Properties and circulation of San Francisco Bay waters

Abstract

Differences in river and waste-water inflow and wind stress create contrasting environments and dissimilar distributions of properties between the northern and southern reaches of the San Francisco Bay system. A conceptual framework describing the physical processes which control these distributions, although still incomplete, is outlined. The northern reach receives 90% of the mean annual river inflow and 24% of the waste-water inflow. It changes from a partially mixed estuary, with a vertical salinity gradient of 10 ‰ during high river inflow, to a well-mixed estuary with a vertical salinity gradient of 3 ‰ during low summer inflow. The southern reach also has seasonally varying water properties. There the variations are determined by water exchange from the northern reach and the ocean and by direct waste inflow (76% of total Bay waste inputs). Salinity stratification is present during winter, whereas during summer the water is nearly isohaline because of wind and tidal mixing. Our knowledge of transport mechanisms is fragmentary. The northern reach has a permanent estuarine circulation cell that is largely maintained by the salinity-controlled density differences between river and ocean waters. Although wind variations alter this circulation, it is largely modulated by the timing and magnitude of the highly seasonal river inflow. This nontidal circulation is nearly equivalent to tidal diffusion in controlling the water-replacement rates in the channels, which vary from weeks (winter) to months (summer). The southern reach, in contrast, has seasonally reversing but sluggish near-bottom and surface nontidal currents that are generated by prevailing summer and episodic winterstorm winds and by winter flows of Delta-derived lowsalinity water from the northern reach. Although the diffusion of substances by the strong tidal currents is notable, the relative importance of diffusion by strong tidal currents and the episodic advective processes in controlling water replacement mechanisms and rates has not yet been fully determined. Studies of transport processes in San Francisco Bay, an estuary surrounded by a heavily urbanized area, could now most profitably focus on water-replacement mechanisms and rates because inflowing water dilutes unfavorable anthropogenic substances and flushes them from the system. Of greatest importance are studies defining the effects of river inflow in modulating water residence time, not only because inflow is perhaps the dominant agent in this modulation but also because man is able to control the inflow through massive river diversions. PROPERTIES AND CIRCULATION OF SAN FRANCISCO BAY WATERS T. JOHN CONOMOS U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 Differences in river and waste-water inflow and wind stress create contrasting environments and dissimilar distributions of properties between the northern and southern reaches of the San Francisco Bay system. A conceptual framework describing the physical processes which control these distributions, although still incomplete, is outlined. The northern reach receives 90% of the mean annual river inflow and 24% of the waste-water inflow. It changes from a partially mixed estuary, with a vertical salinity gradient of 10 % o during high river inflow, to a well-mixed estuary with a vertical salinity gradient of 3 % o during low summer inflow. The southern reach also has seasonally varying water properties. There the variations are determined by water exchange from the northern reach and the ocean and by direct waste inflow (76% of total Bay waste inputs). Salinity stratification is present during winter, whereas during summer the water is nearly isohaline because of wind and tidal mixing. Our knowledge of transport mechanisms is fragmentary. The northern reach has a permanent estuarine circulation cell that is largely maintained by the salinity-controlled density differences between river and ocean waters. Although wind variations alter this circulation, it is largely modulated by the timing and magnitude of the highly seasonal river inflow. This nontidal circulation is nearly equivalent to tidal diffusion in controlling the water-replacement rates in the channels, which vary from weeks (winter) to months (summer). The southern reach, in contrast, has seasonally reversing but sluggish near-bottom and surface nontidal currents that are generated by prevailing summer and episodic winterstorm winds and by winter flows of Delta-derived low-salinity water from the northern reach. Although the diffusion of substances by the strong tidal currents is notable, the relative importance of diffusion by strong tidal currents and the episodic advective processes in controlling water replacement mechanisms and rates has not yet been fully determined. Studies of transport processes in San Francisco Bay, an estuary surrounded by a heavily urbanized area, could now most profitably focus on water-replacement mechanisms and rates because inflowing water dilutes unfavorable anthropogenic substances and flushes them from the system. Of greatest importance are studies defining the effects of river inflow in modulating waterresidence time, not only because inflow is perhaps the dominant agent in this modulation but also because man is able to control the inflow through massive river diversions. San Francisco Bay waters are mixtures of ocean, river, and waste waters. The compositions and relative fractions of these mixtures in the Bay change rapidly in space and time with changes in the amounts and character of the source waters and with differences in the depth and degree of circulation and mixing. The constantly changing process of mixing is, in addition, modified by evaporation and precipitation. Copyright © 1979, Pacific Division, AAAS 47