| An intensive data collection effort was undertaken in a tidal slough network (Napa/Sonoma Marsh Complex) and adjacent riverine channel in northern San Francisco Bay, California, to determine the physical processes controlling circulation patterns of water and suspended sediment. Velocity, water level, conductivity, temperature, and suspended-sediment concentration were measured at 17 sites from September 1997 to March 1998. Data analysis showed that the hydrodynamics of the system is controlled by the development of two types of convergence zones, one driven by barotropic pressure gradients and one created from converging baroclinic pressure gradients.; The slough network is characterized as three separate systems—west, middle, and east. A sill at the entrance to the western system tidally truncates the water levels, preventing a complete tidal range. The eastern system entrance is un-truncated, but asymmetries develop due to friction and off-channel wetland storage. In the middle the east and west asymmetric tidal signals converge to produce a barotropic convergence zone that controls the transport of water and sediment. Higher tidally-averaged water surface elevations on the truncated western side during spring tides create tidally-averaged fluxes of water and sediment to the east. Neap tides create tidally-averaged fluxes in the opposite direction.; The baroclinic convergence zone is created by the phase difference between the currents in two deep tidal channels—Mare Island and Carquinez Straits. The currents in Mare Island Strait turn to flood before those in Carquinez Strait. Therefore, the Mare Island Strait flood will first receive a decreasing salinity from the Carquinez Strait ebb, and then an increasing salinity from the Carquinez Strait flood, creating a local salinity minimum in Mare Island Strait. On a tidally-averaged time scale, converging baroclinic pressure gradients focus on the local salinity minimum, driving a converging near-bed and diverging surface current pattern. This baroclinic convergence zone is shown to have a greater convergence rate than traditional gravitational circulation, and may account for the exceptional rates of sediment accumulation historically observed in Mare Island Strait. |
