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EXECUTIVE SUMMARY The upper Colorado River subbasin (the Colorado River and its major tributaries upstream from its confluence with the Green River; the upper Colorado River basin includes all tributaries upstream from Lee Ferry, Arizona) historically supported populations of four native fishes - humpback chub Gila cypha, bonytail G. elegans, Colorado pikeminnow Ptychocheilus lucius, and razorback sucker Xyrauchen texanus - that are currently listed as endangered under the 1973 Endangered Species Act (ESA), as amended. Self-sustaining, wild populations of Colorado pikeminnow and humpback chub still occur in the subbasin, but wild razorback suckers have not been collected since 1995 and wild bonytails have been extirpated. Repatriation programs are underway for razorback sucker and bonytail. These four species are threatened with extinction because of many factors including habitat loss from dike construction, riparian encroachment in the main channel, and construction of water-diversion structures that restrict movement; regulation of river flow, water temperature, and sediment regimes through construction of large reservoirs; introduction of nonnative fishes that are predators or competitors of the native species; and other human-induced perturbations. One of the most dramatic of these changes has been the alteration of flow regimes by tributary and mainstem reservoirs. These reservoirs inundate large areas. of riverine habitat creating lentic habitat, which is often stocked with nonnative fishes. Riverine: habitats remain downstream of the dams, but are modified by water-release patterns that alter temperature regimes, increase base flows, and decrease peak flows. An equally important change has been the introduction of more than 42 nonnative fishes into the upper Colorado River basin; 21 of these introduced species coexist with one or more of the endangered fishes in at least part of their range in the upper Colorado River subbasin. Recovery of the endangered fishes in the upper Colorado River basin is being addressed by the Upper Colorado River Endangered Fish Recovery Program (Recovery Program). The Recovery Program was initiated under a Cooperative Agreement signed by the Governors of Colorado, Utah, and Wyoming; the Secretary of the Interior; and the Administrator of Western Area Power Administration in 1988. It is a coordinated effort of State and Federal agencies, water users, energy distributors, and environmental groups that functions under the general principles of adaptive management (i.e., management actions are identified, implemented, evaluated, and revised based on results of research and monitoring). The Recovery Program operates in compliance with State and Federal laws related to the Colorado River system, including State water law, interstate compacts, and Federal trust responsibilities to American Indian tribes, thereby recognizing existing water rights. The recovery goals for the endangered fishes require that habitat (including flow regimes necessary to restore and maintain required environmental conditions) necessary to provide adequate habitat and sufficient range for all 'life stages to support recovered populations be provided and legally protected. The Aspinall Unit (Blue Mesa, Morrow Point, and Crystal reservoirs) was built in the upper Gunnison River prior to passage of ESA. Although the Aspinall Unit is located Final Report xviii July 2003