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pikeminnow (Valdez 1990) with significant reproduction observed in recent years (Holden and Wick 1982; Tyus 1991; McAda et al. 1994). In the upper Colorado River, the population of Colorado pikeminnow is widespread, with reproduction and recruitment occurring, but the size and distribution of the population have contracted since historical times (Quarterone 1993). Studies of pikeminnow distribution in the Colorado River indicate that most adults are concentrated in the upper 97 kilometers of the accessible range (above Westwater Canyon), and that few large pikeminnow are found downriver (Valdez et al. 1982, Osmundson et al. 1998) despite more optimal temperature regimes. Juveniles, however, are more numerous in downstream reaches. Evidence to date suggests that larvae drift downstream to lower gradient reaches which afford the best nursery habitat; the young stay there until they become subadults when they then migrate, presumably in search of better adult habitat. Identification of Problem Based upon river-wide distributions and abundance of native forage fish from studies of Valdez et al. (1982) and more recent work, Osmundson et al. 1998 hypothesized that the observed pikeminnow migration to upstream reaches is in response to better food availability and that downstream reaches have low carrying capacities for adult pikeminnow. Due to limited biotic resources, adults may move to upstream reaches because of higher concentrations of larger native food fishes and changes in condition between same size classes in lower versus upper reaches. While several studies are currently underway to assess reproductive success and young-of-the-year (YOY) mortality of Colorado pikeminnow throughout the upper basin (larval drift, YOY monitoring, YOY overwinter survival, transport ofyoung to Lake Powell, etc.), little attention has been paid to factors that may affect the adult population. In addition, most studies to date have focused on life history attributes of pikeminnow and neglected to examine the ecosystem upon which the species depends. Studies concerning adult pikeminnow related to flow regimes generally have been limited to assessing physical habitat preferences (e.g., Hann and Rose 1989; Osmundson and Kaeding 1991; Osmundson et al. 1995). However, there has been a growing need for ecosystem level studies incorporating food-web dynamics and geomorphology studies to aid in developing flow recommendations in the Colorado River (see Stanford 1994). This study is designed as a step toward meeting this need. In this investigation an assessment was made of the potential for changes in the carrying capacity of adult pikeminnow in portions of the upper Colorado River using an ecosystem approach. The work was performed by an interdisciplinary team of fluvial geomorphologists, fish biologists, and stream and systems ecologists. This report describes the results from the investigations of the biological environment (exclusive of fish) for the Colorado and lower Gunnison rivers. Results of the fisheries investigation and geomorphology will be reported by Osmundson (1998) and Pitlick (1998) respectively, under separate cover. 2