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The endangered fish species have been exposed to selenium from irrigation runoff since 1915 with the completion of the first major irrigation project. The primary irrigation projects were completed prior to 1920 (Butler et al. 1996), and selenium concentrations initially were much higher than those observed now. For example, as reviewed by Hamilton (1998), "Historically, selenium concentrations measured in four irrigation drains in the Grand Valley in the 1930s were 320 to 1050 gg/L, whereas two new drains near Mack, Colorado (about 32 km downstream from Grand Junction), contained 1980 gg/L in 1934 and 2680 gg/L in 1935 (Anderson et al. 1961). In 1936, selenium concentrations in the Colorado River at Cameo, Colorado, contained 0 to 1 gg/L, whereas downstream of irrigation influences, but above the Gunnison River, the Colorado contained 0 to 10 gg/L (mean 4 gg/L). At the same time, the Gunnison River at its mouth contained 5 to 55 gg/L (mean 22.5 gg/L). Anderson et al. (1961) also reported that at one point in the 1930s the Gunnison River at Grand Junction contained 80 gg/L and the Colorado River at Grand Junction contained 30 gg/L." Finally, it is worth noting that the present-day populations of the endangered fish are not significantly different above or below Cameo despite the fact that selenium exposure above Cameo, as described above, is much less. Present-day selenium concentrations in the Grand Valley are much lower because most of the readily available selenium already has been leached from the soils. Nevertheless, about 64% of the water samples collected from the lower Gunnison and about 50% of samples from the Colorado River near the Colorado-Utah state line exceed the 5 gg/L Environmental Protection Agency (EPA) criterion for the protection of aquatic life. This investigation also noted that "almost all selenium concentrations in samples collected during the nonirrigation season from Mancos Shale areas exceeded the aquatic life criterion" (Butler et al. 1996). Similar data are reported by Stephens and Waddell (1998) who examined the selenium content of streams in the Green River Basin. One generalization made by these investigators serves as a useful reference point when examining data from backwaters. Stephens and Waddell (1998) note that backwaters that received water primarily from the river were not high in selenium, while those backwaters receiving water from shallow groundwater or irrigation drains contained potentially toxic concentrations of Se. A few examples from their report are the following: • Bitter Creek, near the town of Green River (Wyoming) has up to 10 gg/L of selenium. • In the Little Snake River drainage, tributaries had up to 3 gg/L, and the river itself had typically less than 5 gg/L. • In the Yampa River, the headwaters are in gneiss and granite, but near Craig, Colorado, there are cretaceous outcrops. Several small streams near Craig have 24 to 300 gg/L of selenium and "account for much of the 13 to 17 gg/L measured in the Yampa downstream of Craig." • At Maybell, Colorado, the Yampa has 0 to 11 gg/L with a mean of about 2 gg/L. • The White River at Meeker, Colorado, has selenium concentrations that occasionally exceed 5 gg/L. The White, at its confluence with the Green, has been as high as 8 gg/L. Other creeks draining cretaceous formations into the White River are mentioned. 0 Tributaries to the Blacks Fork River have concentrations as high as 12 gg/L.