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002570 first-order decay coefficients, as described by Willis, Anderson, and Dracup (1975). The first coefficient relates phosphorus losses to benthos, and the second coefficient relates phosphorus use to chlorophyll-a. The temperature- corrected rate coefficients used in the models were 0.45 per day and 0.0' per day for the first and second orthophosphate~decay rates, respectively. The second coefficient relating to chloryphyll-a use was found to have little effect for the model calibrations and therefore WaS neglected. The calibration results of the fecal-coliform and total-coliform bacteria are shown on figures 21 and 22, respectively. Considering the accuracy of the coliform-bacteria determinations and the range of observed diel variations (fig. 9), the modeling results give a general idea of profile variations. The results from the two models for the fecal-coliform bacteria, were nearly identical (fig. 21). No comparison was made for the total-coliform bacteria because the Pioneer-I model did not include an algorithm for its computation. One purpose of the model-calibration analysis was to evaluate the various computational, algorithms in the models. A second purpose was to derive a unique set of model-parameter values. As shown in the preceding discussion of calibration results, some differences in the computed profiles were noted.. The large scatter of the observed field data makes it difficult to derive the unique set of model parameters in the calibration phase for either model. As a result, calibration results are considered only fair. For the simulation phase of the analysis, described later, only the U.S. Geological Survey model was used. This choice was made because of previous experience with the model, the ease with which it can be used, and the form of required input data. FACTORS INCORPORATED INTO MODEL SIMULATIONS Existing Stream-Reach Classification The study reach of the Yampa Ri~from Steamboat Springs to Hayden, has been classified as a cold-water-rtShery secondary-body-contact type B1 (Colo- rado Department of Health, 1974). Stream-quality requirements for this classification include the following: DO concentrations to equal or exceed 6 mg/L, concentrations of fecal-coliform bacteria to be less than 1,000 colonies/100 mL, a pH range from 6.0 to 9.0, water temperature not to exceed 20oC, the maximum temperature increase from ambient water temperature not to exceed 1.lOC, and a recommended requirement that nonionized ammonia concentra-, tions be less than or equal to 0.02 mg/L (Colorado Department of Health, written commun., 1976; U.S. Environmental Protection Agency, 1976). Other variables considered in the standards of the State of Colorado and U.S. Environmental Protection Agency, such as taste, odor, color, and solids, are not included in this analysis. The nonionized ammonia concentrations are not computed directly by the U.S. Geological Surveyor Pioneer-1 models; instead, they are computed manually using reference tables. For the simulation phase of this study, tables in a report by Willingham (1976) were used. These tables express the 35 ),