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[USFWS] 1990ab, 1991, 1998). to addition, recovery goals established by the Upper Colorado Endangered Fish Recovery Program (Recovery Program) include the criterion that recruitment to populations of the endangered fishes must be produced by natural reproduction in the wild (Nester 2000). In the past, recovery efforts focused on maintaining or improving habitat for endangered fish populations in main channel reaches of the Colorado and Green Rivers, and in large tributaries, such as the Yampa and Gunnison Rivers. More recently, there has been increasing interest in other streams, and the potential of tributary streams for aiding fish recovery has been recognized in official recovery plans (USFWS 1991) and by the designation of critical habitat (USFWS 1994). Although it is universally agreed that tributary streams have a potential to aid in recovery of the listed Colorado River fishes, there are concerns that the need to satisfy a continuing demand for water will especially affect tributary streams. Although critical habitat designation provides some protection to identified tributaries, this protection does not extend to all tributary streams that have potential for supporting the recovery effort. Some tributary streams in the upper Colorado River basin provide direct benefits to the listed fishes by providing suitable habitat for one or more of the fish during some part of their fife cycle. For example, razorback sucker utilize the lower portions of some tributary streams as staging areas prior to spawning and for adult habitat (e.g., Duchesne River and Ashley Creek; Tyus 1987). Colorado pikeminnow subadults and adults utilize tributary streams presumably for spawning (e.g., Yampa and Gunnison Rivers; Tyus 1990, Burdick 1995) and also for feeding, growth, and overwintering (e.g., Duchesne, Little Snake, and Price Rivers; Tyus and McAda 1984, Wick et al. 1991, Cavalli 1999). As an example, the need for tributary habitat is evidenced by adults that continue to ascend the White River upstream to a point where their historic habitats are blocked by a dam (Irving and Modde 1994, 2000). Humpback chub seasonally occupy habitats in the Little Snake River, perhaps for several purposes (Wick et al. 1991). Tributary streams also can provide indirect benefits to fish populations in mainstream rivers by influencing river flow regimes, water quality, nutrient concentrations, or other conditions in the downstream receiving waters. Some tributaries (e.g., Yampa River) essentially have unregulated flows and improve habitats in downstream systems that might otherwise be less acceptable for the listed fishes (Tyus and Karp 1989, Modde and Smith 1995). Other tributaries (e.g., Little Snake- River) provide sediment sources needed to maintain habitat features of the natural river ecosystem (e.g. backwaters). However, the present or potential role of most tributaries is not well understood. There are Larger tributary streams in different areas of the basin that have the potential for supporting the endangered frshes, but are virtually unoccupied by them (e.g., Dolores River; Holden and Stalnaker 1975, Valdez et al. 1992). Some streams or stream segments have been isolated by barriers, others may be effectively isolated because of human-induced habitat change. Except for a very few of these streams, their potential for supporting populations of the endangered fishes, or even assisting in the overall recovery-effort remains unknown. 2