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INTRODUCTION General Blockage of Colorado pi keminnow1 movement by dams and water diversion structures has been suggested as an important cause of the decl i ne of thi s 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 this 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 and other Federally-listed endangered species such as the razorback sucker and is an important conservation measure for other non-listed native fishes. The Grand Valley Irrigation Company (GVIC) Diversion Dam is located on the Upper Colorado River 15 miles upstream of the confluence with the Gunnison River near Palisade, Colorado (river mile [RM] 185.3-185.5), at the uppermost end of the '15-Mile Reach' (Figure 1). 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 fish movement only during certain times of the year 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. The GVIC diverts about 640 cfs from the river at this point. 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 Dam is the most downstream of three low-head structures on the Upper Colorado River which precludes the passage of endangered Colorado pikeminnow and razorback sucker into historic habitat further upstream. The two other structures are the Price-Stubb (RM 188.3) and Government Highl ine Diversion (RM 193.7) 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. In the winter of 1998, a series of five pools and five riffles that created a natural channel (approximately 450-feet long) immediately downstream of the diversion dam were constructed. At the same time, a 30-ft wide inverted notch was retrofitted into the existing cement diversion dam at the most upstream, northeastern end of the structure. This newly-constructed notch along with the alternating series of downstream low-gradient pools and riffles comprised the new fish passage at this site. The notch and series of pools and riffles at the GVIC Diversion Dam were designed to provide upstream passage particularly during low flows for endangered fishes because the diversion dam is inundated at high flows (about 15,000 cfs) and fish can pass over the dam at these high flows. The notch and series of pools and riffles were designed to provide passage during March through October with the most important biological period being May through mid-September (FLO Engineering, Inc. 1997). The riffle segments that connect each pool are config- 1 Scientific names and two-letter codes of all fishes mentioned in this report are given in Appendix Table A.l. Only common names for these fishes are used in the text.