A review of circulation and mixing studies of San Francisco Bay, California

Abstract

A description of the major characteristics and remaining unknowns of circulation and mixing in San Francisco Bay has been constructed from a review of published studies. Description of net circulation and mixing over a few days to a few months illustrates best the interactions of major components. The discharge of the Sacramento-San Joaquin Delta into Suisun Bay is approximately 90% of the freshwater inflow to San Francisco Bay. Annual delta discharge is characterized by a winter season of high runoff and a summer season of low runoff. For the period 1956 to 1985 the mean of monthly discharges exceeded 1,000 cu m/s (35,000 cu ft/s) for the months of December through April, whereas for July through October, it was < 400 cu m/s (14,000 cu ft/s). The months of November, May, and June commonly were transition months between these seasons. Large year-to-year deviations from this annual pattern have occurred frequently. Much less is known about the ocean-bay exchange process. Net exchanges depend on net seaward flow in the bay, tidal amplitude , and longshore coastal currents, but exchanges have not yet been measured successfully. The bay is composed of a northern reach, which is strongly influenced by delta discharge, and South Bay, a tributary estuary which responds to conditions in Central Bay. In the northern reach net circulation is characterized by the river-induced seaward flow and a resulting gravitational circulation in the channels, and by a tide and wind-induced net horizontal circulation. During low delta discharges South Bay has nearly the same salinity as Central Bay and is characterized by tide and wind-induced net horizontal circulation. In the northern reach a nontidal current null zone moves rapidly seaward in response to increases in delta discharge, and after runoff events returns landward over a few months. During the low-discharge period the northern reach achieves an approximate salt balance in two to three months. When gravitational circulation penetrates these zones during high discharges, the mean residence time probably is reduced to less than a month. (Lantz-PTT)