Biology Reference
In-Depth Information
Jon Burau of the USGS, who may know more about how and where water
flows within the bay than anyone else.
Offshore currents dictate the type of water that tides push into the bay
from the ocean. Local wind patterns exert the strongest influence on what
enters the Golden Gate. Water heading south from Alaska along the Cali-
fornia Current tends to get pushed offshore by Point Reyes. Local winds
force that water in an arc past the Farallon Islands and toward Half Moon
Bay, then northward again in front of San Francisco Bay. This current may
on occasion deliver southerly plankton species and tar balls from seeps in
the Santa Barbara Channel, but more frequently it mixes with cold, up-
welled water from off Point Arena.
Coastal upwelling, a phenomenon that occurs outside the Golden Gate,
influences not only bay hydrodynamics but also the coastal and estuarine
food supply. During upwelling, winds and the California current drive a
giant river of warmer coastal surface water to the southeast. The rotation of
the earth deflects this water offshore as it travels, and colder, deeper waters
rise up to replace it. These upwelled waters are rich in both nutrients and
oxygen, spurring a flurry of photosynthesis and plankton production.
Local ocean upwelling occurs largely in late spring and summer (from
March through August) between Point Arena and Point Reyes. Upwelling
can depress sea level by six to nine inches, forcing the bay to empty by a
proportional amount. When the winds that drive upwelling stop blowing,
the flow reverses and a large surge of water enters the Golden Gate on the
next tide (see p. 66, “The Power of Upwelling”).
The water on the surface of the bay is not always the same as the water
on the bottom. It doesn't all flow out to sea, either, or simply slosh back
and forth with the tides. Scientists had to do quite a bit of work to figure
this out.
They started their experiments back in the 1970s, when most people
assumed that everything in the bay—water, sediment, sewage, trash, dead
bodies—eventually ended up out in the Pacific. But early USGS scientists
suspected the bay's hydrodynamics were more complicated than that, and
set out to prove it.
A team led by John Conomos, the godfather of bay hydrodynamic
studies, climbed aboard their research vessels and airplanes, headed out
into and over the water, and dropped 1,345 drifters into San Francisco Bay.
Half of these yellow plastic disks, each shaped like a saucer and trailing a
