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• 5 <br />success and survival of larvae was higher. At 20 °C to 26 °C, development and <br />survival through the larval stage was up to 59 percent (Hammon 1981). <br />Juvenile temperature preference tests showed that preferred temperatures <br />ranged from 21.9 °C to 27.6 °C. The most preferred temperature for juveniles <br />and adults was estimated to be 24.6 °C. Temperatures near 24 °C are also <br />needed for optimal development and growth of young (Miller et al. 1982). <br />Only two Colorado squawfish confirmed spawning sites, as defined in the <br />Colorado squawfish Recovery Plan, have been located in the Basin: river <br />mile 16.5 of the Yampa River and river mile 156.6 of the Green River. These <br />areas have the common characteristics of coarse cobble or boulder substrates <br />forming rapids or riffles associated with deeper pools or eddies. It is <br />believed that a stable, clean substrate is necessary for spawning and <br />incubation. Substrates are swept clean of finer sediments by high flows <br />scouring the bed prior to the spawning period. <br />O'Brien (1984) studied the hydraulic and sediment transport dynamics of the <br />cobble bar within the Yampa River spawning site and duplicated some of its <br />characteristics in a laboratory flume study. Based on field observations, he <br />reported: <br />"On the rising limb of the hydrograph, sands are deposited in the <br />cobble interstices. These sands are interchanged between the bed <br />and the suspended zone for discharges less than bankfull. Depending <br />• on the supply-capacity relationship, either deposition or scour <br />could be occurring. When the cobbles move, the sand, of course, is <br />washed from the interstices and may be completely removed from <br />around the cobbles. Rearrangement of the cobbles will result in <br />more stability of the armor layer. On the falling limb, the armor <br />layer becomes a trap for sands until finally, the sand reservoir is <br />again filled. Without cobble movement, sand will be scoured only to <br />a depth of one-half to one median cobble diameter below the cobble <br />bed surface." <br />In the flume experiments, the sand level was observed approximately <br />0.50 to 1 cobble diameter below the surface of the cobble bed, which compared <br />to field observations of sand depth at approximately 0.50 to 1 median cobble <br />diameter. O'Brien reported a cobble size range of 50-100 mm with a median <br />size of 75 mm at the spawning site. Milhous (1982) proposes discharges of <br />approximately 0.50 of that required to initiate cobble movement will be <br />capable of extracting sands and fines from the cobble substrate. Thus, after <br />the supply of sand diminishes, flows of sufficient magnitude and duration are <br />required to scour the cobble bed in preparation for spawning and incubation. <br />Although the location of spawning areas in the Colorado River is not well <br />defined, the presence of larvae downstream of the Walker Wildlife Area, in <br />the Loma to Black Rocks reach and near the confluence of the Dolores River, <br />demonstrates that spawning does occur. Osmundson and Kaeding (1989 and 1991) <br />reported that water temperatures in the Colorado River were suitable for <br />• spawning in the Grand Junction area. In 1986, a year of high runoff, suitable <br />temperatures for spawning (20 °C) occurred in the first week of August. In <br />