FLOOD04816

• County: Statewide
• Basin: Statewide
• Title: Dams and Rivers A Primer on the Downstream Effects of Dams
• Date: 6/1/1996
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

Imperial Dam on the Colorado River
upstream from Yuma, Arizona

But the age of widespread, large.structure dam.building awaited the
arrival of heavy machinery and the high ambitions of industrialized societies.
In the United States, the pace of large dam construction hit its quickest stride
between 1935 and 1965 (Thomas. 1976). In the West. Hoover and Grand
Coulee were completed before World War II; Glen Canyon Dam was finished
in 1963. In the East. creation of the 26.dam Tennessee Valley Authority
system ushered in an era of building dams and managing reservoirs inte-
grated over an entire basin (Cullen. 1962). Currently. there are more than
75,000 dams higher than six feet in the United States; the reservoirs behind
these dams cover about 3 percent ofche nation's land surface (R.F. Stallard,
oral commun.. 1994). Worldwide. 193,500 square miles (mi') of land is
inundated by reservoirs. Now in a given year. 60 percent of the United States'
entire river flow can be stored behind dams (Hirsch and others, 1990). In the
dry American Southwest, dams on the Colorado River can store four years of
typical flow (Andrews, 1991). More dams are being built in developing
countries. but in the United States. Canada. and Western Europe only a few
potential sites for large dams remain under realistic consideration.
Have we benefitted by building these dams? Viewed in one dimension.
the answer is a resounding "Yes:' Hydroelectric powerplants harnessing the
Columbia River and its tributaries produce 75 percent of the American
Northwest's electricity (Palmer. 1991). Each year, 8.2 million acre.feet of
water are diverted from the lower Colorado River to homes and farms in
California. Arizona and Mexico through aqueducts that cross hundreds of
miles of intervening desert. None reaches the gulf of California. Since dams
were built across rivers in the Connecticut River Valley. no floods have
occurred like the ones that killed 108 people and crippled the towns of
Bolton and Hartford in 1927 and 1936 (Leuchtenburg. 1953).
In the simplest sense. we build dams for the same reason we wear coats
in the winter: to exert control over an aspect of an environment that would
otherwise make living difficult or even impossible. If a valley is subject to
destructive flooding. we dam its river. If the desert is dry. we build a lake. The
list of available reasons for building a dam is long and complex - for water
storage to quench municipal. agricultural. and industrial thirsts; for flood
control and improved navigation; for sediment trapping; for improvement of
water quality; for electrical power generation; for recreation, aesthetic. and
wildlife considerations.
As dams became bigger and more expensive, a wider array of benefits
was needed to justify the cost of dam construction. Most dams built after
1950 had many purposes. and sometimes these purposes were in competi.
tion with each other. Glen Canyon Dam in Arizona was initially conceived as
a tool to balance the water allocations between the upper and lower basin
states of the Colorado River (Ingram and others. 1991). But with an initial
price tag of $325 million. construction of the dam needed additional justifica-
tions. Water conservation. downstream distribution. and hydroelectric power
were written into the dam's operation considerations. Recreation and flood
control subsequently were added to the dam's operating criteria.

2