Next consider the salinity distribution. The largest 
freshwater inflows occur in winter and spring. The lowest 
salinity occurs in the upper end of the estuary, in the northern 
reach. You can see this in Figure 2. The first contour on the 
right is for two parts per thousand. You can see salinity is 
depressed into San Pablo Bay, due to the high freshwater in¬ 
flow. In this case, Suisun Bay is becoming more like a river 
than an estuary. 
The salinity near Golden Gate is depressed slightly while 
that in South Bay remains fairly high, again attesting to the 
fact that there is little freshwater inflow into South Bay. 
Typically in summer, you will find that the salinities are very 
much raised in the upper estuary because of the reduction of the 
freshwater inflow. 
I'm going to start at the north end of the estuary, Suisun 
Bay, and come down to Golden Gate, giving an overview of the 
circulation, and then touch upon South Bay. In the upper end of 
the estuary, Suisun Bay, the river input comes from the Delta, 
the confluence of the Sacramento and San Joaquin Rivers. As the 
rivers comes out through Suisun Bay, you can see the sediment 
pattern as it flows out between the islands and passes the 
reserve fleet on the northwest shore. 
There are two types of circulation. One is a horizontal 
circulation pattern that is more or less uniform in depth and is 
driven by the freshwater flows and by tidal effects. Then in 
the vertical, there is a circulation with dense saltwater 
intruding on the bottom and freshwater flowing out on the 
surface. This is called estuarine circulation. Each of the 
different kinds of circulation causes a different kind of 
mixing. In the case of Suisun Bay, there is a net outflow due 
to freshwater inflow. Because of the small residual tidal 
effect, there is also a net counterclockwise circulation, 
through the islands and up the main channel. And because of the 
estuarine circulation, there are also density currents coming up 
the channel. Figure 3 is a schematic representation of how the 
currents flow. Look in particular at the one called "water 
flow". What you can see is the river coming in from the left, 
more or less uniformly with the depth, while the saltwater flows 
from the ocean on the bottom. There is a mixing zone, which 
usually occurs somewhere in Suisun Bay. There is some mixture 
of this fresh- and saltwater which then flows out as a surface 
layer. In San Francisco Bay, because there's so much tidal 
energy, the water column is well mixed in the vertical. There 
is a horizontal decrease in salinity going up estuary, and you 
still see an estuarine circulation. 
26 
