Biology Reference
In-Depth Information
hydrodynamic forces in the system. It rained so hard in 1997 that the re-
sulting flood of fresh water (16 maf in three months) changed the bay as a
whole from 79 percent to 27 percent seawater.
The freshness of the water at any given point in the estuary was once
determined by natural climate changes. Now, it is decided by water man-
agers, who control how much snowmelt and runoff flows out to sea
through the bay. The upper estuary is one of the most managed water sys-
tems in the country, with over half the natural flows diverted into reser-
voirs, aqueducts, and pipelines for human use. What's left to flow down-
stream and mix with seawater is important to many estuarine species.
Little of the fresh water from upstream rivers has made its way down to
the South Bay in recent decades. There the water is so shallow and the
circulation so sluggish that scientists consider the conditions “lagoonlike.”
Only winter and spring storms send enough fresh water downstream from
the Central Valley to influence South Bay salinities. In fact, apart from the
ocean tides and the occasional storm surge from a local creek or river, the
biggest flows into the South Bay come from the plant that treats all of San
Jose's sewage and wastewater.
The two largest rivers delivering fresh water to the bay are the Sacramento
and the San Joaquin. The Sacramento River flows north to south, spring-
ing out of rugged mountain ridges dominated by two active volcanoes:
Lassen and Shasta. The Sacramento's headwaters join the Pit and McCloud
rivers before backing up behind the 602-foot-high plug of concrete in the
river that is Shasta Dam. Below the dam, the Sacramento slows and wid-
ens, meandering across the flats of the great Central Valley and gathering
water from Butte, Battle, and Cache creeks, among others. At Marysville it
merges with the Feather River, and at the state capital with the American
River, before joining the San Joaquin and flowing out to sea. The river
plain in this valley is so flat that natural winter and spring flooding once
lasted for months on end.
The San Joaquin River follows a 742-mile-long bed in the opposite di-
rection from the Sacramento River, flowing south to north. Eight tributary
rivers and 22 streams drain into the San Joaquin, the largest of which are
the Tuolumne, Merced, and Stanislaus rivers. About 20 miles north of
Fresno in the foothills, the flows of the upper San Joaquin are trapped be-
hind Friant Dam, leaving the lower reaches dry until the Merced joins and
rewaters it.
