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that covers all rivers at the same time, using the same methods (such as for the Yampa, Green, White, Colorado,. Gunnison and Dolores Rivers in 1979-81: Miller et al. 1982ac, Tyus et al. 1982b, Valdez 1982), it was judged more acceptable to provide the raw data than to artificially divide the database. -Thus, the capture data are used with caution and they are supplemented with additional information as available. The relative importance of tributary streams for supporting populations of each of the listed species was evaluated using alife-history approach. This approach has both a spatial and temporal component, because- life history stages may occupy different habitats seasonally. Life history information was coupled with distributions of existing populations in order to obtain a geographical perspective on potential roles for tributaries. Next we assessed possible sites for establishing new populations, taking into account the contributions that tributaries might make. We determined the relative value of tributary streams as habitat for the endangered fishes by assessing their direct habitat use, or potential for use above areas that are blocked by barriers. Although native fish diversity is an important component of endangered fish habitat, the presence of other native fishes was not a factor in our ranking of tributary streams. The presence of other native fishes does not guarantee that the four endangered fishes will occur sympatrically, and it remains to be proven whether it will or will not be possible to recover the endangered fishes in the absence of a native fish- community. However, the Endangered Species Act dictates that efforts be made to conserve the ecosystem upon which listed species depend, and we provided general information about native fish abundance to depict the present condition of the streams in our evaluation. We were unable to obtain much information about the relative amount of fish habitat among tributaries, except as expressed in number of miles of river occupied by endangered fishes. To obtain a crude basis for comparing tributaries, we obtained measurements of habitat used by the endangered fishes from USGS topographic maps. We obtained surface area and the volume of physical habitat in each tributary by using a map wheel to measure the distance along the thalweg between adjacent contour lines during the base flow period. Within each section thus delineated we determined channel slope, width, and surface area. Using Manning's equation (Gordon et al. 1992), we computed mean depth at base flow (median across years), assuming that the roughness coefficient was 0.035 at all sites, and that the hydraulic radius is essentially the same as depth for channels that are relatively wide. Mean depth was then used to compute habitat volume and mean water velocity for each tributary reach considered useful for one or more of the endangered fishes. In addition to providing benefits to the endangered fishes, tributary streams also may pose obstacles to recovery, due to a variety of factors. We iden#ified certain obstacles when they were obvious, such as the presence of a barrier to fish movement, potential water quality problems, presence of large numbers of nonnative fishes, and others. However, we do not provide detailed analyses of these potential recovery obstacles. 7