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temperatures in the spawning range of humpback chubs. June flows, however, were necessarily quite high to maintain humpback spawning temperatures, especially if reproductive isolation between humpback and. roundtail chubs was to be maintained. The normal July and August squaw- fish spawning requirements were easily met at the mean monthly discharge levels for those 2 months. The Jensen reach Green River temperatures indicate suitable squaw- fish spawning temperatures only at flows less than the July mean monthly discharges, during July (Figure 19). Even at the July mean monthly discharge, temperatures at Jensen only exceed squawfish spawning criteria by 1° C. The temperature depression here, relative to downstream Green River reaches and to the Colorado River during similar periods, is due to Flaming Gorge releases during the period of record. It is likely that cooling to this extent is no longer apparent, because of increased Flaming Gorge release temperatures. Validation Results of the regression models for the validation gages were compared to results of the physical process model using the mean difference, 6, probable difference range, d, by months for the years 1964 to 1977 (Table 3). The same statistics were calculated for "normal" discharges and meterological conditions. (Table 4). The probable dif- ference range is the 50 percent confidence limits about the mean differ- ence; i.e., 50 percent of the data from the validation gage regression model will fall within 6 + d of the physical process model. One must not assume that the differences represent errors. Where there is a difference, it can only be said that both temperatures cannot be correct. Errors probably exist in both the regression and physical process models. It has been pointed out that the published water temperature data does not represent the average daily water tem- peratures. Therefore, it can only be expected that the regression model predictions generally fall within the minimum/maximum daily water temperatures. Calibration It was possible to calibrate the physical process model to exactly match water temperature data at any of the validation gages by either modifying the regression models at the headwater and point sources or adjusting certain stream channel geometry variables (e.g., stream width). Assuming that the validation gage regression models were accur- ate, another calibration technique involved adjusting the wind speed to account for both the transposition from the weather station to each reach in the basin and to possibly better determine the evaporation coefficient used in the heat flux components. • 28