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<br />. <br /> <br />. <br /> <br />6 <br /> <br />natural ecosystem. They pointed out that dams, impoundments, and water-use <br />practices are probably major reasons for drastically modified natural flows <br />and river channel characteristics in the Colorado River basin. Dams on the <br />main stem have essentially segmented the river system, blocking spawning <br />migrations and drastically changing river characteristics, especially flow and <br />temperature regimes. Channelization, dams, and water-use patterns in the main <br />stem and tributary streams have reduced or nearly eliminated embayments and <br />backwaters in some reaches of river. In addition, major changes in species <br />composition have occurred due to the introduction of non-native fishes, and <br />perhaps as a result of changes in the natural riverine system (i.e., flow and <br />temperature regimes). The decline of endemic Colorado River fishes may be <br />partially related to competition or other behavioral interactions with the <br />non-native species. <br /> <br />Based on early fish collection records, archaeological finds and other <br />observations, the Colorado squawfish was once found throughout wann water <br />reaches of the entire Colorado River basin, including reaches of the upper <br />Colorado River and its major tributaries, the Green River and major <br />tributaries, as well as the Gila River system in Arizona. Colorado squawfish <br />were apparently never found in colder, headwater areas. Seethaler (1978) <br />indicates that Colorado squawfish were abundant in suitable habitat throu9hout <br />the entire Colorado River basin prior to the 1850's. Historically, Colorado <br />squawfish had been collected in the upper Colorado River as far upstream <br />(RM 230) as Rifle, Colorado (Beckman, 1952). <br /> <br />The Colorado squawfish currently occupies about 880 river miles in the <br />Colorado River system (25 percent of its original range) and is presently <br />found only in the upper Colorado River basin above Glen Canyon Dam. It is <br />found inhabiting about 350 miles of the main stem Green River from its mouth <br />to the mouth of the Yampa River. Its range also extends 140 miles up the <br />Yampa River and 156 miles up the White River, the two major tributaries of the <br />Green River. In the main stem Colorado River, it is currently found from Lake <br />Powell extending about 201 miles upstream to Palisade, Colorado, and in the <br />lower 33 miles of the Gunnison River, a tributary to the main stem Colorado <br />River (Tyus, et.al. 1982). <br /> <br />In the main stem Colorado River, the farthest upstream current collection site <br />for Colorado squawfish is RM 185 near Palisade, Colorado. Recent studies to <br />determine the presence or absence of Colorado squawfish immediately upstream <br />from Palisade, Colorado (RM 185), have had negative results (Valdez, 1984; <br />Carlson and Muth, 1984). These studies, utilizing intensive collection <br />efforts during one field season, showed an absence of adult and young Colorado <br />squawfish in the RM 188-196 reach of the upper Colorado River, and in the <br />lower 7 miles of Plateau Creek, a tributary to the Colorado River near Cameo, <br />Colorado. At least two factors may contribute to these results: (1) upstream <br />migration may be blocked by existing barriers that occur just upstream of <br />Palisade, Colorado, and (2) the reach of river above Palisade may not provide <br />sufficient degree-days to support maturation and reproduction. <br />