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study site to or from those at the gage location. Unrecorded inflows and • losses between the gage locations and the study sites were assumed to be negligible. The gage records were obtained for the period of record; the gage with the shortest period of record determined the length of the adjusted period of record. The simulated "natural" flows were obtained directly from CRSS output files. Reclamation defined these flows as those that might have occurred at a particular observation point if there were no human impacts on the basin. These flows were adjusted to the locations of the six study sites using the technique described above. Natural flow calculations were performed only through 1978. These calculations were a major effort and are only updated once every five years due to the costs and time involved. Flows simulated for the 1980 level of development also came from the CRSS model. Natural flows were calculated for the period 1907 to 1974 (this was the period for which Reclamation has prepared the input data base). The natural flow series was then run with the water demands held constant at the 1980 level of development. This produced an output time series reflecting the probable range and frequency of flows at selected sites under 1980 conditions given historic hydrologic conditions. These flow time series may be translated to habitat time series using the library of procedures and programs described below as soon as final WUA vs flow relations for each species and life stage at each study site are prepared by CRFP. L? RESULTS AND DISCUSSION Analyses of WUA Because of extreme changes in bed elevations between low and high measured flows, three separate IFG-2 HABITAT simulations were used at the Potash, Ouray and Mineral Bottom reaches. In such instances, WUA was calculated for a range of flows within the predictive range of each simulation model for the reach. For example, at the Potash reach, simulation models were calibrated against data collected at approximately 4400, 8000 and 16000 cfs. Because of sand transport and scour, dis- tinctly different bed elevations existed at these three flows. Predic- tions made using one simulation model therefore were limited to a small range of flows around the calibration flow. To allow a broader range of simulations under these conditions, the WUA values for a range of flows around each calibration flow were calcu- lated. At points of obvious discontinuity the WUA's were adjusted to form a continuous curve for the entire range of flows (Figures 4 and 5). To calculate WUA for a time series of flows, these final WUA vs. flow curves were digitized and the coordinate pairs input to the com- puter. An interpolation program was then used to calculate WUA for any requested flow within the range of the initial curve. This process 13