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11/14/01 draft report, Schmidt and Box Introduction Aquatic species may develop survival strategies that exploit specific segments (sensu Graf 2(01) of a river system at specific life stages and at specific times. The most obvious example of this is the return of spawning salmon to specific reaches of headwater streams, despite long and difficult passage. Gaining insight about the role of specific river segments in the life history of endangered endemic species requires integration of several scientific disciplines within a geographic context. An interdisciplinary perspective is necessary, because the ecological importance of specific river segments depends on the life history strategy of the species in relation to watershed hydrology, sediment transport, and geomorphology, as well as temperature, food base, nutrients, and non-native species. Analysis of the underlying ecological importance of specific river segments also has practical importance to river management. In some cases, populations of endemic species decline to the point of extirpation. When endangered aquatic species are affected by dams, federal regulatory programs require that actions be taken to mitigate those adverse impacts that threaten the survival of these species in segments designated as "critical habitat." Thus, it is essential to understand the controlling factors that cause specific segments to be "critical" to species survival. Mitigating actions required of dam operators are potentially costly, and it is essential to understand the role of the biotic and abiotic factors that influence the survival of endangered species, because only some abiotic factors are affected by dam operations. The Colorado pikeminnow (Ptychocheilus lucius) is one endangered species whose survival has been adversely affected by transformation of river hydrology, fragmentation by dams, de- watering caused by diversions, and introduction of competitive and predatory non-native species. The fish is endemic to the Colorado River basin and was common in all of the basin's large rivers prior to extensive water development. The species is the largest minnow native to North America (Tyus 1991) and was listed as endangered by the U.S. Bureau of Sport Fisheries and Wildlife in 1967. There is a longstanding conflict between water development and endangered species preservation in the Colorado River basin, because the watershed also has the largest proportion of res~rvoir storage to river discharge of any large basin in North America (Hirsch et al. 1990). Presently, some of the large dams in the upper Colorado River basin are required to release flood flows and/or base flows to improve the long-term success of the Colorado pikeminnow. These releases of water are intended to achieve specific target discharges in specific target segments of the Colorado River system. In this paper, we evaluate the longitudinal distribution oflarval Colorado pikeminnow nursery habitat in 185 Ian of the middle Green River of Colorado and Utah (Fig. 1). This evaluation is based on analysis of ecological sampling data for 1990-1995 and on a simulation model that we developed as a tool to understand the abiotic factors that control the longitudinal 3 ....