Investigating Causes and Consequences of 150 Years of Channel Morphology Evolution in San Pablo Bay, California

The Delta is an area where rivers draining the Central Valley and Sierras of California, including the Sacramento and San Joaquin Rivers, meet before discharging into the northeastern end of the San Francisco Estuary. San Pablo Bay, a sub-embayment in the northern Estuary, is circular with an area of about 250 km2 and an average tidal range of about 1.5 m. It is rather shallow (depths generally less than 4 m, average depth <2 m) and muddy apart from the main channel that transects the Bay from East to West conveying the river flow seaward. Bathymetric surveys from the 1850s to 1980s on a 30-year interval show that the morphology of San Pablo Bay has changed markedly since the Gold Rush. Deposition of more than a quarter billion cubic meters of hydraulic gold mining debris reduced the average depth of San Pablo Bay by 85 cm in the middle and late 1800s. In the late 1900s the intertidal flats narrowed and the major channel in the Bay deepened as more sediment was lost to the sea than entered from rivers. Processes of sediment redistribution caused the main channel to become narrower as well, a trend observed over the last 150 years. It is not clear what is causing the change in channel geometry and the implications of the change in geometry on the seaward transport of sediment through San Pablo Bay. This study investigates the cause of this channel geometry development and its impact on the conveyance of sediment through and distribution within San Pablo Bay using a process-based, numerical model (Delft3D). The Delft3D model developed for this study is a 3D model that includes the k-ɛ turbulence model, wind, waves, multiple mud and sand fractions and salt-fresh water density differences, as well as schematized tidal and river flow boundary conditions. The approach is to perform different runs with equal forcing on different historic bathymetries. By keeping the bed in a fixed, non-erodible state, we can analyze the impact of the evolving San Pablo Bay morphology on the conveyance efficiency of water and sediments. Model results show what happens with sediment supplied by the Sacramento River and San Joaquin River as well as the behavior of different sediment classes on different historic bathymetries and allow for further analysis of the governing mechanisms in sediment redistribution. Estimates are given on the impact of sea level rise and reduced upstream sediment supply on the sediment transport through San Pablo Bay. A final research question to be answered is whether or not, over the last 150 years, the San Pablo Bay morphology has evolved to a state in which water is being discharged more efficiently by lowering shear stresses and decreasing friction to allow better conveyance of water in the tidal prism.