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12 Only two Colorado squawfish confirmed spawning sites have been located 1n the Upper Bas;n--RH 16.5 of the Vampa River and RH 156.6 of the Green River. These areas have the common characteristics of coarse cobble or boulder substrates that form rapids or riffles associated with deeper pools or eddies. It 1s believed that this combination of habitats and a stable, clean substrate is necessary for spawning and incubation. Substrates are swept clean of finer sediments by high flows during spring runoff scouring the bed prior to the spawning period. Suitable spawning habitats appear to be present throughout the portion of the San Juan River inhabited by Colorado squawfish. O'Brien (1984) studied the hydraulic and sediment transport dynamics of the cobble bar within the Vampa River spawning site and duplicated some of its characteristics in a laboratory flume study. Based on field observations. he reported: On the rising limb of the hydrograph, sands are deposited in the cobble interstices. These sands are interchanged between the bed and the suspended zone for discharges less than bankfull. 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 a~proximate1y 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 mm at the spawning site. Hilhous (19B2) reported that discharges of approximately one-half that required to initiate cobble movement would 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 San Juan River are not well defined, the capture of young-of-year specimens in 1987 and 1988 documents that successful Colorado squawfish reproduction does occur when conditions are favorable (Platania 1990). Miller et a1, (1982) and Archer et al. (1986) demonstrated that Colorado squawfish often migrate considerable distances to spawn in the Green and Vampa Rivers and similar movement has been noted in the mainstem San Juan River. A fish captured and tagged in the San Juan Arm of Lake Powell in April 1987 was later recaptured in the San Juan River approximately 90 miles upstream in September 1987.