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. -, 7 In the flume experiments, the sand level was observed approximately one-half to one cobble diameter below the surface of the cobble bed, which compared to field observations of sand depth at approximately one-half to one median cobble diameter. O'Brien reported a cobble size range of 50 to 100 mm with a median size of 75 on at the spawning site. Milhous (1982) proposes discharges of approximately one-half 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 near the confl uence of the Gunn i son Ri ver, in the lema to Black. Rock.s reach, and near the confluence of the Dolores River demonstrates that spawning does occur. Kaeding and Osmundson (1987) 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 tefllleratures for spawning (200) occurred in the first week of August. Miller et al. (1982) and Archer et al. (1986) demonstrated that Colorado sQuawfish often migrate considerable distances to spawn in the Green and Yilf1lla Rivers and similar DDvement has been noted in the mainstem Colorado River. Miller et al. (1982) concluded from collections of larvae and young-of-year below known spawning sites that there is a downstream drift of larval Colorado sQuawfish followillg hatching. Extensive studies in the Yampa and upper Green Rivers have demonstrated downstream distribution of young Colorado sQuawfish from k.nown spawnillg areas (Archer et al. 1986, Haynes et a1. 1985). Miller et al. (1982) also found that young-of-year Colorado sQuawfish, from late summer through fall. preferred natural backwater areas of D velocity and less than 1.5-foot depth over a silt substrate. Juvenile Colorado sQuawfish habitat preferences are siaililr to that of young-of-year fish, but they appear to be lOObile and IIIOre tolerant of lotic conditions away from the sheltered back.water envi rOllllent. Information on r~o-tagged Colorado sQuawfish that have been observed during fa 11 suggests that fish seek out deepwater areas in the Colorado Ri ver (Mi II er et al. 1982), as do EIlY other riverine species. River pools, runs, and other deepwater areas, especially in upstream reaches, are important winter habitats for Colorado sQuawfisll. Very little infol1liltion is available on the influence of turbidity on the endangered Colorado River fishes. It is assumed, however. that turbidity is important, particularly as it affects the interaction between introduced fishes and the endeaic Col~o River fishes. Since these endemic fishes have evolved under natural coMntions of high turbidity, it is concluded that the retention of these highly turbid conditions is an illllortant factor for these endangered fishes. ReductiOl of turbidity may enable introduced species to gain a c~etitive edge llIIiB could further contribute to the decl ine of the endangered Colorado River fi~.