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002577 Protection Agency, 1977), there has been a large variation in the population projections ~or the Steamboat Springs area, both in terms o~population and o~ the per-capita use of water. For the study reported here, the peak-daily population was used for the December modeling condition and the permanent population projection was used for the September modeling condition. The population projections considered in this analysis (Gathers and Asso- ciates, 1976; U.S. Environmental Protection Agency, 1977) are considered only as indications of future growth. Because of the uncertainty of population projections, estimates for 2010 were used for the entire,analysis. This pro- moted a maximum loading viewpoint of the stream reach from the proposed re- gional wastewater-treatment plant (U.S. Environmental Protection Agency, 1977). MODEL SIMULATIONS Effluents from Existing Wastewater-Treatment Plants and 7-Day, 10-Year Streamflow Model simulations were conducted using the U.S. Geological Survey model for effluents from existing wastewater-treatment plants in the study reach, assuming low-flow conditions and population projections for 2010, The nonaugmented Q7,10 flow of 28 ft3/s (0.78 m3/s) was considered. ,Also con- sidered was a flow augmentation of 20 ft3/s (0.56 m3/s) from the proposed Yamcolo Reservoir. The analysis assumed the same effluent conditions measured during the September 23-24, 1975, sampling period. The results of this analysis using both flow conditions for selected water-quality variables are shown on figures 24 to 27. For the simulation phase, it was,assumed that all model parameters were only temperature dependent. Therefore, for example, the simulation for December basically assumed the same biologic and pH conditions as in September. The amount of error induced because of this and other assumptions is not known. The traveltime through the system, for the simulation phase of the study only, was estimated using a procedure described by Boning (1974). This procedure utilizes a technique of estimating traveltime as a function of discharge and mean streambed slope. After the traveltime data were computed, they were used in computations of subreach transit times and, also, of the stream-reaeration rates. A subsequent study of the traveltime and reaeration rates for the Yampa River has been completed (D. P. Bauer, R. E. Rathbun, and H. W. Lowham, written commun., 1977). Traveltime determined in the subsequent study is within 5 percent of the traveltime used in this study. Simulated profiles for concentrations of CBODU are shown on figure 24. The largest concentrations, as expected, occur immediately downstream from the main wastewater-treatment plant at Steamboat Springs. No criteria concerning in-stream concentrations of this constituent have been proposed to date. 42