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loss or habitat isolation due to flow reductions (flow/habitat relationship) can be accurately predicted usirrghydraulic simulation models (Bovee 1982). However, due to concerns related to estimation of suitability curves (Le., few observations, life stage dependency, concerns regarding whether observations truly reflect suitable habitat [Tyus 1992], and other criticisms of the PHABSIM approach [Stanford 1994], etc.), the primary basis for developing low flow needs were changes in river channel characteristics (curve break analysis). In our approach, the use of weighted useable areas estimates for RHABSIM was used only as a comparison to the curve break analysis approach. The curve break analysis is a threshold approach using hydraulic simulation that was used to estimate channel characteristics at various baseflow scenarios. The application of this approach assumes that the Yampa River channel is in equilibrium. Our approach used telemetry data to identify which mesohabitat types (runs, riffles, or pools) were important to Colorado pikeminnow Ptychocheilus lucius and humpback chub Gila cypha. In addition, riffles were emphasized in the analysis because of their importance to invertebrate production (Brown and Brussock 1991) as well as habitat use (as discussed in Chapter 4). Guidance from the RIP expert panel (Drs. Stephen Ross, University of Southern Mississippi; Robert White, Montana Cooperative Fish and Wildlife Research Unit, and William Trush, McBain and Trush, Inc. and Humboldt State University) during two meetings in March 1997 and January 1998 emphasized the importance of riffles in the analysis because they are the most sensitive mesohabitat to reductions in discharge. The curve break analysis was used to define the flow below which the greatest loss in habitat availability occurred for each mesohabitat type (Le. maximum degradation in stream profile of the mesohabitat type). The rationale behind the curve break analysis is: 1) native fishes, including listed species, in the Yampa River are adapted to local channel characteristics as they have existed historically, and 2) that baseflows below the curve break flow represent the greatest loss of habitat and is, therefore, detrimental to recovery of the fishes. However, because variability in low flows as well as high flows is recognized as essential in maintaining native fish communities (Poff et al. 1997), we recognize that endangered fishes have maintained persistent populations in the Yampa River despite periods of very low flow. Therefore, the recommendation of this report is to maintain flows above levels that were identified by habitat availability simulations integrated with an understanding of variability within the context of the historical flow record. Habitat Availability Testing the physical properties of the six study strata showed differences: however, only wetted width indicated a geographical pattern which segregated the three Yampa Canyon strata from the three upper 9