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INTRODUCTION The long-term impact of the operation of Flaming Gorge Dam on fish populations in the Green River is unknown. However, dams may alter historic flow and temperature regimes and eliminate flood events (Tyus et aI., 1987; Graf, 1980). In response to reduced peaking conditions, the Green River bankfull channel width has decreased substantially following construction of the Flaming Gorge Dam (Andrews, 1986), and the abundance of endemic fish species in the Green River has decreased subsequent to the construction of Flaming Gorge Dam in 1962 (Tyus et aI., 1987). These species included the endangered Colorado squawfish (Ptchoecheilus lucius), the humpback chub (Gila cypha), and the rare but unlisted razorback sucker (Xyrauchen texanus). Studies conducted by the FWS (U.S. Fish and Wildlife Service) concluded that backwaters in the Green River are preferred nursery habitat for yoy (young-of-the-year) Colorado squawfish (Holden and Stalnaker, 1975; Holden, 1977; Tyus and McAda, 1984; Tyus et aI., 1987). Colorado squawfish spawn during mid- to late-summer and their larvae become distributed in shallow backwater habitat reducing predation and protecting larvae from adverse flow events (Tyus et aI., 1987). This evidence suggests that maximizing backwater habitat during summer will increase the survival rate of young Colorado squawfish and, therefore, would be an important factor in the management and preservation of this species. A technique was required to accurately map backwater habitat in response to various Green River flows. The IFIM (instream flow incremental method) has not proven adequate to calculate backwaters on large, turbid, hydrologically complex rivers such as the Green River (Rose and Hann, 1989). IFIM uses PHABSIM (physical habitat simulation models) to calculate physical habitat, assuming that velocity, depth, and substrate in a river system behave as independent hydraulic variables. On the contrary it is believed that these variables interact continuously along the length of a river by varying turbulence and shear stresses and it is doubtful that they may be considered independent (Gore and Nestler, 1988). In addition, PHABSIM uses velocity to calculate physical habitat and, because backwaters have little or no velocity, it is impossible to model them accurately. Attempts to model backwaters using IFIM on the Green River were not successful (Rose and Hann, 1989). Therefore, it was decided to use remote sensing techniques and a GIS (Geographic Information System). Pucherelli et a1. (1987) used large-scale aerial photography and a GIS to map backwater habitat and changes in river morphology as a function of riverflow. This study examined the relationship between flow and the number and area of backwaters on the Green River in Utah. The objective was to establish a data base to assist in determining the optimal Green River flows needed for maximum backwater availability during summer and fall. STUDY SITES Five study sites (fig. 1) were selected by the FWS and the Bureau of Reclamation because they were known to contain important backwater habitat for yoy Colorado squawfish (Tyus et aI., 1987). The Island Park site is located just upstream from Split Mountain in Dinosaur National Monument; the Jensen site is located just downstream from Split Mountain; the Ouray site is located in Ouray National Wildlife Refuge; the Sand Wash site is just above Desolation Canyon; and the Mineral Bottom site is located about 80 kilometers upstream from the confluence of the Colorado and Green Rivers.