Suspended particle transport and circulation in San Francisco Bay - an overview

Differences in the relative magnitude and timing of wind stress and river inflow in the northern and southern reaches of San Francisco Bay create different sedimentary conditions. The northern reach is a partially to well mixed estuary receiving most of the total annual fresh-water input (840 m³ sec⁻¹) and suspended sediment input (4 × 10⁶metric tons) into the bay; more than 80% of the sediment is received during winter. Density-driven nontidal estuarine circulation (~5 cm sec⁻¹) maintains a turbidity maximum which changes seasonally in particle concentration (40 to 80 mg litre⁻¹). Strong tidal currents (≤225 cm sec⁻¹) and wind-generated waves resuspend sediment from the shallow bay floor: some of the riverborne sediment deposited during winter is resuspended during summer and transported landward to the turbidity maximum. Long-term sediment data (extrapolated from bathymetrie charts) indicate that the northern reach is an effective sediment trap. In contrast, long-term sediment data suggest that the southern reach is experiencing net erosion. The southern reach receives little river inflow or riverborne suspended sediment, and the average nontidal circulation is weak (≤2 cm sec⁻¹). The principal source of suspended sediment (25 mg litre⁻¹) in the southern reach is the shallow bay floor (average depth 6 m).