USGS o 23 science for a changing world Summary of Findings About Circulation and the Estuarine Turbidity Maximum in Suisun Bay, California by David H. Schoellhamer and Jon R. Burau Suisun Bay, California, is the (landward) and ebb (seaward) cur­ the tidally-averaged (residual) move­ most landward subembayment of San rents. Tidal currents are strongest dur­ ment of water caused by river inflow Francisco Bay (fig. 1) and is an impor­ ing full and new moons, called spring or wind. Tidal and residual currents tant ecological habitat (Cloern and tides, and weakest during half moons, carry and mix (transport) salt, others, 1983; Jassby and others, 1995). called neap tides. This sloshing back sediment, plankton, and other constit­ During the 1960s and 1970s, data col­ and forth is usually much greater than uents. Saltwater is heavier than lected in Suisun Bay were analyzed to develop a conceptual model of how water, salt, and sediment move within 122°30' 122°00' 121°30' 38°30' and through the Bay. This conceptual model has been used to manage fresh­ water flows from the Sacramento-San Joaquin Delta to Suisun Bay to improve habitat for several threatened GRIZZLY BAY Reserve Fleet and endangered fish species. Instru­ Channel / mentation used to measure water IONKERB AY $ Carquinez SACRAMENTO- velocity, salinity, and suspended- Strait SAN JOAOUIN RIVER DELTA solids concentration (SSC) greatly 5A<- improved during the 1980s and 1990s. The U.S. Geological Survey (USGS) Suisun \ 38°00' Cutoff Mallard has utilized these new instruments to Island collect one of the largest, high-quality hydrodynamic and sediment data sets DELTA available for any estuary. Analysis of these new data has led to the revision of the conceptual model of circulation and sediment transport in Suisun Bay. A primer on estuarine physics 37'30' - Estuarine scientists use many terms to describe the complicated physical processes in estuaries, where freshwater from the rivers mixes with saltwater from the sea. The salinity of freshwater is 0 practical salinity units 10 15 MILES (psu) and the salinity of seawater is 35 0 5 10 15 KILOMETERS psu. The gravitational pull of the sun and moon generates tides with flood Figure 1. Location of study area, Suisun Bay, California. U.S. Department of the Interior U.S. Geological Survey freshwater; therefore, saltier water lected during research cruises. River has significant statistical relation with tends to be near the bottom of estuar­ flow transports suspended sediment many estuarine communities (Jassby ies. The difference in the amount of and other suspended material, such as and others, 1995) and is used as a salinity between the top and bottom of plankton, seaward near the water sur­ basis for regulation of freshwater flow the water column (stratification) can face. Laboratory studies suggest that into Suisun Bay. be great enough to prevent the top and when fine sediment particles from the bottom waters from mixing. Salinity rivers encounter small amounts of is greatest near the ocean and smallest salt, they adhere to other particles New technology near the rivers. This difference in lon­ (flocculate} and sink more rapidly gitudinal salinity (gradient} from the (Arthur and Ball, 1978). These parti­ Technological advances during river to the ocean can cause the tidally cles (floes) descend to near the bottom the 1980s and 1990s have improved averaged currents to flow landward of the water column, where the resid­ our ability to measure water velocity, along the bottom and seaward along ual current is landward; thus, the floes salinity, and suspended sediment. the surface (gravitational circulation) become "entrapped" in Suisun Bay Acoustic Doppler current profilers (fig. 2). The null zone is the region in and form an ETM in the null zone. (ADCP) can measure vertical profiles the estuary where the residual, near- Certain species of plankton and larval of water velocity at 1-meter (or less) bottom, landward current reverses and fish also accumulate near the ETM in intervals every 10 minutes for as long flows in the seaward direction as a Suisun Bay and the western Delta as 3 months (fig. 3). The resulting result of river inflow. In many (Arthur and Ball, 1979). In this estu­ time series of vertical velocity profiles estuaries, the null zone contains an ary, the ETM and the region of can be analyzed to determine how estuarine turbidity maximum (ETM) increased abundances of certain gravitational circulation changes as where SSC and turbidity are greatest. aquatic organisms is known as the salinity and the spring-neap tidal entrapment zone. Increasing river cycle change. Conductivity- flows push the entrapment zone sea­ temperature-depth (CTD) sensors can Existing conceptual model of ward and decreasing river flows allow automatically and continuously mea­ gravitational circulation in Suisun the entrapment zone to move land­ sure salinity at any location and depth Bay ward. Results of water-sampling pro­ in Suisun Bay for several months. grams in the 1970s suggested that the Optical backscattering (OBS) sensors The existing conceptual model of entrapment zone is associated with measure the amount of suspended circulation and entrapment in Suisun surface salinities that range from 1 to material in the water. Output from Bay is based on the aforementioned 6 psu and provided indirect evidence these sensors is converted to SSC with general characteristics of estuaries, that gravitational circulation is calibration curves developed from laboratory studies, water-velocity data responsible for the entrapment zone analyses of water samples (Buchanan from meters deployed during a few (Arthur and Ball, 1978, 1979). The and Schoellhamer, 1996). The OBS tidal cycles, and water samples col­ position of the 2-psu bottom salinity sensors can be deployed with other Profile of net residual current ^__ Net sea ward «- _ _ _ currents^ _ _ _ \^ Net land ward currents Entrapment zone Zone of gravitational circulation Figure 2. Existing conceptual model of the entrapment and null zones, Suisun Bay, California. It has been measured in the lower Sacramento River when local, near-bottom salinities have exceeded about 2psu(Nichol, 1996); It is weakest during spring tides and strongest during neap tides (Burau and others, 1993); and It can occur as landward pulses of water that develop along the bottom at the beginning of flood tides during weak neap tides, when the water column is stratified (Monismith and others, 1996). These observations differ from the existing conceptual model of gravitational circulation. For example, gravitational cir­ culation is not dependent on a particular salinity, it varies in strength in Suisun Bay, and it is altered by the spring-neap tidal cycle. A revised conceptual model (table 1) was developed using results from other studies and quantitative scaling of stratification Figure 3. Acoustic Doppler current profiler (ADCP). and mixing to explain these observations (Burau and others, in press). The revised conceptual model includes the following changes: instruments to continuously measure time series of SSC. These time series can be analyzed to deter­ Gravitational circulation increases with water depth mine how SSC varies with salinity, freshwater flow, wind, and the spring-neap tidal cycle (Schoell- (Walters and others, 1985); hamer, 1996). High-quality, long-term data sets A semipermanent null zone is located near the Benicia from ADCPs, CTDs, and OBS sensors provide the Bridge during spring (Burau and others, in press), and necessary information to better understand the tidal other geographically fixed null zones may be located and seasonal variability of salinity intrusion, gravi­ elsewhere in Suisun Bay, where deep channels become tational circulation, SSC, and entrapment in Suisun shallower in the landward direction (see bathymetry in Bay. Some results of the analyses of these data are fig. 4); summarized below. Gravitational circulation is suppressed by increased ver­ tical mixing during spring tides (Walters and others, 1985); and Revised conceptual model of gravitational The horizontal salinity gradient (rate of change of salin­ circulation ity along the estuary), not salinity, drives gravitational Velocity data collected during the 1990s and a circulation (Hansen and Rattray, 1965). complete review of all historical current-meter data collected in Suisun Bay and the Sacramento River Effect of gravitational circulation on salt and suspended have been used to develop a revised conceptual sediment transport model of gravitational circulation in Suisun Bay. The data suggest that gravitational circulation has According to the existing conceptual model, gravitational the following characteristics: circulation transports salt and suspended sediment similarly, with residual landward transport near the bottom and residual seaward It dominates residual transport in Carquinez transport near the surface (fig. 2). Data from concurrent deploy­ Strait unless freshwater inflows are so high ments of ADCPs, CTDs, and OBS sensors for several months, that the waters in the strait are completely however, demonstrate that salt and suspended-sediment transport fresh (Burau and others, 1993; Monismith are different. Landward pulses that develop along the bottom at and others, 1996); the beginning of flood tides during weaker neap tides greatly increase the residual landward salt flux (mid-August and mid- It is rare in the southern ship channel during October, fig. 5). SSC, however, is smallest during these neap tides the spring, but