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Hamilton 346 kIn of drains in use. Water temperatures in the return flows were often 10 degrees warmer than the main stream, and were thought to contain a higher concentration of salts, and presumably dissolved elements such as selenium, due to evaporative loss. A recent example of this attenuation of selenium concentrations in warmed drainage water was given in Setmire et al. (1993), who reported that irrigation water from the Colorado River contained an average of 2 Ilg/L selenium, but was increased due to evaporation to a median of 44 Ilg/L in the Salton Sea area. The Gunnison River basin and the Grand Valley are major contributors to selenium loading in the lower Colorado River basin (Engberg, 1999). An analysis of selenium loading in the Colorado River at the CO-UT state line indicated that about 13% of the load was from the Colorado River basin above irrigation influences, 44% was from the Gunnison River basin including the Uncompahgre Valley, and the remaining 43% from the Grand Valley (Butler etal.,1989). ENDANGERED FISH AND IRRIGATION One historical observation overlooked until recent times is the fact that Colorado pikeminnow, bonytail, and razorback sucker were found in irriga- tion drainage ditches throughout the basin in the 194Os, based on widespread accounts (Quartarone, 1993). These ditches included those adjacent to the Green River near Green River, UT. and Linwood, WY (now under waters of the Flaming Gorge Reservoir), and in Hartland Ditch on the Gunnison River near Delta, CO. Miller (1961) noted that until 1911, Colorado pikeminnow, bonytail, and razorback sucker were so abundant in the lower Colorado River that fish got into irrigation drains and supply canals and were pitchforked out onto the banks by the hundreds for use as fertilizer or died when they were unable to reenter the river from the irrigated lands. Prior to making a spawning run, adult razorback sucker typically stage in backwater areas (low-velocity, high water temperature, high productivity ar- eas) usually found at stream mouths. Two of these staging sites in the Green River basin are the lower 0.8 kIn of Ashley Creek and Stewart Lake outfall (Tyus, 1987; Tyus and Karp, 1990), which are adjacent to each other. Ripe razorback sucker were found there during 1978 to 1989. Tyus (1987) found that 73% of the ripe razorback sucker captured in 1981 to 1986 were in the Jensen area, which includes Ashley Creek and Stewart Lake outfall. Both of these sites are known to have the most elevated selenium concentrations in water and food organisms in the Green River basin. Stephens et al. (1988) reported selenium concentrations in water in Ashley Creek were 25 to 731lg/L and in Stewart Lake outflow were 6 to 10 Ilg/L in 1986 to 1987, which exceed the criterion for the protection of freshwater organisms of 5 Ilg/L (USEPA, 1987a). Razorback sucker have also been found in the past at backwater sites in the upper Colorado River that in recent years have been documented to have elevated selenium concentrations. McAda (1977) reported that 22 razorback 1168 Hum. Eco\. Risk Assess. Vol. 5, No.6. 1999