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INTRODUCTION Background Populations of Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) have diminished since historic times. The range of the Colorado squawfish has been reduced by 80% (Tyus 1990); the razorback sucker, a similar amount. This has compelled the U.S. Fish and Wildlife Service (USFWS) to list each as endangered species. Both species are endemic to the Colorado River basin and were formerly widespread and abundant (Girard 1856, Jordan and Evermann 1896, Miller 1961). Riverine populations are now confined to the upper basin (upstream of Glen Canyon Dam). There, the Colorado, Green and San Juan rivers and associated tributaries comprise the remaining range of these species. Currently, the Green/Yampa river system supports the most viable population of Colorado squawfish and also contains the largest number of adult razorback sucker remaining in the upper basin (Holden and Wick 1982, Lanigan and Tyus 1989). The San Juan system contains the most diminutive population of Colorado squawfish, and no razorback sucker have been found there in recent years (Ryden and Pfeifer 1993). This report focuses on the Colorado River. There, a small remnant population of razorback sucker persisted up through the mid-1980's; since then only a few individuals have been captured (Valdez et al. 1982, Osmundson and Kaeding 1991, Burdick 1992, USFWS unpublished data). Colorado squaw- fish continue to persist but distribution and abundance have declined to the point that long-term survival is far from assured. Reduction in range can generally be attributed to dams and diversion structures. Large dams and associated cold-water releases render downstream reaches uninhabitable. Range is also reduced where large or small structures prevent young and adults from returning upstream after they have migrated downstream. In those nonfragmented reaches where habitat for all life phases still exist, it is difficult to quantify the factors that negatively effect remaining populations. Factors that have been implicated include predation or competition from nonnative fishes, mortality from ingestion of spined prey, angler-associated adult mortality, reproductive problems associated with environmen- tal contaminants, low egg-hatching success due to infrequent flushing of spawning substrates, a reduced food base, low availability of quality nursery habitat, and degradation or simplification of adult habitats. Though some or all of these factors may act in concert, the relative importance of each for each species within each river is unknown. Indeed, several of these factors are strongly suspected but have not been demonstrated. With the exception of angling mortality and problems associated with environmental contaminants, many of the suspected problems listed above have been caused or exacerbated by flow regimes that have been significantly altered during the past 100 years. Even problems associated with nonnative fish can in part be linked to reduced flows that have allowed the colonization and continued proliferation of certain species (Osmundson and Kaeding 1991, Muth and Nesler 1993). Legal protection of sufficient instream flows to support self-sustaining populations of the endan- gered fish is one of the primary goals of the Recovery Implementation Program' (USFWS 1987, USFWS 1993). Instream flow needs are based upon the habitat requirements of rare fish species at various life stages. Identification of habitat requirements and instream flow needs of the rare fish is perhaps the most important element of the research effort expended by the Recovery Program.