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14 <br />Although the river downstream of the Grand Valley also ,supports adult <br />Colorado pikeminnow, the primary importance of these downstream <br />reaches is in providing nursery areas for larvae and rearing areas for <br />juveniles. Concentrations of larvae and young-of-year occur in <br />backwaters in the 65-mile, low-gradient reach between Moab, Utah and <br />the confluence with the Green River (McAda et al. 1994). These <br />backwaters are especially important during the Colorado pikeminnow's <br />critical first year of life. Juveniles dwell in these downstream <br />reaches until they are five or more years old. Then many begin <br />extensive upstream migrations seeking habitats more suited to needs of <br />subadults and adults (Osmundson et al. 1998). The entire river, from <br />the confluence with the Green River upstream to Palisade, Colorado, <br />provides important habitat for sub- and young adults. <br />The razorback sucker, an endemic species unique to the Colorado River <br />Basin, was historically abundant and widely distributed within <br />warmwater reaches throughout the Colorado River Basin. The razorback <br />sucker is the only sucker with an abrupt sharp-edged dorsal keel <br />behind its head. It has a large fleshy subterminal mouth that is <br />typical of most suckers. Adults often exceed 3 kg (61bs) in weight <br />and 600 mm (2 ft) in length. <br />Historically, razorback suckers were found in the main stem Colorado <br />River and major tributaries in Arizona, California, Colorado, Nevada, <br />New Mexico, Utah, Wyoming, and in Mexico (Ellis 1914; Minckley 1983). <br />Bestgen (1990) reported that this species was once so numerous that it <br />was commonly used as food by early settlers and, further, that <br />commercially marketable quantities were caught in Arizona as recently <br />as 1949. In the Upper Basin, razorback suckers were reported in the <br />Green River to be very abundant near Green River, Utah, in the late <br />1800's (Jordan 1891). An account in Osmundson and Itaeding (1989) <br />reported that residents living along the Colorado River near Clifton, <br />Colorado, observed several thousand razorback suckers during spring <br />runoff in the 1930's and early 1940'x. In the San Juan River <br />drainage, Platania and Young (1989) relayed historical accounts of <br />razorback suckers ascending the Animas River to Durango, Colorado, <br />around the turn of the century. <br />A marked decline in populations of razorback suckers can be attributed <br />to construction of dams and reservoirs, introduction of nonnative <br />fishes, and removal of large quantities of water from the Colorado <br />River system. Dams on the main stem Colorado River and its major <br />tributaries have segmented the river system, blocking migration <br />routes. Dams also have drastically altered flows, temperatures, and <br />channel geomorphology. These changes have modified habitats in many <br />areas so that they are no longer suitable for breeding, feeding or • <br />