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METHODS Many of the methods and analytical techniques used in the previous study by Pitlick et al. (1999) have been repeated here, with less of an emphasis on evaluating historical trends, and more of an emphasis on measuring existing conditions within the Colorado River. Accordingly, we have shortened the discussion of techniques involved in analyzing aerial photographs, and lengthened the discussion of the methods and assumptions involved in modeling flow and sediment transport. GIS Analysis of Channel Change in the Rifle-DeBegue Reach In order to provide a more complete view of the geomorphology of the Colorado River, and to complement results from our previous studies (Pitlick et al., 1999), we analyzed two sets of black and white aerial photographs covering the reach between Rifle and DeBeque Canyon. Historically, this reach supported populations of Colorado pikeminnow and razorback sucker (Kidd, 1977; Burdick, 1992; Quarterone, 1993), but a series of diversion dams now prevent fish from migrating upstream. The idea of modifying these structures to allow fish passage has been discussed, but the physical characteristics of the reach have not been studied in detail until now. Changes in channel morphology were determined by digitizing specific features (e.g. channel banks, islands, secondary channels) on black and white stereo photographs taken in 1937 and 1995. Figure 4 shows how these features were differentiated. Secondary channels that form where the flow splits around an island were distinguished from the main channel on the basis of their smaller size. Backwaters and other areas of slow moving water were often associated with side channels, thus we grouped them as one feature. 1937 1995 RM 220 RM 220 RM 219 RM 219 0 1000 m RM 218 RM 218 Figure 4. Digitized channel maps of a section of the Colorado River near Parachute, CO. 9