8155

• UCREFRP Catalog Number: 8155
• Author: Minckley, W. L.
• Title: Executive Summary
• USFW Year: 1985.
• USFW - Doc Type: Native Fishes and Natural Aquatic Habitats in U.S. Fish and Wildlife Service Region II West of the Continental Divide.
• Copyright Material: NO

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structures regulate the mainstream and others have been constructed on
major tributaries. Flooding ha.; been eliminated e:{cept in period.::. of
unusual runoff, silt and sand are caught by quiet waters of reservoirs, and
:~mperatures are ame1 iorated b;1 hypol imnetic relea.~es of ~oJater -from
:-:;idroelectr:c dam~. (Dolan et 3.1. 1974; ~1jndle/ 197?a; iurnerand V:<.r;:::::':3.1<
1982). Biological characteristics. of the lower Colorado Ri'.Je!'" ha'.Je :,een
c~anged e\Jen more .~pectac!Jlarl;' (Dill 1944; ~1il1er 19,51; ~1incKley 1973~
:979a, 1982; Nicola 1979; Carothers and ~.~inckle/ :'?81; and others::'; th~
~ative fish fauna has been essentially replaced by introduced species.

Reaches downstream from Grand Canyon

Much of the watercourse downstream from LaKe Head is channel ized,
dredged, rip-rapped, diKed, or otherwise directly altered from the original
state. Harnessed water resources are used for irrigation, generation of
hydroelectric power, municipal and domestic water supp1 ies, and recreation,
and impounding structures also function in flood control. Under average
watershed conditions the entire volume of flow is withdrawn several times
for use before all water is ultimately consumed. Progressively smaller
downstream flows are maintained by return of agricultural and domestic
wastewaters, seepage, and tributary inflow, until no surface flow reaches
the Sea of Cortez in periods of average precipitation.

Altered flow regimes, increased water clarity, and creation of large
reservoirs ~ echulon alternating with cold tailwater reaches, are habitats
heretofore unKnown to the lowermost Colorado River downstream from Grand
Canyon. These laKes and interconnecting streams provided suitable homes
for a variety of introduced fishes such as trouts, common carp, a number of
catfishes, mosquitofish and other poeci1 i ids, sunfishes (including large-
and sma1lmouth basses), striped bass, and tropical cichl ids in ~oJarmer
places. Concommitant with physical-chemical alterations and estab1 ishment
of an exotic fauna was a dramatic decl ine in abundances, diversities, and
distributions of native fishes. Once plentiful species such as C01orado
squawfish, bony tail , razorbacK sucKer, and desert pupfish are extirpated or
reduced to tiny fractions of their former populations in only a few places.
Other natives of lesser historic abundance, woundfin, roundtail chub,
flannelmouth sucKer, are gone. Only marine forms sporadicallY entering
from the Gulf of Ca1 ifornia continue to represent the former native fauna
at any level of abundance.

Modifications of the lowermost Colorado River mainstream are so vast
and the system is so large that most reaches now may be unsui table for
reclamation for management of native fishes. Only the 101)Jermost portion of
the reach below ParKer, Arizona, has yet to be severely modified, and plans
have long e:<isted for channelization and installation of control structures
in that area. On the other hand, we do not Known why native big-river
fi shes disappeared, and if it resul ted from an event or events now passed,
the;- might I,'Jel1 re-es~abl ish i": d.,)O\ila.ble to colonize the area. Calorac::)
SGuawfish from Willow Beach and De~ter NFHs stocKed in the upper Colorado

River bas;n h~ve survived at least three ;ears (Valentine :983 I ~n~ nc

prep?r'",:::-, :-

~ere ~ade prior to their re-introduc~:on. Natural

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