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. ~ . ,. . . 002855 Conditions at Y-76 appear to be an artifact of the high velocities asso- ciated with the outflow of Yampa Reserv0ir (fig. 1). The total biomass of 99 g/m2 (wet weight) measured at this site consisted of 99 percent (by num- bers) black fly larvae. The next highest biomass value measured in the basin on a Surber sample was 21.6 g/m2, and 59 of 61 determinations in the basin were 12.6 g/m2 or less. The low value of d (0.21) at site Y-48 may at first be thought to result from the coal-stripping operation above this point, and indeed this may be at least partly true. However, at site Y-49, above the stripping operation, the value of d is still only 1.19. These generally low values of d probably are influenced by the very low streamf10ws (estimated to be 0.1 and 0.01 ft3/s or 0.003 and 0.0003 m3/s, respectively) at sites Y-48 and Y-49. At site Y-24, high values of periphyton biomass were noted in addition to the low value of d. However, none of the nutrients occurred in excessive concentrations at this site. Three sites (Y-26, Y-46, and Y-68) were designated previously as anoma- lous based upon the outlier analysis of observed trace-element concentrations in water. The d value at site Y-26 was 2.28, giving it a "moderate pollution" designation based upon the Wilhm-Dorris index. The bottom conditions at sites Y-46 and 68 made the collection of a Surber sample impossible. AMBIENT STREAM QUALITY Based on the preceding data analysis, the maximum likely ambient concen- trations of selected chemical constituents in streams in the Yampa River basin have been determined. Application of this procedure in this context was first used by Wentz (1974b) for detecting impacts of metal mining on Colorado streams. The maximum likely limit for ambient concentrations in streams is defined herein for each constituent as x + 2s (mean plus twice the standard deviation for data obtained at unperturbed sites during low-flow conditions in August and September 1975. A summary of these maximum concentrations of con- stituents, so determined, is given in table 1. RELATED ONGOING STUDIES Knowledge of ambient stream quality in the Yampa River basin forms the basis from which to evaluate potential water-quality impacts of coal-resource and related economic development. The following is a brief overview of relat- ed ongoing studies designed to examine these potential impacts. Specific re- sults from these investigations will be topics of subsequent Yampa River basin-assessment proj ect reports. Additional studies supported by other Fed- eral, State, and local agencies will serve to complement the basin-assessment studies mentioned here (see, for example, U.S. Geological Survey, 1976; Eddy, [976) . Based upon a range of projected coal-mining production levels in the basIn for the year 1990, coal has been allocated among the following uses: (1) Utilization by thermal-power generation or coal-gasification plants, (2) transport out of the basin by rail or slurry pipeline, and (3) in-basin 22 , ~ ! ;;h<;-;li