7040

• UCREFRP Catalog Number: 7040
• Author: Miller, R. R.
• Title: Man and the Changing Fish Fauna of the American Southwest
• USFW Year: 1961
• USFW - Doc Type: Papers of the Michigan Academy of Science, Arts, and Letters
• Copyright Material: YES

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Changing Fish Fauna of the Southwest 367
HABrrAT CHANOEs
i The aboriginal habitats have become modified in various ways.
There has been a shift from clear, dependable streams to those of
intermittent flow subject to flash floods that carry heavy loads of silt.
As a result of loss in volume and destruction of vegetation, there has
been a trend toward rising temperatures in the surviving waters. The
smaller creeks, springs, marshes, and lagoons have disappeared, due
in part to severe lowering of the water table. There has been destruc-
tion of trees, grasses, and aquatic plants; pollution from industrial
and domestic wastes; deep channeling (arroyo cutting) of stream
r beds; and gully erosion on bare hillsides. Examples of these types of
a modifications are considered in this section.
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a COLORADO RIVER BASIN IN ARIZONA
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Lower Colorado River.-The lower Colorado River, from the Lake
Mead area to the Gulf of California, flows through a desert region of
low plains and narrow valleys broken by short mountain groups. The
source of nearly all of its water is in the high mountains of the upper
basin. In its natural condition the river was swift, silt-laden, and sub-
ject to spectacular fluctuations in volume and turbidity, annually
carrying thousands of tons of silt into the Gulf of California. Its maxi-
mum flow usually was attained in June, diminished greatly in August,
and was very low from September to February. At Yuma, Arizona,
a peak flow of 250,000 c.f.s. occurred in 1916 and a minimum daily flow
of only 18 c.f.s. was recorded in the drought year of 1934 (Dill, 1944,
p. 123). With the completion of Hoover Dam in 1935, control of the
river began. The succession of dams lying between Lake Mead and
Yuma has transformed the Colorado into a series of deep, placid
reservoirs with connecting channels of clear water and regulated vol-
ume. The water temperature has been strikingly modified. Originally
it varied from about 50° to 90° F. or more annually. Now, below
Hoover Dam and Davis Dam the fluctuation is from 54° to 62° F.,
providing temperatures suitable for the development of an important
trout fishery. Below Parker Dam, however, the water is clear but
warm. Dredging of the river channel has further modified aquatic
t habitats by the elimination of marshes, lakes, and sloughs and by
producing a straight sluice that is poor in food and cover and with
waters of relatively high turbidity (Beland, 1953a). Introduction of
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