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:~ 002854 the Yampa River just downstream from the town of Craig (fig. 1) exhibited llilusua11y high concentrations of Cr (40 to 45 and 70 to 80 ~g/g) in bottom sediments (see fig. lOB). Because Cr values just upstream from Craig fall within the median range (5 to 10 ~g/g), the source of Cr appears to be relsted to some industrial process in Craig discharging to the Yampa River. NUTRIENTS . Dissolved and total nitrogen, phosphorus, and organie carbon were deter- mined for water samples collected during August~September 1975. An examina- tion of the data using the outlier-analysis technique indicates several areas with anomalously high levels of one or more of these nutrients. The follow- ing sites and variables are delineated: (1) Y-26 , 3.0 mg/L as N, dissolved nitrite plus nitrate; (2) Y-46, 0.17 mg/L as P, dissolved phosphorus; (3) Y-54, 0.46 mg/L as P, total phosphorus; and (4), Y-52, Y-4, and Y-17 with 27, 33, and 18 mg/L, respectively, of dissolved organic carbon. DISSOLVED OXYGEN AND AQUATIC BIOLOGY . Dissolved oxygen was measured at all sites sampled during the August- September 1975 reconnaissance. Nine measurements were below the lower allow- able limit of 7 mg/L established by the Colorado Department of Health (1976, table 1, p. 1) for cold-water biota; whereas, only two measurements were less than the lower limit of 5 mg/L applicable to warm-water biota. Of course, at higher altitudes, dissolved-oxygen solubility decreases for a given tempera- ture. For example, at 150C, the altitude of a sampling site would have to ex- ceed 10,000 ft (305 m) before the saturation level of dissolved oxygen would fall below 7 mg/L. . A Surber sampler (American Public Health Association, 1976, p. 1063-1064) was used to collect samples of the aquatic macroinvertebrate populations at 72 stream sites in the Yampa River basin during August-September 1975. As part of the analysis performed on the benthic macroinvertebrate samples, di- versity indices were computed using the formula given by Slack, Averett, Greeson, and Lipscomb (1973): d = -it C~) 10g2 C~], where d is the diversity per individual, n is the total number of individuals, ui is the number of individuals in each taxon, and t is the number of taxa. The calculations were made down to the genus level. When plotted in the form of a histogram (fig. 11), the distribution of d for the 72 sampled sites is seeu to be negatively skewed, and most d values fall in the range from 2.0 to 4.0. . According to Wilhm and Dorris (1968), values of d "* * * less than 1 have been obtained in areas of heavy pollution, values from 1 to 3 in areas of moderate pollution, and values exceeding 3 in clean water areas." Forty- five percent of the d values measured as part of the basinwide reconnaissance were greater than 3. Ooly three values of d were less than 1--at sites Y-24, Y-48, and Y-76. 20 ,.:. ,~<-;;;;L: