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<br />. ' <br /> <br />~~ <br /> <br />. <br />-~ <br /> <br />- <br /> <br />Unit and postulated their effects on the endangered fishes. The <br />important changes they described are: <br /> <br />1. Reduced water temperature in the Gunnison River within habitat <br />occupied by Colorado squawfish--about 2oC. Water temperature <br />in the Colorado River is unaffected by the Aspinall Unit. <br /> <br />2. Reduced sediment load in both the Colorado and Gunnison rivers. <br /> <br />3. Reduced spring runoff in the Gunnison and Colorado rivers, with <br />a concomitant increase in fall and winter streamflow. Mean- <br />monthly flows during May and June, the primary runoff months, <br />have declined 25 and 32% (respectively) in the Gunnison River <br />and 12 and 13% in the Colorado River as a result of the <br />Aspinall Unit. <br /> <br />McAda and Kaeding (1991) developed nine hypotheses to guide studies to <br />assess the impact of these changes on the endangered fishes: <br /> <br />Hypothesis 1. Relative abundance of age-O Colorado squawfish in <br />the Colorado River is greatest during years with a natural <br />hydrograph and maximum-annual discharges of 30,000 to 40,000 <br />cfs (measured at the Cisco stream gage). Relative abundance is <br />reduced in years with higher or lower maximum-annual <br />discharges. <br /> <br />Hypothesis 2. length of growing season affects over-winter <br />survival of age-O Colorado squawfish. <br /> <br />Hypothesis 3. High spring flows and a natural hydrograph reduce <br />non-native fish populations. <br /> <br />Hypothesis 4. The Gunnison River above Redlands Diversion <br />contains a small, reproducing Colorado squawfish population. <br /> <br />Hypothesis 5. Redlands Diversion Dam restricts up and <br />downstream movement of all life stages of Colorado squawfish. <br /> <br />Hypothesis 6. The Gunnison River contains habitat suitable for <br />reintroducing razorback sucker, augmenting the Colorado <br />squawfish population, and establishing a new population of <br />humpback chub. <br /> <br />Hypothesis 7. Higher spring flows and a natural hydrograph will <br />increase flooded areas in spring for adult razorback sucker and <br />Colorado squawfish use in both the Colorado and Gunnison rivers <br />and will improve quality of young-of-the-year habitat. <br /> <br />Hypothesis 8. Higher spring flows and a natural release pattern <br />from the Aspinall Unit will improve the ability of the Gunnison <br />and Colorado rivers to clean spawning substrate, to maintain <br />sand and silt substrate in nursery areas, and to maintain <br />natural channel characteristics. <br /> <br />3 <br />