5001

• UCREFRP Catalog Number: 5001
• Author: Nicola, S. J.
• Title: Fisheries Problems Associated With the Development of the Lower Colorado River.
• USFW Year: 1981.
• USFW - Doc Type: \
• Copyright Material: YES

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Nevertheless, it was only a matter of time before even larger water
development projects were undertaken. In 1928, the U. S. Congress
authorized legislation and appropriated funds to begin construction of what
is now known as Hoover Dam. Completed in 1936, it has an active storage
capacity equivalent to twice the annual discharge of the entire Colorado
River. Two more storage reservoirs soon followed: Parker Dam (Lake Havasu)
and Davis Dam (Lake Mohave) in 1938 and 1946, respectively. Hoover Dam
serves primarily to generate electricity and store winter runoff for
release into the downstream reservoirs. Water is diverted from Parker
Dam to the greater southern California metropolitan area where it now
provides a full or supplemental supply for nearly 11 million people.
Several smaller agricultural diversion dams have also been constructed,
including Morelos Dam below Yuma, so that at present eight major structures
(including Hoover Dam) span the river below Lake Mead.
Changes in the River
As is the case with most rivers in the north temperate zone, there is
usually a complete change following impoundment in-the temperature regime
below the dams. This is due to the fact that water is drawn from the reservoir
from the bottom layers, which in deep reservoi=s, remain cool during the
summer. Below Hoover and Davis dams, for example, the water temperature
remains close to 13°C throughout the year,. whereas prior to impoundment it
varied from 10°C to around 30°C. Parker Dam restores the natural temperature
regime to the river somewhat because Lake Havasu is shallower and lacks
the deep layer of cool water.
All three of these dams imparted more profound changes to the river than
their effects on temperature. The peak period of discharge was changed
from winter, as flood flows were brought under control, to the summer, as
stored water was released in large amounts during the agricultural growing
season. With large quantities of water being diverted from the river for
irrigation and domestic consumption, and with the reduction of flood flows,
silt accumulated in the river bed,. reducing the capacity of the river channel
and causing the water table along-the flood plain to rise. Compounding
this problem are the high and fluctuating summertime irrigation releases
which raise the water level as much as 5 feet, scouring the river bed
immediately below the reservoirs. The dislodged silt is deposited in the
channel downstream in areas where the flow decreased such as in backwaters
or at the upper ends of reservoir.
These man-caused changes in bedload hydrodynamics caused two major problems
shortly after completion of the major lower river dams--insufficient drainage
from irrigated fields, and flooding. The town of Needles was flooded by
the rising waters of Lake Havasu soon after the Parker Dam was completed.
Silt had settled out in the river channel in the vicinity of the town,
causing the impounded waters to back upstream farther than planned. While
the construction of Glen Canyon Dam and other large dams upstream from Lake
Mead have since virtually eliminated the threat of natural runoff exceeding
the storage capacity of the reservoirs, the problem of the degrading and
aggrading river channel remains to this day. To cope with this problem,