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INTRODUCTION General Blockage of Colorado pikeminnow) 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 this species in the upper basin is unknown, providing fish passage past instream barriers has come to be considered an important means to aiding recovery. Similarly, it is important for the recovery of other Federally listed species such as the razorback sucker. Providing passage is also an important conservation measure for other non-endangered native fishes. Large dams and diversion structures are also effective instream barriers that fragment stream reaches and reduce the range of native species by precluding both young and adult fish from returning upstream after they have migrated downstream. Barriers are particularly harmful to species that migrate long distances to fulfill life history requirements such as the potamodromous Colorado pikeminnow (Tyus 1984; Tyus 1990). In the Upper Colorado and Gunnison rivers, Colorado pikeminnow may have been precluded from historic upstream spawning sites in canyon-bound reaches since the early part of the 20th century when low-head diversion dams were constructed. These low-head diversion dams have altered Colorado pikeminnow movement patterns, disrupted reproductive behavior, and reduced the number of spawning sites available. 'These dams precluded movement into what had been occupied habitat. The warmwater reaches of the Gunnison River are historical habitat for the Colorado pikeminnow and razorback sucker. The Gunnison River from the confluence of the Colorado River upstream to the confluence with the Uncompahgre River (56 river miles) was designated critical habitat for both the Colorado pikeminnow and razorback sucker in 1994 (59 FR 13374-13400) under authority of the Endangered Species Act. This 56-mile reach plus a 3-mile reach upstream from the Uncompahgre River confluence to Hartland Diversion Dam is considered occupied habitat for both Colorado pikeminnow and razorback sucker by the U. S. Fish and Wildlife Service (FWS). Other fishery studies conducted over the past 15 years indicate that Colorado pikeminnow occupy both up- and downstream reaches from the 1 Scientific names and two-letter codes of all fishes mentioned in this report are given in Appendix Table A.1. Only common names for these fishes are used in the text.