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~ C-J ("j C\l c:. ' c: 20 the suspended zone for discharges less than bank full. Depending on the supply-capacity relationship. either deposition or scour could be occurring. When the cobbles move. the sand. of course. is washed from the interstices and may be completely removed from around the cobbles. Rearrangement of the cobbles will result in more stability of the 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 one-half to one median cobble diameter below the cobble bed surface." In the flume experiments. the sand level was observed 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 range of 50-100 mm with a median size of 75 mm at the spawning 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 (200 C) occurred in the first week of August. In 1989. a year of low runoff. the mean temperature reached 200 C during the last week of June. Miller et al. (1982) and Archer et al. (1986) demonstrated that Colorado squawfish often migrate considerable distances to spawn in the Green and Yampa Rivers. and similar movement 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 downstream drift of larval Colorado squawfish occurs following hatching. Extensive studies in the Yampa and upper Green Rivers have demonstrated downstream distribution of young Colorado squawfish from known spawning areas (Archer et al. 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 1 ess than 1. 5- foot depth over a sil t substrate. Juvenil e Colorado squawfi sh habitat preferences are similar to that of young-of-year fish. but they appear