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becoming more pronounced beginning in the 1950s and culminating with the major dam and road construction projects in the 1970s. These habitat alterations also were favorable to the proliferation of nonnative species that competed with the native fish. Interviews with long-time residents provided some insight but no clear record regarding the decline of the native fish. The fish were used. for subsistence in the 1930s and 1940s, a practice that ceased in the 1950s. By that time, the endemic species were considered "trash" fish as emphasis was placed on providing lakes and streams for trout fishing. This attitude was exemplified by the use of rotenone in the Green River during the 1960s. In summary, the fish began a long-term decline sometime in the 1930-1950 period. With the advent of the 1970s, either that long-term decline became evident or the newly completed dam projects and road construction projects provided the mortal blow to the populations. 2. SELENIUM CYCLING IN THE ENVIRONMENT 2.1 Source Because of the problems at Kesterson, the source of selenium in irrigation water in California's San Joaquin Valley has been studied extensively (Martens and Suarez 1997). The explanation applicable to the San Joaquin is believed to explain selenium availability in other regions as well (Seiler 1998). In general, the primary sources of selenium in irrigation waters in the western United States are pyrite-rich cretaceous marine sediments. Selenium substitutes for sulfur in pyrite (FeS) forming ferroselite (FeSe). Selenium also substitutes for sulfur in chalcopyrite (CuFeS2). Supporting this explanation is the fording that selenium migration is inhibited in fractured rocks because of reactions with pyrite (Yllera et at.. 1996). The initial stages in mobilizing selenium are erosion of selenium-containing pyrites into alluvial fans and debris flows. As rainwater and snowmelt seep through these alluvial fans and debris flows, there is oxidation of the pyrites, a process manifested. by acid sulfate seeps that contain high concentrations of selenium. The water from the acid seeps is neutralized by the soils lower in the valleys where the high pH and oxidizing conditions mobilize selenium. Because of the semi-arid climate, leaching and migration of the selenium is ephemeral and evaporite minerals are formed. In these evaporite deposits, selenium is associated with Na, Mg, and Ca. Two common selenium-containing evaporite minerals are mirabilite, a sodimn sulfate with ten waters of hydration, and bloedite, a sodium/magnesium sulfate with four waters of hydration. In these and similar highly soluble minerals, selenate substitutes for sulfate. These minerals provide an instant source of mobile selenium when wetted. Meanwhile, the continued upslope erosion continues to add selenium to the system (Presser 1994). Soils in the San Joaquin Valley were too saline for agriculture until a series of drains were installed. The irrigation water, therefore, leaches salts from the fields and. into the drains that conduct them to collection areas, such as Kesterson (Presser 1994; Seiler 1998). The highest concentrations of salts usually occur where the groundwater flows upward, either in response to local topography or seasonal cessation of irrigation (Fio 1997).