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Below Westwater Canyon the river flows through shallow canyons and open valleys and <br />then through steep sandstone canyons above and below Moab. <br />Habitats are comprised of deep runs and pools with several rapids formed by side <br />canyons. Many backwaters with sand/silt substrate occur between Moab and the <br />confluence with the Green River during low flow periods (Valdez et al. 1982b). Between <br />the confluence with the Green River and Lake Powell the Colorado River flows through. <br />Cataract Canyon where the river has deep swift runs, major rapids, large eddies, and <br />pools. Lake Powell now inundates the lower end of Cataract Canyon where there is a <br />transition zone between riverine and lacustrine habitat. <br />Primary Constituent Element - Water <br />The quantity of water in the Colorado River has been reduced by water development <br />projects. Flows regimes have been altered significantly in the Colorado River: in <br />addition to the alteration caused by the Aspinall Unit, flow in the Colorado River has <br />been altered by numerous upstream reservoirs and water projects, many of which <br />transport large volumes of water out of the Colorado River basin. <br />Elevated selenium concentrations associated with irrigation drainwater were found in the <br />Colorado River during National Irrigation Water Quality Program investigations (Butler <br />et al. 1994, 1996; Butler and Osmundson 2000). These elevated selenium concentrations <br />still occur in water, sediment, and biota, and continue to pose a risk to this PCE. The <br />Colorado River below the confluence with the Gunnison to the State line and associated <br />tributaries appear on the State of Colorado's 303(d) list of impaired waters because of <br />selenium. Studies show that selenium concentrations in water and fish tissue are <br />inversely related to flows; the lower the flows the higher the selenium concentrations <br />(Osmundson et al. 2000). <br />Primarv Constituent Element - Physical Habitat <br />Westwater and Cataract Canyons provide movement and migration corridors between the <br />other relatively flat water habitats. Floodplain habitats between the canyons provide <br />warm water, low velocity, feeding and nursery habitats. Many backwaters between <br />Westwater Canyon and Lake Powell provide nursery habitat. The Service has developed <br />flow recommendations for the Colorado River below the confluence with the Gunnison <br />River (McAda 2003) designed to maintain spawning and backwater habitat. Under <br />current conditions these recommended flows are achieved only in naturally wet years. <br />Primary Constituent Element - Biological Environment <br />This PCE is impaired by the presence of nonnative fishes common in this reach of the Colorado <br />River. Nonnative fishes occupy the same backwaters that are very important for young Colorado <br />pikeminnow and razorback sucker. Largemouth bass (Micropterus salmoides) and green sunfish <br />(Lepomis cyanellus) are the most common large-bodied fishes that occupy backwater habitats <br />year-round (Osmundson 2003). The three most common small-bodies fishes found in <br />backwaters are fathead minnow, sand shiner, and red shiner, comprising 80 to 100 percent of the <br />fish found in Colorado River backwaters (McAda 2003). <br />29