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 Lower Colorado River
With a river elevation at Hoover Dam of 170 m, the average gradient of
the lower Colorado is only 0.3 m/km. Under natural conditions the lower
Colorado meandered through the alluvial valleys of the Colorado Desert
depositing a heavy silt load acquired as it cut through the sandstone
canyons (the most notable of which is the Grand Canyon) of the upper
river. It has been calculated that as much as 130 x 106 m3 of silt per
day was transported by the river past the town of Yuma on the United
States-Mexican border during peak flows.
Unimpaired runoff was seasonal and highly variable. Peak flows occurred
in the winter and spring with low flows in late summer. Maximum and
minimum recorded flows prior to development were 6,796 m3/sec and 0.51 m3/sec,
respectively. The concentrations of dissolved solids varied from around
1,800 ppm during periods of low flow to less than 300 ppm during larger
floods.
Virtually all the flow of the Colorado originates in the upstream areas
aid in the headwaters of the lower tributaries. Rainfall- in the lower
river area varies from 7.5 to 15 cm/year. Thundershowers in both-the
upper and lower river are common during the summer and can suddenly and
temporarily increase the flow and turbidity significantly.
Water temperatures in the lower river ranged from 12.2°C in the winter to
23.8°C in the summer. Winter air temperatures vary from 4°C to 21°C,
while maximum summer air temperatures range from 32°C to 49°C.
Prior to the arrival of white man, some half-dozen Indian tribes occupied
the banks of the lower Colorado River. They hunted small game, such as
rabbits and quail, and large game such as burro-mule deer (OdocoiZeus
hemionus eremicus) and bighorn sheep (Ovis canadensis neZsoni). They
also gathered seeds and raised squash and beans along the fertile flood
plain. Some tribes also fished for the ten-pounder (EZops affinis), a
member of the tarpon family that came up the river from the Gulf of California,
and for some of the larger native fish that could be found in the lower
river, such as the Colorado squawfish (PtyehoeheiZus Zueius) and humpback
sucker (Xz~rauchen texanus).
Native Fishes
Sixteen species of fishes (representing eight families) are native to the
Colorado River. Eight of these are believed to have occurred in the lower
Colorado (Table 2). -Two marine fishes, the mullet and ten-pounder, are
known to have ascended the lower river on brief feeding forays. The four
strictly freshwater forms show remarkable adaptations to life in the main
Colorado. The humpback sucker and the bonytail (Gila eZegans) and hump-
back (G. eypha) chubs show extreme adaptations in body morphology (Figure 2).
One of these morphological features is a nuchal hump, an adaptation that
is thought to allow the humpback chub and the sucker to more effectively
hold their position in strong currents. It is an enlarged muscle mass which,
in addition to acting as a stabilizer, apparently also provides added strength
needed in swimming against the strong currents. Possibly the most significant