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,M~Y-15-1998 15:48 OOR LULND 9706633212 P.08/24 ,.. ~ armor layer. On the falling limb, the armor layer becomes a trap for sands until finally, the sand reservoir is again filled. Without cobble movement, sand will be scoured only to a depth of ana-half to one median cobble diameter below the cobble bed surface." In the flume experiments, the sand level was obseryed approximately 0.50 to 1 cobble diameter below the surface-of the cobble bed, which compared to field observations of sand depth at approximately 0.50 to 1 median cobble diameter. O'Brien reported a cobble size ~ange of 50-100 mm with a median size of 75 mm at the spawnin9 site. Milhous (1982) proposes discharges of approximately 0.50 of that required to initiate cobble movement will be capable of extracting sands and fines from the cobble substrate. Thus, after the supply of sand diminishes, flows of sufficient magnitude and duration are required to scour the cobble bed in preparation for spawning and incubation. Although the location of spawning areas in the Colorado River is not well defined, the presence of larvae downstream of the Walker Wildlife Area, in the Lorna to Black Rocks reach and near the confluence of the Dolores River, demonstrates that spawning does occur. Osmundson and Kaeding (1989 and 1991) reported that water temperatures in the Colorado River were suitable for spawning in the Grand Junction area. In 1986, a year of high runoff, suitable temperatures for spawning (20 'C) occurred In the first week of August. In 1989, a year of low runoff, the mean temperature reached 20 'C during the last week of June. Miller et al. (1982) and Archer et al. (1986) demonstrated that Colorado squawfish of tin migrate considerable distances to spawn in the Green and Vampa Rivers, and similar movement has been noted in the main stem Colorado River. 7 ~ Miller et al. (1982) concluded from collections of laryae and young-of-year below known spawning sites that there is a downstream drift of larval Colorado squawfish following hatching. Extensive studies in the Vampa and upper Green Rivers have demonstrated downstream distribution of young Colorado squawfish from known spawning areas (Archer et a1. 1986; Haynes et al. 1985). Miller et al. (1982) also found that young-of-year Colorado squawfish, from late summer through fall, preferred natural backwater areas of zero velocity and less than I.S-foot depth over a silt substrate. Juvenile Colorado squawfish habitat preferences are similar to that of young-of-year fish, but they appear to be mobile and more tolerant of lotic conditions away from the sheltered backwater environment. Information on radio-tagged adult Colorado squawfish during fall suggests that fish seek out deepwater areas in the Colorado River (Miller et al. 1982), as do many other riverine species. River pools, runs, and other deep water areas, especially in upstream reaches, are important winter habitats for Colorado squawfish. Very little information is available on the influence of turbidity on the endangered Colorado River fishes. It is assumed, how8yer, that turbidity is important, particularly as it affects the interaction between introduced fishes and the endemic Colorado River fishes. Because these endemic fishes have evolved under natural conditions of high turbidity, it is concluded that the retention of these highly turbid conditions is an important factor for .