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Ou2583 130C for September and 20C for December were assumed and were based on average yearly harmonic-mean water temperatures for Steamboat Springs (T. D. Steele, written commun., 1977). The regional treatment-plant discharge was based on projected population and 134 gal (0.509 m3) per capita per day for the plant' location (see page 40) 188 river-miles (303 km) upstream from the,mouth. The computed profiles for CBODU are shown on figure 28 and for DO on figure 29. The profile patterns were distinct for the September and December conditions. The effects of flow augmentation were negligible in both instances. The simulated DO profiles indicated only a slight DO, sag downstream from Steamboat Springs (fig. 29). The differences ,between the September and December DO profiles can be attributed to the different water- temperature conditions. The sharp increase of DO on the December profile at the start of the curve is a result of an assumed 8.5-mg/L initial concentration, much below the saturated DO concentration. The nutrient species modeled included ammonia nitrogen (fig. 30), total nitrogen (fig. 31), and orthophosphate (fig, 32). The profiles for ammonia- nitrogen and orthophosphate concentrations indicate that large differences between September and December seasonal conditions and the effects of augmented flow conditions are discernible. The ammonia-nitrogen concentrations in the Yampa River for the two periods were computed assuming the treatment-plant effluent contained 9.0 mg/L ammonia nitrogen, the proposed standard for December 1978; and 2.8 mg/L, the proposed standard for September 1978 (table 5). For each of the four conditions modeled, the maximum concentrations of ammonia nitrogen, total nitrogen, and, orthophosphate occurred downstream from the proposed regional wastewater-treatment plant to the confluence of the Elk River in December. The profiles of nonionized ammonia-nitrogen concentrations for the four assumed conditions are shown on figure 33. The concentrations of nonionized ammonia nitrogen shown of the two profiles for the augmented and nonaugmented flow conditions computed for December exceed the proposed 0.02-mg/L concentration for a type B1 stream classification with maximum concentrations of 0.042 mg/L for augmented flow and 0.065 mg/L for nonaugmented flow. These concentrations will vary depending on the actual water-temperature and pH values. For example, for the acceptable pH range of 6 to 9 and a water temperature from 00 to 200C, the nonionized ammonia-nitrogen concentration could range from 0 to 0.27 mg/L for augmented flow and from 0 to 0.42 mg/L for nonaugmented flow in December. The assumed ammonia-nitrogen concentration of the proposed treatment- plant effluent was varied in the model to evaluate its effect on the nonionized ammonia-nitrogen concentrations in the Yampa River. The results for December are shown on figure 34. An ammonia-nitrogen concentration for the plant effluent of 3 mg/L resulted in a maximum of 0.025 mg/L of nonionized ammonia nitrogen for nonaugmented flow. The results for a flow augmentation of 20 ft3/s (0.56 m3/s) in December also are shown on figure 34. An ammonia- nitrogen concentration of 4 mg/L in the effluent resulted in a maximum of about 0.02 mg/L of nonionized ammonia nitrogen in the study reach. The analysis indicates that flow augmentation may allow an approximate 25-percent increase in the permissible ammonia-nitrogen waste loading in the study reach, relative to the Q7,10-flow statistics. 48 ~