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Chan es in nurse habitat availabili within five alluvial reaches of the re ' g ry ty G en River were quantified by Pucherelli et al. [1990]. They used remotely gathered data from five different discharges over a 4-month period in 1987 to develop a correlation between habitat availability and dischazge, but found the relationship between habitat availability and discharge at Ouray to be the weakest of the five reaches. Backwaters next to the bank were much more abundant than midchannel-baz backwaters, and there were more very large (>1000 m2) backwaters in the Ouray reach than in the other sampled reaches. Stanford [1994] noted that the relationship developed by Pucherelli et al. [1990] for a single yeaz's topography would only be valid for that yeaz, and would likely change in subsequent years as the channel morphology changed in response to flood peaks. Habitat availability at base flow is a product of channel morphology, and within-channel morphology in this dynamic system may vary greatly from year to year. Thus, from a geomorphic point of view, annual variability in habitat availability is expected. Need for a New Methodology Multiscale methodologies are necessary to determine flows that maintain low-flow habitat availability and the large magnitude floods that form habitats. Neither PHABSIM nor the methodology of Pucherelli et al. [ 1990) accounts for the dramatic changes in channel morphology that occur from year to yeaz in alluvial rivers, nor do they consider competition and predation pressures [Mathur et al., 1985] that are presumed to be significant in the Green River [Tyus, 1992]. These methods are inappropriate tools for the Green River [Tyus, 1992]. Thus, a new methodology must be developed to determine the effect of low-flow variation on habitat availability. The 2-dimensional HEC-2 modeling and historic hydrology do not provide information on channel response to flood passage, but a 3-dimensional flow and sediment transport model can simulate the response of channel topography to flood passage. The long- lived endemic Colorado River fishes, including the Colorado pikeminnow, are adapted to not require successful recruitment each year for species survival [Minckley and Deacon, 1991 ]. Consequently, the short-term high-dischazge disturbances necessary to maintain long-term channel structure maybe viewed as a viable river management tool. ~~ OVERVIEW OF GREEN RIVER PHYSIOGRAPHY, HYDROLOGY, AND RESPONSE TO FLAMING GORGE DAM Physiography The Green River is the longest tributary of the Colorado River, draining approximately 115,800 km2 (Figure 1). The mainstem of the Green River has its headwaters in the Wind River Range of Wyoming. The Yampa River, the Green River's largest tributary, has its headwaters in the Park Range of Colorado and supplies almost half of the Green River's water. Flaming Gorge ' Dam, located 662 river kilometers upstream from the Green River's confluence with the Colorado River, has regulated the flow of the main stem of the Green River since October 1962. The Yampa River remains lazgely unregulated. A-12