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INTRODUCTION
<br />The Colorado River is one of many rivers in the United States where populations of native fish are
<br />endangered and nearing extinction. Currently four federally listed endangered fishes reside in the
<br />upper Colorado River basin: the Colorado squawfish (Ptychocheilus lucius), razorback sucker
<br />(Xyrauchen texanus), humpback chub (Gila cypha), and bonytail (Gila elegans). The former two
<br />species were once abundant in warm-water reaches of the lower Gunnison River and upper
<br />Colorado River (we define the "upper" Colorado River as that segment of the river upstream from
<br />the Green River confluence; historically this segment was referred to as the "Grand River").
<br />Populations of Colorado squawfish and razorback sucker are now very small, however, and they
<br />continue to decline. The reasons for this decline are often cited as (a) competition with non-native
<br />species, (b) changes in water quality, and (c) reductions in the amount of in-stream habitat due to
<br />reservoir operations, flow diversions and channel modifications (USFWS, 1987; Stanford, 1994).
<br />Over 40 species of non-native fish have been introduced into the upper Colorado River basin
<br />(Tyus, 1991), and some introduced species, such as channel catfish (Ictalurus punctatus), present
<br />particular problems for native fishes (Osmundson et al., 1997). Changes in water quality due to
<br />agricultural practices and urban development near Grand Junction are an on-going concern. Two
<br />decades ago, there was much interest in the problem of diffuse source salinity (Laronne and Shen,
<br />1982), but now attention has turned to the environmental effects of heavy metals, such as selenium
<br />(Butler et al., 1993). Finally, there is the issue of reservoirs and diversions: there are 24 reservoirs
<br />with a capacity greater than 5,000 acre-feet (6,168,000 m3) upstream of the Colorado-Utah State
<br />line, and almost as many flow diversions (Liebermann et al., 1989). The reservoirs in the upper
<br />Colorado River basin are relatively small in comparison to other reservoirs in the Colorado-Green
<br />River system, but collectively they alter the annual hydrograph significantly (Liebermann et al.,
<br />1989). Flow diversions have less of an impact on peak discharges, but at certain times of the year,
<br />especially in late summer, these structures can divert a high proportion (> 50%) of the river's flow.
<br />Large diversions have the added impact of blocking fish migration, thereby limiting access to
<br />habitats upstream.
<br />It has generally been assumed that reservoirs and diversions have altered stream flows of the upper
<br />Colorado River significantly, and that this has caused important changes in the amount, diversity
<br />and quality of habitats used by the endangered fishes (USFWS, 1987; Tyus and Karp, 1989;
<br />Osmundson and Kaeding, 1991; Stanford, 1994). It has further been assumed that flows can be
<br />managed to maintain or improve existing fish habitats, and thereby restore self-sustaining
<br />populations of the endangered species. These are reasonable assumptions, given what is known
<br />about the downstream geomorphological and ecological effects of dams (Williams and Wolman,
<br />1984; Lignon et al., 1995; Collier et al., 1996; Stanford et al., 1996), but only recently have
<br />specific studies been initiated to characterize the hydrological and geomorphological effects of river
<br />regulation on fish habitats in the Grand Valley. Osmundson and Kaeding (1991) analyzed changes
<br />in streamflow of reaches of the Colorado River near Grand Junction, CO, and concluded that, in
<br />the period since the upper basin reservoirs were constructed, peak daily discharges of the Colorado
<br />and Gunnison Rivers have been only about half of the long term average. These authors also
<br />speculated that without high spring discharges the Colorado River would continue to narrow and
<br />become more simplified, a point that was supported in subsequent studies by Osmundson et al.
<br />(1995) and Van Steeter (1996). The maintenance of complex, braided-channel reaches is thought
<br />to be a key habitat requirement for Colorado squawfish because adult fish are found more often in
<br />these reaches than in other, less heterogeneous reaches (Osmundson and Kaeding, 1991).
<br />The present study was undertaken to develop a more thorough understanding of historic and recent
<br />changes in the geomorphology of the upper Colorado River and lower Gunnison River, and to
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