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c°. EXECUTIVE SUMMARY The size and number oflow-velocity eddies that may be preferred habitat for the endangered humpback chub (Gila cypha) in the Green-River in Desolation and Gray Canyons changes with discharge and has changed since the early 1900Os. We determined the present extent and distribution of these habitats by mapping low-velocity eddies at a scale of 1:5000 at 5 or 6 discharges between 2,100 and 27,000 ft3/s in four 8-km reaches that are regularly sampled by the Utah Division of Wildlife Resources. We also mapped the surficial geology of these study reaches and the distribution of bed substrate that are emergent at 2000 ft3/s. We analyzed the distribution of these areas within a geographic information system. The availability of habitat prior to completion of Flaming Gorge Dam was estimated by matching old oblique photographs, analyzing old aerial photos, and recomputing habitat availability prior to channel change. The total area oflow-velocity eddies, when summed for the 4 study reaches, does not change with discharge, but the relative distribution of these eddies among the 4 study reaches does change. Also, the type of eddies changes with discharge. At low discharge, the greatest proportion of the total area oflow-velocity eddies occurs as small shoreline eddies, but the greatest proportion occurs in a few large eddies at higher discharges. At low discharge, the river bank is highly contorted and is dominantly bare sand and gravel. At high discharge, the river bank has a simpler shape, and much of the shoreline is inundated vegetation. The Green River channel is 19% narrower today than it was in the 19200s, and riparian vegetation has established itself at low elevations and on formerly active mid-channel islands. We estimate that the substrate of most nearshore habitats was sand or gravel prior to channel- narrowing. For purposes of developing flow recommendations, our results must be integrated with the results of ecological studies which identify the relative importance of the different habitats in the life history of the target species. LIST OF KEY WORDS Green River, geomorphology, flow, Desolation Canyon, Gray Canyon, habitat, Gila 4-' A-viii