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Hypothesis of Historical Effects from Selenium 1957). The lake was filled from water from several canals carrying irrigation drain water for reuse in irrigation in the Uncompahgre Valley. Barnhart (1957) evaluated several possible causes of the fish die-offs, and conclude that selenium was probably the cause. In one portion of his research, fish held in cages in the lake survived 4 months, but those released in the lake failed to survive longer than 2 months. Barnhart (1957) may have been the first inves- tigator to attribute fish die-offs to selenium uptake through the aquatic food chain. Although flannelmouth sucker were abundant in the lake, he did not find any reproduction occurring, but rather reported several pathological abnormalities in the species. Birkner (1978) reported elevated selenium in water, and in aquatic inver- tebrates and fish samples (reported on a dry weight basis) from Highline Resen'oir, Grand Junction, CO, Sweitzer Lake, Delta, CO, and Desert Reser- voir, GrandJunction, CO. At Highline Resen'oir, water samples had 4.2Ilg/L, aquatic invertebrates had 4 to 28 Ilg1 g, and plankton had 8 Ilg1 g. At Sweitzer Lake, water had 9.4 Ilg/L, aquatic invertebrates had 20 to 45 Ilg/g, plankton had 42 Ilg1 g, and fish had 26 to 79 Ilg1 g. At Desert Resen'oir, water had 12.5Ilg/L, aquatic invertebrates had 18 to 58 Ilg/g, plankton had 31 Ilg/g, and fish had 611lg/g. Selenium concentrations at these three sites show that selenium was elevated in food organisms and fish in the 1970s. HYPOTHESIS In 1890 native fish in the Colorado River basin were widespread and abundant, including currently endangered fish. Many physical changes to the rivers in the basin have resulted from dams and water diversions for domestic and agricultural use, as well as biotic changes from non-native fish introduc- tions. Chemical changes in rivers in the basin have occurred due to widespread use of irrigation and the concomitant disposal of drainwater starting in about the 1890s. Documentation from several DOA and university studies of very high selenium concentrations (55 - >2000 Ilg/L) in drainwater, tributaries, and many rivers in the upper and lower Colorado River basin in the mid-1930s, combined with the disappearance of adult endangered fish and the striking historical absence of young fish from both the upper and lower basins before influences from several mainstem reservoirs occurred. suggests the hypothesis that historical selenium contamination in the 1890 to 1910 period caused the decline of native fish inhabiting big rivers such as Colorado pikeminnow, razorback sucker, and possibly others such as bonytail in the 1910 to 1920 period in the upper basin and in the 1925 to 1935 period in the lower basin (Table 1). University studies in the 1970s and NIWQP studies in the 1990s have documented elevated selenium concentrations in water, sediment, and biota throughout the entire Colorado River basin. Recent DOl studies designed to determine the biological effects of selenium on lan'al and adult endangered fish in the upper Colorado River basin, have shown adverse effects at concen- Hum. Eco\. Risk Assess. Va\. 5. No.6, 1999 1173