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002836 . ! INTRODUCTION Energy resources, primarily coal, are being developed at increasing rates in the Yampa River basin (fig. 1) of northwestern Colorado and south-central Wyoming. For example, annual coal production in the Colorado part of the basin is expected to increase from 4.4 million tons (4.0 million t) in 1975 to more than 20 million tons (18 million t) by 1990 (Steele, 1976). Other energy resources in the basin include gas, oil, oil shale, uranium, and geo- thermal springs. Impacts from their development are anticipated to be rela- tively minor compared to those resulting from the development of the coal reserves. Alternative plans for development of coal resources in the basin have been delineated (Steele, 1976). This development will lead to increased discharges of residuals (non-economic byproducts) to air, water, and land; and the residuals, in turn, will affect environmental quality. Attempts to modify or reduce residual discharges will affect both the quantity and quality of the basin's water. A 2~-year project designed to evaluate the impacts of energy-resource development on the water resources of the Yampa River basin currently is underway. The various basin-assessment investigations are described in two work-plan reports (Steele and others, 1976a; 1976b). t The present paper describes ambient stream-quality conditions in the basin. Details of the abbreviated results given herein are contained in an expanded technical report currently being prepared by D. A. Wentz and T. D. Steele (written commun., 1976). Historical data have been evaluated, and complementary basin-assessment investigations have been implemented to deter- mine water-quality implications of the proposed alternative coal-development plans. These complementary investigations include a basinwide reconnaissance of 85 stream sites (fig. 1) during low-flow conditions in the last week of Au- gust and the first week of September 1975 (Steele and others, 1976a). Three of the sites--Y-3, Y-5, and Y-38A--were dry. Streamflow, temperature, pH, dissolved oxygen, and specific conductance were measured in the field at the remaining 82 sites. Laboratory analyses of other physical, chemical, and biological variables were performed as discussed under individual sections of this paper. The data from the basinwide reconnaissance have been tabulated in a basic-data report by T. F. Giles and R. E. Brogden (written commun., 1976). Details regarding data-analysis techniques are given in the expanded technical report. . STREAM TEMPERATURE ~ Seasonal stream temperature patterns are typically cyclical in nature and repeat themselves annually. As examples, seasonal patterns for the 1963 water year at Yampa River near Maybell, Colo., and Little Snake River above Lily, Colo. (see fig. 1 for locations), are shown in figures 2A and 28, re- spectively. Because of their cyclical nature, stream-temperature data were subjected to harmonic analysis as proposed by Ward (1963) and Collings (1969); the procedure has been incorporated in a computer program documented by Steele (1974). Harmonic coefficients estimated by the program include amplitude, phase angle, and mean, all of which provide a general characterization of seasonal stream-temperature variability. The form of the harmonic-analysis function is given in the figures. All harmonic coefficients subsequently referred to are presented in the expanded report. . 2 i l ~