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Oll11ft tion' on each study is documented in reports as in- dicated in the discussion. The analysis" of -the waste-load assimilative capacity indicated that concentrations of non- ionized ammonia nitrogen in the Yampa River from Steamboat Springs to Hayden (fig. 2) mayex- ceed the State's proposed stream standards of 0.2 mg/L on peak-population days (Bauer and others. 1978). The computed concentrations were based on population projections for the year 2010 derived from several planning studies (Gathers and As- sociates, Inc.. 1976; U.S. Environmental Protection Agency, 1977). December and September streamflow conditions and different population projections for 2010 were considered (Bauer and others, 1978). Several major factors-for example, existing stream-reach classifications for aquatic life, sug- gested standards for effluent from a proposed regional wastewater-treatment plant, and Steam- boat Springs area population projections-were considered in the model simulations. For the model-simulation phase, it was assumed that all water-quality variables conaidered by the model were only temperature dependent. For example, the simulation for December basically assumed the same biologic and pH conditions as assumed for September, which introduced some degree of un- certainty into the model-simulation results. Nonionized ammonia concentrations were com- puted separately. with due consideration given to prevailing conditions of temperature as well as pH. Concentrations of non ionized ammonia deter- mined from computed ammonia concentrations ex- ceeded the proposed State standard for existing wastewater-treatment-plant effluent when 7-day low flows with a 1O-year recurrence interval (Q7,1O) of 28 ft'ls (0.9 m'/s) were assumed (fig. 6). Possible augmented flows of 20 ft'ls (0.6 m'/s) released from a proposed upstream reservoir such as the Yamcolo Reservoir (fig, 2) would reduce the in-stream ef- fects of wastewater-treatment-plant effluents (fig, 6). The fraction of ammonia nitrogen that exists in nonionized form increases with increasing pH. When pH exceeds 8.5, nonionized-ammonia con- centrations may become great enough to be toxic to fish (Willingham, 1976). Stream-reaeration coefficients also were com- puted for several reaches of the Ya mpa River between Steamboat Springs and Hayden, Colo. (Bauer and others, 1979). Reaeration, the physical absorption of oxygen from the atmosphere, is the primary process by which a stream replaces the oxygen consumed in the biodegradation of organic wastes. For the reaeration-coefficient computation procedure, ethylene and propane were used as tracer gases and rhodamine-WT dye was used as dispersion tracer. The basic premise of the reaeration-coefficient computation is that the ratio of the rate coefficient for a tracer gas desorbing from water to the rate coefficient for oxygen being absorbed by the same water is a constant and is in- dependent of mixing and water temperature. Measured reaeration coefficients, adjusted to 20oC. range from 33.4 to 6.04 d-1 for the stream reaches. These direct measurements were then compared with coefficients computed using several semiem- pirical and empirical functions (Bauer and others, 1979). The model used for the waste-load assimilative- capacity analysis similarly could be used in other stream reaches in which steady-state assumptions of streamflow and waste inputs are acceptable. Steady-state assumptions may be acceptable in many situations. particularly when the focus of a study is on critical-flow conditions, usually as- sociated with low-flow periods for a given stream. A semiempirical reaeration-rate formula by Tsivoglou and Neal (1976) compared the closest to the measured reaeration data for the Yampa River. A technique described by Bennett and Rathbun (1972) was used for the earlier waste-load assimilative-capacity analysis (Bauer and others, 1978). The amount of error difference because of the use of the Bennett and Rathbun (1972) formula is not known. Because of the many available em- pirical and semiempirical reaeration formulas, completion of reaeration studies prior to making a waste-load assimilative-capacity analysis would result in a more accurate analysis. As part of a second surface-water investigation, traveltime, unit-concentration, and longitudinal- dispersion characteristics were measured for stream reaches on the Yampa and the Little Snake Rivers. Two sets of data were collected for the Yampa River and three sets of data were collected for the Little Snake River. lnformation on travel- time and unit concentrations provides a convenient means of predicting the timing and peak con- 27