Biology Reference
In-Depth Information
cattle-grazed grasslands of Contra Costa County “are like walking on con-
crete. The water runs off the landscape like it's practically pavement.”
Settlers made conditions worse by diking the marshes that once ab-
sorbed overflows. Then urbanization began paving over soils, leaving
water fewer places to slow and seep into the ground.
Overloaded creeks incised (cut deeper) or burst their banks to accom-
modate the torrents from storms, setting of a domino effect of further
erosion, bank undercutting, and channel deepening. This damage sent the
network of trees and other riparian plants that once held the soil in place
tumbling downstream. Stream waters filled with dirt and debris, muddy-
ing fish spawning habitat and clouding the bay. Alarmed at their crum-
bling stream banks, both individuals and communities sought to armor
creeks against the water's assault.
By the twentieth century, local flood control districts and the Army
Corps of Engineers were routinely demolishing natural stream reaches in
favor of earthen or concrete channels, which were thought to reduce the
risk of flooding to neighborhoods and businesses built on natural flood-
plains. Smaller versions of the Los Angeles River setting for car chases in
movies like
Grease
, these channels were designed to deliver water down-
stream in the fastest, most efficient manner possible.
Unfortunately, poorly planned projects that put creeks in artificial
channels and armor their banks tend to worsen rather than prevent flood-
ing. Development has virtually eliminated places for rivers to spill over
their banks and spread out. The construction of dams and reservoirs at the
headwaters of local creeks has further altered water flow patterns. In a
storm, natural creeks experience a gradual increase in flows that peter out
over many hours. But creeks encased in artificial channels swell rapidly to
torrent strength in heavy rains, then slacken to nearly average levels within
a brief span of time. Gone is the “sustained tail” seen in natural creek flows
that may have mediated flooding, pushed out channel obstructions, and
rinsed the mud from gravel beds used by spawning salmon and trout.
Summer conditions along creeks are equally distorted from their origi-
nal state. The region's Mediterranean climate, marked by a long stretch of
rainless months, once dried up many creek reaches from midsummer to
late fall. Now, many previously intermittent streams receive reservoir re-
leases (for fish) and urban runoff all summer long—remodeling vegeta-
tion and stream ecology.
“The Bay Area is very diverse in terms of geology, rainfall, and climate.
So it's not a surprise that streams would look so different in different
places, but this variety is masked by urbanization and homogenization.
Every stream winds up being, at best, a narrow channel with some trees
along it,” says historical ecologist Robin Grossinger of the San Francisco
Estuary Institute.
