Biology Reference
In-Depth Information
Carquinez and formed the Carquinez Strait. A clue to the timing of this
breakthrough can be found in the bay mud, where layers of volcanic ash
from a Mount Lassen eruption about 579,000 years ago suggest the time
when the Central Valley began draining through the Golden Gate.
It wasn't until much more recently that the bay itself was formed. Sloan
writes: “About 10,000 years ago, the rising ocean entered the Golden Gate
and began to fill the Estuary basin—advancing at first at a rapid rate of 100
feet per year, and then slowing about 5,000 years ago when glaciers
reached their present size.” Sea level has since been slowly rising, and cli-
mate change promises to accelerate this inland advance very soon. Scien-
tists say global warming will create new bays farther inland, and perhaps
one day a new inland lake like the one that once filled the Central Valley
(see p. 285, “Climate Change and the Bay's Future”).
Regardless of sea level rise and fall, the bay has remained relatively
shallow over the last few centuries. As part of the United States' earliest
attempt to define its navigable waters, the Coastal Geodetic Survey first
mapped the bay floor in the 1850s using a boat, lines, and a sextant. Sur-
veyors would pick points on the shoreline, and sight through the sextant
for positioning, using this highly accurate instrument and three points to
triangulate their location. They'd throw a lead-weighted line out in front of
the boat, catch up to it, and take a reading when vertical. Then they'd pull
it up and do it again. In shallower areas of the bay, surveyors would stick
poles in the water and measure to the quarter foot.
These 1800s depth measurements were actually more accurate than
those gleaned today. Early surveyors reviewed their data every night, and
if they saw a discrepancy between two independent readings, they'd go
back the next day and redo their work. Scientists today tow a sidescan
sonar device behind a boat that bounces multiple beams of sound of the
bay floor. The data produced go straight into a computer and don't get
checked until months later, when the surveyors are more likely back in the
lab than on the boat. As such, historic bathymetric maps can be more ac-
curate than modern ones.
Beneath the surface, the bay today is a blend of unusual topography and
marine and riverine waters. But its most defining feature is that it is an
estuary. The most telltale sign of the presence of estuarine conditions is the
change in salinity as the water flows from inland out to the ocean. Up-
stream in headwater rivers and creeks, the water is fresh, with the amount
of salt in the water measuring about 0.5 parts per thousand (ppt). By the
