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III. Study Background/Rationale and Hypotheses Blockage of Colorado squawfish movement by dams and water diversion structures has been suggested as an important cause of the decline of this species in the Upper Colorado River Basin (Tyus 1984; Burdick and Kaeding 1990). Although the actual role that blockage might have contributed in the decline of these species in the upper basin is unknown. providing fish passage past instream barriers has come to be considered an important means to aid the recovery of this species. The GVIC Diversion is located on the Upper Colorado River 15 miles upstream of the confluence with the Gunnison River near Palisade. Colorado. Initially constructed in 1883, it is the oldest major irrigation project in the Grand Valley and is privately owned by the Grand Valley Irrigation Company. The GVIC diversion dam is an upstream barrier to endangered fish movement because of the 3 to 4 feet of head differential it creates at low to moderate flow conditions. This . difference in upstream and downstream water surface elevations functions to divert water into the GVIC canal. As flows increase, the head differential across the dam decreases and the structure no longer creates a barrier to upstream fish passage. The GVIC Diversion structure is the most downstream of three low-head structures on the Upper Colorado River which precludes the passage of endangered Colorado squawfish and razorback sucker into historic habitat further upstream. The two other structures are the Price-Stubb and Government Highline Diversion dams. Whereas the GVIC diversion is only a. barrier at low and moderate flows, the other two diversion structures prevent passage at all flow conditions. The Recovery Implementation Program has proposed fish passage at all three of these diversion dams on the Upper Colorado River to allow endangered fish unimpeded movement to historic stream reaches upstream. Providing passage at GVIC will only extend the range of endangered fishes upstream about 3 miles to Price Stubb Dam. However, providing passage at both Price-Stubb and Government Highline Diversion dams will open up an additional 50 miles of historic habitat upstream. The U. S. Fish and Wildlife Service documented a radio-tagged adult Colorado squawfish moving upstream to the GVIC Diversion Dam in mid- July 1986. The fish eventually passed the structure and moved upstream to the base of the Price-Stubb Dam. This fish either moved upstream over the diversion dam or could have passed around the diversion dam via the Orchard Mesa Irrigation Diversion (OMID) bypass canal. Flows GVIC 2