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In context of recovering endangered species, the flow objective is to maintain enough habitat to support the fish population at a level near carrying capacity. Adult carrying capacity (or biomass) may be associated with the long term base flow condition, which presumably limits habitat availability. The median base flow in the study area is about 180 to 200 cfs. CHAPTER 3: HABITAT ISOLATION Methods Habitat isolation was defined for adult Colorado squawfish. Two approaches were used to determine the minimum flows needed to allow squawfish to swim across shallow riffles, observation and modeling. Observed movements of radio implanted fish across riffles during low flow periods (under 200 cfs), will be correlated to flows recorded by the gage during the period the fish moved. A bed profile will be taken across the shallowest part of the riffle to determine riffle depths at the flow when fish were observed to move across it. (This was not done in 1996, but should in done by the telemetry crew during the 1997 field season). This data identifies the capabilities of individual fish to move across particular riffles at known flows. However, observation data is difficult to collect because the observer must be on site when the fish moves and the flow at the time of a movement indicates a successful effort, not the flow which precludes a movement. If flow does not drop low enough to be restrictive to movements, direct observation will not help identify the flow that is limiting. In other words, if flows in 1997 do not drop below 200 cfs, this method will only indicate if movement was observed or not at 200 cfs. The modeling approach is to determine the stage/discharge relationship across a number of riffles and simulate depths over a range of flows. This approach obviates the need for low flows during the observation period, because depths can be determined across the bed by the stage/discharge relationship. This approach requires a criterion for minimum depth of a passable riffle. Burdick (1996), for the fish ladder study on the Gunnison river, used a minimum depth of one foot, and we adopted that standard for this study. However, this criteria may change if fish are observed to move across riffles a <1.0 ft. A cross section was placed across the shallowest part of the cluster. Since controls are located at the break in slopes between a run and riffle, they do not represent the shallowest part of the riffle. However there is usually a riffle just downstream of the control, and another riffle located at the upper terminus of the sequence. Cross sections were placed across these riffles to determine their bed profile and the maximum depth and water surface elevation at a known flow. The hydraulic model was used to determine the flow that provided a depth of 1.0 ft. Results Strata 2 41