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1 1 15 Colorado squawfish increases as one moves up river with the 15-mile reach containing the greatest proportion of large adult Colorado squawfish. The 15-mile reach appears to provide the optimum balance between temperature and food abundance for adult Colorado squawfish. Colorado squawfish and razorback sucker were historically found as far upstream as Rifle, Colorado (55 miles upstream of Palisade). The construction of fish ladders at the diversion structures upstream of the 15-mile reach would allow these fish to access this historically occupied upstream reach. However, this upper reach, the fringe or outer margin of their range, was and is suboptimal habitat. Native suckers and chubs are abundant in this reach and would provide plentiful forage for Colorado squawfish. However, the thermal regime of this reach is marginal. This reach forms the transition zone between the cold-water fishery upstream of Rifle, dominated by salmonid species such as trout and mountain whitefish, and the warm-water fishery downstream of Palisade, historically dominated by Colorado squawfish and native chubs and suckers. Temperatures necessary for Colorado squawfish spawning are not always met in the Rifle-Palisade reach, and when they are met, the duration of the first-year growing season for larvae is relatively short. The lower temperatures of the Rifle-Palisade reach also would affect growth rates of Colorado squawfish. The total of accumulated annual degree-days at Rulison, Colorado was measured to be approximately half of the total at the state line. The complete life cycle of the razorback sucker, in terms of its entire river habitat use pattern, is largely unknown but may be similar to that of Colorado squawfish because adults are generally concentrated in upstream reaches of the Colorado and Green Rivers and larvae from spawning sites in the upper Green River have been found to drift long distances down river. Other evidence suggests that bottomlands in both upstream and downstream reaches historically served as nursery areas for larvae when these habitats flooded during the historically higher spring runoff period. .~~ The 15-mile reach also is important for the survival and recovery of endangered fishes because it provides a refuge for the Colorado River basin ~ ~~ ~ populations should a catastrophic event such as an oil spill or chemical spill ~;,~<<<~~ F~ affect the Gunnison River and the Colorado River below the Gunnison River s c~ confluence. ~'~ ~, `~ , ~ ~ In summary, the 15-mile reach is important for several reasons: ~~~, ~,`~ 1. The 15-mile reach provides valuable spawning habitat for Colorado ~~ ~~ squawfish and razorback sucker. ~~~ ~~ `~ 2. The 15-mile reach provides an optimum balance between temperature and food availability for adult Colorado squawfish in the Colorado River. ~ ~.-~ 3. The 15-mile reach provides an important refuge for endangered fishes ~ ='~ should a catastrophic event cause a loss of populations in the Gunnison River and in the Colorado River below the Gunnison River confluence.