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Table 11. Mean catch-per-unit-effort (CPUE, fish/100 m2) in 1987 of -_ three exotics in backwaters in the Colorado, Green and San Juan rivers. Data for the Colorado and Green rivers is ~ from the Interagency Standardized Monitoring Program, U.S. Fish and Wildlife Service, Colorado River- Fishery Project, Grand Junction, Colorado. CPUE (fish/100 m~) ~ Colorado River Green River San Juan River Grand Cisco Upper Lower Valley to Species Confluence red shiner 236 305 156 852 81 ~ sand shiner 63 30 * 6 10 fathead minnow 72 28 4 85 51 *Less than 1.0 Distribution of backwaters and other slow-water habitats seined, - was not uniform along the entire length of the San Juan River, Utah. Aerial observation led us to "block" the river into three distinct geomorphologic areas. The upstream-most area is in a broad floodplain - with braided channels (RMI 136.2-78). A middle area (RMI 78-60) was ~ identified where the morphologic change in the river is striking, as it changes from the upper braided area and enters the San Juan Canyon. Conditions within this middle area are not as uniform as in the canyon below Mexican Hat. About seven miles above Mexican Hat the canyon broadens as it enters Soda Basin. The evaporite formations within Soda Basin have been dissolved, resulting in broadening of the ~ canyon (Huber 1974). The most downstream area is a deep, narrow canyon area between Mexican Hat, Utah and Grand Gulch (RMI 60-16) where the river is confined to a single, relatively uniform channel. The fish community characteristics of backwater habitats varied longitudinally. Backwaters in the upper two geomorphologic areas of ~ the river had similar percent species compositions. In both areas red shiner constituted about 50% of the fishes in seine hauls and fathead minnow 20 to 30% (Table 12). Catch rates (fish/100 m2), however, were much higher for red shiner and fathead minnow in the braided channel area than in the middle, canyon area (Table 12). In the lowei° canyon area fathead minnow was the dominant species seined (73%), and the ~ catch rate for this species was comparatively high (Table 12). Young catostomids were seined almost exclusively in the upper braided chan- nel area (Table 12). s - 31 -