Biology Reference
In-Depth Information
Sidescan sonar of the bay floor under the Golden Gate. (Patrick Barnard)
and around the Golden Gate headlands—testimony to the narrowness and
frequent obscurity of this opening into the world's greatest natural harbor.
Indeed, annual averages indicate that fog obscures the Golden Gate for
several hours a day, especially in summer.
In the middle of the Central Bay, however, the bottom warps into a
field of underwater sand dunes. These are not like the diminutive ripples
in the sand often seen on the beach. Central Bay sand dunes can reach 25
feet in height and hundreds of feet in length. And they aren't static: they
move, much like tractor treads, says U.S. Geological Survey (USGS) scien-
tist Bruce Jaffe, who spends a lot of time following sediments around the
bay floor. Depending on the strength of tidal currents and the size of the
sediment, sand grains hop a certain distance along the bottom and then
build up over a slight bump. Once formed, they keep moving back and
forth, like the waves above.
Up on the surface, the Central Bay contains several visible islands—Al-
catraz, Angel, Yerba Buena, and Treasure—and more invisible ones. Four
large rocks—Shag, Harding, Arch, and Blossom—have all been dynamited
over the years to keep them from pricking the hulls of supertankers and
container ships.
The third sub-bay lies to the north between the long span of the Rich-
mond Bridge and the short span of the Carquinez Bridge. Called San
Pablo Bay, or sometimes the North Bay, this compartment empties two
small rivers, the Petaluma and the Napa. Here, the city skylines and dra-
matic topography of the Central Bay fade, and horizons grow more dis-
tant and rural. San Pablo Bay opens wide into shallows created by the
