deep, whereas mixing increases with current speed and with de¬ 
crease in depth. 
Next, I'd like to talk about the actual physical processes 
that drive the circulation. These include the inflow of fresh¬ 
water, the propagation of tides, and other sea level variations 
through the Bay, and wind stress on the water surface. The 
first, the one with the greatest controversy, is the amount of 
freshwater inflow into the upper end of the Bay. At the seaward 
boundary there is saltwater, which is relatively dense, ap¬ 
proximately 2 percent heavier than freshwater. By contrast, 
there is freshwater introduced through the Delta, in through 
Suisun Bay, down through San Pablo Bay, and out through the 
Golden Gate. A good conceptual model of this system is one of a 
partially mixed estuary; that is, salt and freshwater mixed in a 
continuous manner from the Delta down through the Golden Gate in 
the northern reach. South Bay is what is called a tributary 
estuary, that is, like an appendage hanging on to the main 
estuary. In fact, the main estuary determines the type of 
circulations and exchanges that go on there. This is very 
similar to the Chesapeake, for instance. Chesapeake Bay is the 
main stem and there are the tributary estuaries such as the 
Potomac. 
The second physical forcing process is the sea level changes 
at Golden Gate. It's important to separate different time 
scales when you're talking about sea level changes. 
San Francisco Bay, unlike East Coast estuaries, is very much 
dominated by tides. If you were to measure sea level and call 
that the signal, you would find that approximately 95 percent of 
that signal is the tide. The system is essentially dominated by 
tides. But the tides go roaring in and the tides go roaring out 
leaving a small average circulation. This little difference is 
what is important to the long-term effects, that is, seasonal 
patterns and the way the biology responds over the seasonal 
cycles. 
So we can consider sea level to be broken into two frequency 
ranges. One is the tidal period variations, which we'll dispose 
of shortly. The other is what we call the low frequency or 
subtidal variations. Now, in perspective, this would include, 
for example, the 10-day period in the weather that blows up and 
down the coast of California on the continental shelf. It would 
involve storms passing through the system, setting up sea level 
and causing sea level changes. It also can be related to the 
local rise of sea level. 
The third forcing function we should talk about is the wind 
stress on the water surface. We have considered the seaward 
boundary condition, mainly sea level, and the landward boundary 
condition, namely the input of freshwater. We must now examine 
the surface boundary condition, the wind stress. 
