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1)0'2575 Flow Augmentation of Proposed Yamcolo Reservoir The Upper Yampa Water Conservancy District and the Colorado Water Conser- vation Board have proposed the construction of the Yamcolo Reservoir on the Bear River, a tributary of the Yampa River, in Garfield County (fig. 1). An engineering feasibility and operational study of the proposed reservoir was conducted by Western Engineers, Inc. (1975). Of particular interest to the analysis of waste-load assimilative capacity is the possibility of streamflow augmentation from the Yamcolo Reservoir in the study reach of the Yampa River. The operating rules of the proposed reservoir would include possible augmenta- tion of flow in the Yampa River by approximately 20 ft3/s (0.6 m3/s) during low-flow periods; On the basis of streamflow-data evaluation for the State of Colorado (Livingston, 1970), the computed Q7,10 flow of the Yampa River at Steamboat Springs was 28 ft3/s (0.78 m3/s) with a standard deviation of 27 ft3/s (0.76 m3/s) and a standard error of 58 percent. The regionalized Q7,10 flow estimated for tributaries flowing into the study reach should be consid- ered only as fair to poor in accuracy (Livingston, 1970). The augmentation from the proposed Yamcolo Reservoir could increase the Q7,10 flow in the Yampa River at Steamboat Springs to 48 ft3/s (1.3 m3/s). The option of flow augmen- tation of 20 ft3/s (0.6 m3/s) was used in evaluating the effects of treated wastewater effluent from the proposed regional treatment plant during Septem- ber and December flow conditions on the Yampa River. population Projections for Steamboat Springs A substantial seasQnal variation in population in the Steamboat Springs area occurs each year because of summer and winter recreational activities. Because of this variation, two population indexes,have been used in this study: peak-day and permanent population. The peak-day population is defined as the maximum daily population for a given year, and permanent population is defined as the average number of year-round residents in the Steamboat Springs area.. Population projections and extrapolations for 2010 based on studies by Gathers and Associates (1976) as revised by the Routt County Planning Commis- sion and the U.S. Environmental Protection Agency (1977) (fig. 23) were used for the model-simulation phase of this study. The projected population for 2010 was recommended by the Routt County Planning Commission (P. E. Stark, written commun., 1976) for use in the model- simulation phase of this report. The projected peak-day population for.2010 is approximately 26,000 and. the projected permanent population is 11,500. These population estimates were determined by extending the graphical rela- tionships in figure 23 to 2010. An assumed water use of 134 gal (0.509 m3) per capita per day was used in conjunction with the projected peak-day popula- tion of 26,000 and a maximum-design capacity of 3.5 Mgal (13.1xl03 m3) per day for the proposed wastewater-treatment plant (F. J. Vogel, written commun., 1975). As noted in the wastewater-facilities plan (U.S. Environmental 40