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Hamilton Wash, and Big Salt Wash. Butler et ai. (1994) reported that in 1991, water samples from Reed Wash contained 120 ~g/L of selenium, Little Salt Wash 16 ~g/L, and Big Salt Wash 24 ~g/L. Average selenium concentrations in whole-body of several fish species at these sites ranged from 8 to 11 ~g/ g (all sediment and tissue concentrations are given as dry weight). Consequently, adult Colorado pikeminnow that congregated at these three sites may have been exposed to substantial amounts of selenium in water and in prey fish. In the summer of 1995, two Colorado pikeminnow were caught in an irrigation drain that emptied into the Colorado River just upstream of Adobe Creek near GrandJunction (B. Osmundson, personal communication, 1996), which showed these fish continue to use drains. mSTORICAL SELENIUM CONTAMINATION IN THE COLORADO RIVER BASIN Because of concerns about selenium toxicosis in livestock and accumula- tion in irrigated agricultural crops in the Western states in the early 1900s, numerous DOA and university studies were conducted (reviewed by Trelease and Beath, 1949). Ancillary to selenium measurements in soil, plants, and animals, concentrations were also measured during the 1930s in various water samples, but not in aquatic biota, of the Colorado River basin including irrigation drains. The 1930s reports of the DOA used a measurement tech- nique adequate to recover quantitatively 0.01 mg of selenium, as sodium selenite, from 4 L of distilled water, which corresponds to 2.5 J..lg/L and qualitative detection as low as 0.25 ~g/L (Byers et al., 1938). Above irrigated areas, water in the upper parts of the Uncompahgre, Gunnison, and Colorado rivers contained less than 1 J..lg/L (Byers, 1936). Trace amounts of selenium were also found in the Dolores River at Cisco, UT, and the Green River at Green River, UT. However, below irrigated areas, water from these rivers contained high concentrations of selenium. In samples collected in 1935, the Uncompahgre River above its confluence with the Gunnison River contained 220 ~g/L, the Gunnison River near its confluence with the Colorado River contained 80 J..lg/L, and the Colorado River near Grand Junction contained 30 J..lg/L (Williams and Byers, 1935; Byers, 1936). The high selenium concentrations in the Uncompahgre, Gunnison, and Colorado rivers were the result of irrigation return flows. Two drains entering the Uncompahgre River near Montrose, CO, had 320 and 700 J..lg/L selenium, and a drain near Chipeta, CO, had 1050 ~g/L. Cow Creek in the upper Uncompahgre River contained 2 J..lg/L, which Byers (1936) concluded, showed the first traces of selenium because it received drainage from about 486 ha of irrigated land. In the Grand Valley, one drain near Mack, CO, contained 1980 ~g/L in December 1934, and 6 months later in May 1935, it contained 2680 ~g/L. A main drain near Loma, CO, contained 630 ~g/L. It is interesting to note that Byers (1936) concluded "... irrigation, with under irrigation [i.e., tiled drains], is a potent influence in decreasing the injurious effects produced 1170 Hum. Eco\. Risk Assess. Vo\. 5, No.6, 1999