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<br />K-UlntJy EFFECTS OF. }'i.~TER DEVELOP)::NTS
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<br />Da vi d M. Kennedy
<br />~' Jerry C. Tash ~
<br />ar"1-~."""Ari zona~ Cooperat i ve Fi shery Res ea rch Uni t
<br />. 0 The Uni vers i ty of Ari zona ~
<br />Tucson, Arizona 85721
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<br />ABSTRACT.
<br />
<br />The lower Colorado River was originally one of the most erratic and slit-laden rivers in
<br />the world. Seasonal discharge and water quality ~ere enon~us1y variable, and the rigo-
<br />rous environment limited the numbers and kinds of aquatic species to a few specialized
<br />types. In less than Tive decades, the river was co~letely changed by dam construction
<br />and formation of large reservoirs for hydroelectric power and irrigation, and by channel-
<br />ization, meanders of the original stream bed that were cut off by dikes became man-~de
<br />backwaters, which added to the natural backwa~crs already present along the river. The
<br />lower Colorado River system now consists of three distinct but interacting major habitat
<br />types - reservoirs, inter-reservoir mainstrea~ seg~ents, and natural and man-made back
<br />wa ters ~
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<br />The specialized "endemic biota of the Lower Colorado River and its backwaters has undergone
<br />much change since man modified the river environment. Endemic fishes are rare or extinct,
<br />and entirelY new'biotic communities have developed in each of the major habitats. In an
<br />attempt to increase the recreational potential of the new system, man has introduced ~~ny .
<br />of the fishes he believes are desirable for sport fishing_ Currently, the entire sport
<br />fishery of the Lower Colorado River system is supported by introduced species.
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<br />INTRODUCTION
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<br />The Lower Colorado River system is an important hydrological resource exploited by the
<br />rapidly growing human population in the arid American Southwest. According to the U.S.
<br />Bureau of Census (Newspaper Enterprise' Assoc. 1975), the populations of Arizona and
<br />Californ1a numbered 1,772,482 and 19,953,134 in 1970; these figure~ re!)resent increases
<br />of 36 percent and 27 percent since the 1960 census. To meet increasing water needs ,the
<br />U.S. Water and Power Resources Service (foTiilerly the Bureau of Reclamation) has controlled
<br />the flow of the;Colorado River by constructing several storage reservoirs. The Water and
<br />Power Resources Service releases water from tne reservo; rs pri~~ri ly to meetrequirerrents
<br />for irrigation and for municipal and industrial users (U.S_ Dept; of the lnterior 1964).
<br />Under these operating conditions, water releases cannot always accommodate the ecological
<br />needs of fish_ In addition to controlling river discharge rates, the Water and Power
<br />Resources Se~vice has channelized and stabilized the river to reduce flooding (caused in
<br />part bY"ID2andering of the stream and de!Josition of sediment in the main channel) and in-
<br />crease water salvage. This program red~ced the value of the mainstream as fish habitat
<br />because pools and riffles were eliminated (Beland 1953). Channelization also created faster
<br />currents that deepened the main channel and lcwered the water table, causing the" loss of
<br />many backwater fish habitats.
<br />
<br />The lower Colorado River system between Oavis Dam and the Arizona-Sonora border (Figure 1)
<br />comprises three major types of habitat: large reservoirs. segments of river between
<br />
<br />lCurrent address: U.S. Fish and Wildlife Service, Columbia National Fisherie~ Research
<br />laboratory, Field Research Station, Route 2, Box 5210, Dixon, CA 95620
<br />2Current address: Wisconsin Department of Natural Resources, 3550 Permon Cou11e Road
<br />la Crosse, WI .54601
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<br />CAl-NEVA WILDLIFE
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<br />d~p. /I 0 - I'.;
<br />TRANSACTIONS~1980. ,J
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<br />100
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