Biology Reference
In-Depth Information
“I'm always amazed at the shear at the Golden Gate: what's on the sur-
face goes 20 miles offshore; what's at the bottom goes in. The sucking
power is enormous,” says venerable USGS oceanographer David Peterson,
who worked with Conomos on the early drifter experiments.
Scientists have been tossing similar drifters in the bay and upper estu-
ary ever since, and their results fill in some of the details about why bot-
tom and surface waters behave differently from one another and often
flow in different directions. In some cases, salinity differences in water
masses are responsible; in others, the bathymetry of the bay floor is the
deciding factor.
Fresh water is lighter than salt water. Though the two mix in shallower
portions of the bay, in deeper areas they often flow in opposite directions.
In other words, regardless of tides, a two-layer flow called
gravitational
circulation
occurs: fresh water on the surface flows toward the sea, and
salty water flows underneath upstream, as shown in the sea daisy studies.
As the fresh water flows downstream, gathering ocean waters along the
way, deeper ocean waters move upstream to replace it.
In the upper bays and straits, the fresh water coming down on top
and the salt water coming up along the bottom encounter each other in
much more confined and bumpy conditions than those in the Central
Bay. Studies using current meters conducted by Jon Burau of the USGS
in the 1990s showed that the strength of this two-layer flow is highly
dependent on the shape of the bottom of the estuary. In particular, sills
and shoals (bumps on the bottom) effectively shut down this two-layer
flow, creating localized “entrapment zones,” where sediments and organ-
isms concentrate.
San Francisco Bay has a sequence of these zones associated with sills,
whereas most other estuaries only have one zone where fresh water and
salt water meet. This difference is entirely due to San Francisco Bay's
unique shape and bathymetry, according to Burau. Scientists call their re-
sulting concept of how water moves through the estuary a “sills and cells”
model, with sills being bumps such as the Pinole Shoal and cells being
areas between sills where the two-layer flow is strongest, such as in Car-
quinez Strait (see Figure 2).
Burau developed his concepts by deploying current meters and by
chasing drifters around most of the critical bottlenecks and confluences in
the northern estuary and delta. He is the modern-day Conomos, but the
water movements he's studying are finer than the gross to and fro of water
under the Golden Gate. Burau's drifters look nothing like the sea daisies.
Instead of waiting for someone else to return them, Burau follows the de-
vices in a boat.
Burau's first drifters comprised a plastic dish carrying an early GPS
chip, a big underwater sail, and a tiny flag above the water. “You want min-
