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~nry5~7 U~L~~ State of Colorado, 12 regional councils of government have been established, and among their responsibilities are the coordination of studies dictated by section 208 of Public Law 92-500. This report was prepared by the U.S. Geological Survey in cooperation with the Routt County Department of Environmental Health. The study reported herein was coordinated with the staff members of the, North West Colorado Council of Governments. Both the Yampa River basin assessment by the U.S. Geological Survey (Steele and others, 1976a, 1976b) and the areawide wastewater-management study by the North West Colorado Council of Governments involve evaluating primary and secondary impacts of coal-resource development in their respective study areas (U.S. Department of the Interior, 1976). The study described herein was initiated because of impending population growth in the Steamboat Springs area and the concern for maintaining the regional envi- ronmental esthetic quality in order to meet demands for various forms of recreation (including water-based uses) of the area. The specific area of interest for this study, is a reach of the Yampa River, approximately from Steamboat Springs to Hayden, located in Routt County, Colo. (fig. 1). Purpose and Scope The study was conducted, in part, in fulfillment of guidelines for implementing section 208 of Public Law 92-500. The primary purpose of the study was to aid in evaluating the waste-load assimilative capacity of the study reach for a minimum mean 7-day low flow and 10-year recurrence interval (Q7,10). The results of this analysis will provide planners and managers in Routt County with information for determining the waste-load capacities of the study reach for possible design and operational alternatives of future wastewater-treatment plants. The report presents an analysis of one set of data collected during a 24- hour period. These data were used to calibrate two water-quality models: A U.S. Geological Survey model and a modified Pioneer-I model developed by Battelle Pacific Northwest Laboratories. Calibration of the two models was done to evaluate the adequacy of the various model computational algorithms. The report also discusses simulation results using the U.S. Geological Survey model for alternative levels of waste loadings, Q7,10-flow conditions, and seasonal conditions during September and December. Discussion of possible relationships between measured concentrations of 'nutrients and observed populations of macroinvertebrates and periphyton also is included. Acknowledgments The authors are grateful to Phillip E. Stark and his staff, Routt County Department of Environmental Health, for assistance in the planning and execu- tion of the field-data collection and in providing supplemental information; and to Ronald M. Eddy, Aquatic Biologist, Technical Investigations Branch, Surveillance and Analysis Division, U.S. Environmental Protection Agency, 2