Biology Reference
In-Depth Information
shallows of murky water spread over a muddy bottom, but in a few spots
such as the Central Bay—bounded by the Richmond, Bay, and Golden
Gate bridges—the water clears and deepens over a sandy and rocky bot-
tom.
Moving these waters are visible forces on the surface like winds and
storms, as well as subsurface forces like tides, currents, and differences in
salinity that interact with bottom topography, also known as bathymetry.
In general, water moves in one direction down rivers and creeks, and back
and forth in the lower bays due to ocean tides. Beyond these underlying
basics, scientists have found that the water—not to mention everything in
it—can move in any number of directions, because so many forces are at
work in this complex estuary.
In the last century, humans have greatly changed the estuary's natural
hydrodynamics and bathymetry, straightened and dammed its rivers and
creeks, and filled and diked its shores. More recently, however, those in
charge of protecting California's water supply and fisheries have learned
that the naturally variable hydrodynamics of the bay and its watershed re-
main key to the health of the larger ecosystem.
As estuarine science veteran Bruce Herbold of the U.S. Environmental
Protection Agency once put it, “The system is much more complex than a
bunch of blue lines and blobs on a map.”
Most rivers get wider and deeper the closer they are to the mouth of their
estuaries. This is the geography of a big river like the Mississippi, which
widens and deepens before dumping its water and mud into a vast trian-
gular delta extending out into the sea.
The San Francisco estuary looks nothing like the Mississippi. The bay
empties into the Pacific not through a vast delta hundreds of miles wide,
but via a one-and-a-half-mile-wide keyhole break in the coastal mountain
range, leading
Wikipedia
to label it “an inverted river delta.” The bay itself,
meanwhile, consists of several smaller interior bays rather than one big
curve in the coast.
From downstream to upstream, the estuary looks more like one of
those long balloons sold by sidewalk vendors, twisted and turned at vari-
ous points: open in the ocean, narrow at the Golden Gate, open again into
the Central and San Pablo bays, narrow again at Carquinez Strait, open yet
again into Suisun Bay, then finally narrowing into a labyrinth of river
channels and an inland “delta” miles from the ocean. At each narrowing
