general characteristics of estuaries, laboratory studies, water-velocity data from meters deployed during a few tidal cycles, and water samples collected during research cruises. River flow transports suspended sediment and other suspended material, such as plankton, seaward near the water surface. Laboratory studies suggest that when fine sediment particles from the rivers encounter small amounts of salt, they adhere to other particles (flocculate) and sink more rapidly (Arthur and Ball, 1978). These particles (flocs) descend to near the bottom of the water column, where the residual current is landward; thus, the flocs become "entrapped" in Suisun Bay and form an ETM in the null zone. Certain species of plankton and larval fish also accumulate near the ETM in Suisun Bay and the western Delta (Arthur and Ball, 1979).

In this estuary, the ETM and the region of increased abundances of certain aquatic organisms is known as the entrapment zone. Increasing river flows push the entrapment zone seaward and decreasing river flows allow the entrapment zone to move landward. Results of water-sampling programs in the 1970s suggested that the entrapment zone is associated with surface salinities that range from 1 to 6 psu and provided indirect evidence that gravitational circulation is responsible for the entrapment zone (Arthur and Ball, 1978, 1979). The position of the 2-psu bottom salinity has significant statistical relation with many estuarine communities (Jassby and others, 1995) and is used as a basis for regulation of freshwater flow into Suisun Bay.

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