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002852 . Site Y-46 is on Sage Creek downstream from a coal-fueled electric-power generating plant near Hayden, Colo. During the basinwide reconnaissance, the pH of the stream was determined to be 2.1. In addition, the water contained abnormally high concentrations of total and dissolved Cu, and dissolved Pb and V. The high trace-element concentrations may have originated in cooling- tower blowdown water discharged to the stream. According to Stratton and Lee (1975), high concentrations of chemical constituents in blowdown water can result from several processes, including (1) a natural concentration of from two to eight times due to evaporative cooling, (2) corrosion of pipes, and (3) chemicals sdded to inhibit scaling, corrosion, and algal growth. Subse- quent water sampling at this site has indicated no recurrence of the anoma- lous water-quality conditions noted during the reconnaissance. This might have been expected, due to curtailment of. the plant-effluent discharge above the sampling site since the reconnaissance. The water at site Y-26, originally selected as a control site above pos- sible oil-brine contamination, somewhat unexpectedly contained high concentra- tions of total Cd, Pb, Zn, and total and dissolved Se. Although the stream above the sampling point drains primarily the Upper Cretaceous Mancos Shale, other streams draining this formation (for example, sites Y-28, 35, 52, 70, and 75) did not exhibit similar conditions. . Concentrations of Fe and Mn exceeding currently applicable recommended drinking-water standards (U.S. Public Health Service, 1962) occurred relative- ly frequently (at 40 sites). Excessive concentrations were found in numerous streams draining areas of active coal-mining and past metal-mining (gold, sil- ver, copper, lead, and zinc). However, at a few sites in the Yampa River basin, no ready explanation may be given at present for the high levels ob- served during low-flow conditions. ) An analysis of the trace elements associated with stream-bottom sediments has the potential of providing information not obtainable from concentrations of trace elements in water collected at discrete times from the same site. Stream sediments are exposed to varying conditions over an e~tended period of time; as such, the sediments should act as integrators and reflect some aver- age of upstream water-quality conditions during the period of exposure. Un- der certain circumstances bottom sediments may reflect the extreme conditions. The underlying assumption in this analysis is that bottom-sediment transport rates are substantially lower than average stream velocities. The trace-element frequency distributions in stream-bottom sediments were analyzed with the same techniques used for trace-element distributions in water. Generally speaking, most distributions were approximately symmet- rical. For example, the Fe distribution shown in figure lOA is typicsl of Sb, As, Cu, and Hg distributions. However, the positively skewed Cr distri- bution (fig. lOB) characterizes only the Pb and Ni distributions. , Stream sites determined from trace-element concentrations in bottom sediments to be indicative of upstream sources of water-quality degradation are as follows: Y-68 (3 outliers), Y-46 (2 outliers), and Y-26 (1 outlier), all of which also were detected by water-quality sampling procedures. More detailed discussions of conditions at these sites is given in the expanded technical report. In addition to the above, samples from the three sites on 18