ESTUARINE CIRCULATION AND MIXING 
Roy A. Walters 
U.S. Geological Survey 
Abstract 
Tidal-period and low-frequency variations in sea level, 
currents, and mixing processes in the northern and southern 
reaches of San Francisco Bay lead to contrasting characteristics 
and dissimilar processes and rates in these embayments; the 
northern reach is a partially mixed estuary whereas the southern 
reach (South Bay) is a tidally oscillating lagoon (tributary 
estuary) with density-driven exchange with the northern reach. 
The mixed semidiurnal tides are mixtures of progressive and 
standing waves. The relatively simple oscillations in South Bay 
are nearly standing waves, with energy propagating down the 
channels and dispersing into the broad shoal areas. The tides 
of the northern reach have the general properties of a progres¬ 
sive wave but are altered at the constrictions between embay¬ 
ments and gradually change in an upstream direction to a mixture 
of progressive and standing waves. The spring and neap varia¬ 
tions of the tides are pronounced and cause fortnightly varying 
tidal currents that affect mixing and salinity stratification in 
the water column. 
Wind stress on the water surface, freshwater inflow, and 
tidal currents interacting with the complex Bay topography are 
the major local forcing mechanisms creating low-frequency 
variations in sea level and currents. These local forcing 
mechanisms drive the residual flows that, with tidal diffusion, 
control the water replacement rates in the estuary. In the 
northern reach, the longitudinal density gradient drives an 
estuarine circulation in the channels, and the spatial variation 
in tidal amplitude creates a tidally-driven residual circu¬ 
lation. In contrast, South Bay exhibits a balance between wind- 
driven circulation and tidally-driven residual circulation for 
most of the year. During winter, however, there can be suffi¬ 
cient density variations to drive multilayer (2 to 3) flows in 
the channel of South Bay. 
Residence times of the water masses vary seasonally and 
differ between reaches. In the northern reach, residence times 
are on the order of days for high winter river discharge and of 
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