months for summer periods. The residence times for South Bay 
are fairly long (on the order of several months) during summer, 
and typically shorter (less than a month) during winter when 
density-driven exchanges occur. 
The subject that I would like to deal with is circulation 
and mixing in the San Francisco Bay estuary. From an overall 
perspective, perhaps, the physics of an estuary is not quite as 
glamorous as the chemistry and biology. Nonetheless, it is a 
very necessary foundation on which distributions in chemistry 
and biology rest. 
The U.S. Geological Survey initially became involved in 
studies of San Francisco Bay in approximately 1969 through 
investigations in marine geology. They did one of the first 
things to study water movements; that is, they dropped a bunch 
of drifters into the Bay to see where they would go. Immedi¬ 
ately, they were involved in controversy because their findings 
had impact upon possible water deliveries by the California and 
Federal water projects. Since those controversial beginnings, 
we have expanded into a comprehensive research group studying 
aspects of the physics, chemistry, and biology of San Francisco 
Bay. 
What I would like to do now is take you through the present 
understandings of the circulation and mixing in the estuary. 
In the beginning, there was the Ice Age, which had direct 
implications for San Francisco Bay. The most notable thing was 
that water held as ice led to the lowering of sea level by as 
much as 100 meters or so. At this time, the Bay was a river 
plain, cut by several river channels. As sea level rose, the 
drowned river valley became t:he Bay and the relict river chan¬ 
nels became the shipping lanes. 
What I would like to bring to your attention in Figure 1 is 
the deepest part of the Bay. You will see immediately that 
there is a channel that comes entirely down the northern reach 
of the Bay and out through the Golden Gate. There is also a 
channel that starts in the southern end and goes out through the 
opening at the Golden Gate. These are relict river channels. 
Most of the freshwater flow comes down the northern reach -- 
about 90 percent of the total flow into the Bay. The channel 
left from the last ice age made this area; this area was dry at 
one time. The ocean shore was out past the Golden Gate on the 
continental shelf. 
The shoals contrast with the rest of the Bay. The depth is 
of the order of a few meters in the shoals and up to 10-15 met¬ 
ers in the channel, except near the Golden Gate where the depth 
is about 100 meters. 
The bathymetry has a profound affect on the circulation and 
mixing. High current speed tends to occur where the water is 
23 
