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O~2539 Region VIII, for coordinating an extensive evaluation of macroinvertebrates in the study reach of the Yampa River. The authors also thank the many r~sidents of Routt County for access to the Yampa River and tributary sampling sites during the study. PreviOus Investigations A study by McCall-Ellingson and Morrill, Inc. (1974), which was com- pleted for the Colorado Department of Health, developed a water-quality management plan for that part of the Green River basin in, Colorado. Their study was designed to fulfill the requirements of section 303(c) of Public Law 92-500. In their study, the reach of the Yampa River from Steamboat Springs to the confluence with the Elk River (fig. 1) was classified as an area with significant pollution problems, and an extensive computer-model analysis was made. Conclusions of the study by McCall-Ellingson and Morrill, Inc., were: Concentrations of ammonia nitrogen exceeded the recommended standards of the U.S. Environmental Protection Agency (National Academy of Sciences-National Academy of Engineering, 1973) for almost the entire study reach, concentrations of dissolved oxygen exceeded the minimum recommended standards of the State for the entire reach, and concentrations of fecal-coliform bacteria did not exceed recommended standards of the State. Their analysis was based upon data collected at relatively high flows during July 1972, and did not include data for low-flow conditions. In 1974, Wright-McLaughlin Engineers conducted a study in the Mount Werner ski area, located northeast of Steamboat Springs. The purpose of the study was to evaluate the use of treated sewage effluent for snow-making on the ski slopes. Results of this study (Wright, 1976) were favorable toward the use of treated sewage in this manner. A separate summer land-treatment study was completed by Wright-McLaughlin Engineers (1974) for the Timbers Water and Sanitation District, also located near Steamboat Springs. DESCRIPTION OF STUDY REACH The 38-mi (61-km) study reach of the Yampa River begins approximately 3 mi (4.8 km) east of Steamboat Springs and ends 2 mi (3.2 km) west of 'Hayden (fig. 1). Daily-mean discharge of the Yampa River for the 1975 water year for the gaging station at Steamboat Springs is shown on figure 2 and for the gaging station near Hayden on figure 3. Mean-annual discharge for the Yampa River at Steamboat Springs for the period of record (1905-06 and 1910-77 water years) is 464 ft3/s (13.1 m3/s) (fig. 4). Mean-annual discharge for the 1975 water year was 9 percent higher than the long-term average (fig. 4). The variability of mean-month1y discharges for the Yampa River at Steamboat Springs for the period of record is shown on figure 5. Low flows generally occur from September to February (fig. 5). The streamflow regime for the study reach of the Yampa River during lOw-flow periods is pool and riffle. The slope of the channel (11 to 26.4 ft/mi or 2 to 5 m/km) results in long 4 .