The horizontal circulation is in fact a very effective 
mixing mechanism. The point I want to bring out is that mixing 
is very complicated, much more complicated than most East Coast 
estuaries. When you anticipate or plan modifications in the 
estuary, it is really difficult to say with any certainty what 
is going to happen in more than a qualitative sense. 
We skipped over San Pablo Bay, which is very similar to 
Suisun Bay in that there is still a large horizontal, rotating 
residual current. There are the outflowing currents which occur 
at the surface in the channel. There are density currents going 
up the channel. But the density currents are interesting in San 
Pablo Bay, and they're very similar to those in South Bay. So 
I'll point to this example. 
One of the important features here is the shoal area in the 
center of the channel in San Pablo Bay. The density currents 
that come into the Bay are stronger where the water is deeper. 
This density current moves up through the northern reach of the 
Bay. Because the water is so shallow in the center of San Pablo 
Bay, it can't really sustain this density current. What in fact 
happens is that the density current more or less vanishes on the 
shoal, and then re-forms on the other side of the shoal toward 
Carquinez Strait. 
It's really an interesting feature. You might ask yourself 
how the salt is getting past the shoal. Apparently, it's doing 
this by tidal pumping. That is, on the flooding tide, saline 
water flows over the shoal, into the channel. On the ebb, less 
saline surface water flows out over the shoal. So there is a 
tidally induced exchange over the shoal. 
In South Bay, something very similar occurs. Because South 
Bay is a tributary estuary with little freshwater inflow, the 
freshwater has to come from the north end, from Central Bay. 
For most of the year, South Bay is at oceanic salinity. It's 
just sitting there equilibrated with Central Bay. 
During winter, with the big freshwater flows coming down 
from the northern reach, the salinity in Central Bay is de¬ 
pressed and the water in South Bay then drains out as a density 
current. When freshwater inflows decrease, the salinity starts 
to go up in Central Bay; the water in Central Bay then drives 
back into South Bay as a density current in the opposite direc¬ 
tion. There is again the dispersion mechanism of tidal pumping 
over the shoal. 
In fact, +*hat is the big event of the year in South Bay, 
especir-'.^y for mixing. Figure 4 depicts this. After freshwater 
inflow peaks and South Bay salinity is increasing, there is a 
density current being driven into the channel. But because of 
the tide, when there are spring tides, there's a lot of vertical 
mixing, and the currents are very sluggish and slow. During 
neap tides, when the tidal energy is low, there is less vertical 
mixing and the density currents really pick up speed. You can 
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