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some cases, these decisions maybe relatively arbitrary, as in the case of the Colorado River above its confluence with the Green River, which was formerly known as the Grand River (until it was changed by legislative action). Flows of the Colorado and Green rivers are almost equally divided at the Gunnison River in the UCR basin and the Yampa River in the Green River basin. For purposes of this study, the Green River above the Yampa River, and the Colorado River above the Gunnison River are considered to be tributaries. All of these stream reaches once supported large standing stocks of the endangered fishes. Numerous other tributary streams (e.g, Little Snake, White, Duchesne, Price, San Rafael, Dolores, Dirty Devil and Escalante rivers, and Plateau Creek) are potentially important due to their proximity to occupied habitat in the UCR basin. A comprehensive view of the existing and potential roles for tributaries was developed by drawing on information about the role of other important tributaries (e.g., the Little Colorado River) that have merit in providing information useful to recovery of the fishes, even though they are beyond the geographic confines of the upper basin. For our evaluation, we divided the UCR basin into six geographic areas: (1) Yampa River and tributaries, (2) Green River and tributaries above its confluence with the Yampa River, (3) Green River from the Yampa River to the Colorado River confluence, (4) the Colorado River and tributaries above the Gunnison River, (5) the Colorado River from confluence with the Gunnison to confluence of the Green River, and (6) the Colorado River below confluence with the Green River to Lake Powell. The main focus of this study was to evaluate the role of tributaries and not the mainstream rivers, but it was deemed necessary to provide some information about mainstream areas to aid in understanding what, if any, indirect effects tributaries may have on habitats located downstream of them. A review of existing published and unpublished information, historical and present, about use of tributary streams by the endangered fishes and the role of tributary streams in maintaining endangered fish habitat in downstream areas was a major portion of our evaluation. Present and historical distributions, habitat use, life histories, and life history strategies of the four Colorado River fishes provided a basis for: (a) evaluating direct and indirect effects of tributary streams on extant populations of the fishes, and (b) assessing future recovery potential. Examples of important reviews that provide excellent background information and extensive references include recovery plans prepared by the USFWS for all four of the listed species (USFWS 1990a,b; 1991, 1998), Critical Habitat designation and supporting documents (Maddux et ai. 1993; USFWS 1994), Recovery Action Plans (e.g., USFWS 2000), Flaming Gorge Studies Muth et al. 2000), and biological recovery goals and criteria development (Nester 2000). Published compendia, such as works edited by Miller et al. (1982b), and Minckley and Deacon (1991), which provided critical reviews and voluminous references to published and unpublished works also were useful. Numerous archival (unpublished) reports (available,through the Recovery Program) review, summarize, and present much 5