Biology Reference
In-Depth Information
ANATOMY OF A WATERSHED
The land that makes up the drainage area of the bay, each of its rivers, and
every creek, is a watershed. To visualize a watershed, place a piece of paper in
your palm and crumple it loosely. Then imagine rain falling into the loose bowl
of paper you've created, and collecting at the lowest point in your palm. The
paper bowl is like a watershed. Place several such bowls side by side, and their
edges represent the ridgelines and hills dividing one watershed from another.
At least 85 watersheds, defined as having a mouth connected to the bay,
border the bay proper, and have a total drainage area of about 3,500 square
miles within the nine-county Bay Area. A region roughly equal to the size
of Puerto Rico, this represents just 6 percent of the drainage of the total
Sacramento-San Joaquin river system. The largest stream in the area by far
is Alameda Creek, which drains 700 square miles of the Diablo Range and
the East Bay hills. Its watershed alone comprises 20 percent of the bay's
total drainage. Second in size is the Napa River in the North Bay, and the
third is Coyote Creek in the south, which drains out the Diablo Range.
Scientists divide San Francisco Bay into four basic sub-embayments (sub-
bays for short), using bridges as the most convenient borders on a fluid
surface (see Map 1, p. 16). Distinct hydrological conditions and geography
are what define these smaller bays, not the bridges themselves.
At the southernmost end of the estuary, between the San Mateo Bridge
and the city of San Jose, lies the lagoon of the South Bay. The Central Bay
is what most people see from their hilltop homes or San Francisco office
towers, or from the three main commuter routes across the water: the
Golden Gate Bridge, the Bay Bridge, and the Richmond Bridge. It's much
deeper and colder in the Central Bay than in other sub-bays, because
ocean tides surge back and forth and big pulses of fresh water roll down
from the mountains.
The waters under the Golden Gate itself are the deepest part of the
bay—they form an underwater canyon dropping 330 feet below sea level.
Here, high-speed tidal currents continually sweep the bay floor clean of
sand and mud, leaving boulders and rock exposed. Down in these dark,
cold, fast-moving waters, few things can linger. The exceptions may be the
engine blocks of historic shipwrecks, Before the advent of modern-day
navigation tools, more than 90 ships crashed into the rocks and sank in
