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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the government agency given responsibility by Congress to develop the
river, the Bureau of Reclamation, has been dredging and channelizing
various sections of the river since 1949.
As the flooding and drainage problems have been brought under control,
the Bureau has extended its dredging and channelization activities for
the purpose of making more water available for agricultural, domestic,
and industrial consumption. The growth of population and agriculture in
the American Southwest, stimulated by the availability of water and power
provided by the Bureau's dams, has exceeded the supply of water that was
believed needed at the time the projects were authorized by Congress.
Dredging and channelization have probably caused greater changes in river
hydrodynamics and morphology than have. the dams. Silt that is dredged
from the channel is deposited along the shoreline and in backwater marshes,
lagoons, and lakes (Figure 3); unstable banks are lined with rocks;. and
fn areas where the river is judged to be "too wide", current deflectors or
"training" structures are constructed in the river to direct the flow
toward the center of the channel. The Bureau may or may not fill the
area behind the current deflectors with dirt and gravel, but if they do
not, silt eventually accumulates behind them anyway; cattails (Typha sp.)
and other vegetation encroaches; and, in the absence of flooding, the area
eventually forms a stable shoreline "naturally".
Another measure employed by the Bureau to obtain more water for agricultural
and domestic use is removal. of riparian vegetation. Introduced salt cedar
(Tcancrrix spp.) transpire large quantities of water daily. As the cost
of controlling these species have climbed, the Bureau is now attempting
to reestablish cottonwoods (Populus fremontii), once the dominant native
riparian hardwood, with the hope that they will prevent the exotics from
becoming reestablished. The cottonwoods,- which expire much less water
than tamarisk, were decimated in the 1850s and '60s to supply fuel for
paddlewheel steamers that plied the lower Colorado.
Land in the Palo Verde and Imperial valleys is fertile, but high in
mineral content. The standard farming practice in these areas requires
large quantities of water to leach the soils and prevent accumulation of
minerals to levels harmful to crop production.. As a result, the water
in the main channel of the river is steadily increasing in dissolved
solids, and, as might be expected, the concentrations gradually increase
as one moves downstream.
Effects on Fish
As best we are able to determine, it appears that native fishes of the lower
river were able to maintain their numbers despite the inroads made on their
spawning populations by local settlers and farmers. There is some indication
that drought conditions in the early 1930's may have caused a decline_ of
the native fishes in the lower river during this period-(Dill 1944). Unfortunately,
the numbers of these fish was not monitored by biologists or others, so their
status in the lower river can only be inferred. Regardless of whether there
was in fact a drought-induced decline in the early 1930's, it is reasonably