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Curve breaks for all cross sections were grouped by mesohabitat-type, i.e. riffle, run, and pool and compared between variables and between habitats for differences. Physical Habitat Simulation Calculation ofWUA for multiple cross sections was modeled using the step-back module ofRHABSIM (Bovee 1982). Cross sections were placed to represent an entire run/riffle sequence with the downstream cross section in each cluster on a hydraulic control. Habitat criteria for adult Colorado pikeminnow and "humpback chub were used for modeling WUA. Depth and velocity habitat criteria were used for Colorado pikeminnow and shoreline habitat (20 feet outward from each shoreline) criteria were used for humpback chub. Only shoreline habitat was used for humpback chub because the majority (71 %) of these fish have been caught in shoreline eddy habitats with most fish being collected adjacent to large shoreline substrate (unpublished data, Colorado River Fish Project, Vernal, UT). Habitat suitability criteria for Colorado pikeminnow are presented in Appendix 2. The frequency distribution developed from these observations represented the likelihood that fish would select a specific habitat type. Diurnal Colorado pikeminnow habitat was restricted to pools over 2 ft deep, while nocturnal Colorado pikeminnow habitat and includes shallower swifter areas. Humpback chub were found in habitat associated with large cobble and boulder substrate in shoreline eddy habitat. Passage The hydraulic equation (stage-discharge relationship) was used to define minimum passage criteria. The minimum passage depth of a riffle is considered to be the maximum body depth of the largest fish in the community. Body depth is defined as the distance from the tip of the extended dorsal fin to the lowest portion of the body cavity. A large Colorado pikeminnow in the Yampa River is 32 inches (80cm) long and weighs about 11 pounds (2.4 kg) (Interagency Standardized Monitoring Program data) with a body depth of about 9 inches, of which the dorsal fin is about 3 inches. Using this logic, average depth of riffles need to be at least 0.75 ft to allow passage. We also used the depth criteria defined by Burdick (1996), who felt that a maximum depth of one foot would satisfy unrestricted movement of adult Colorado pikeminnow. Statistical Testing. Tests for significant differences between means of the seven parameters were used to determine if the physical properties of aquatic habitats varied between strata. Data from riffles were tested at flows of 80, 150 and 300 cfs. Eighty cfs represented minimum flows that infrequently have occurred (<25%), 150 cfs represents the minimum flow that commonly occurs (<50%), and 300 cfs represents median flows that 44