Peak populations of 12,000 wintering waterfowl were observed on the water-filled ponds with many more on the surrounding habitat. The reservoir was planned to ultimately cover a 4,700-acre area with an eventual discharge into the valley's river system. But that never happened. In 1981, San Luis NWR complex manager Zahm sensed something was wrong. Routine tests indicated high selenium content in the pond water.
Selenium is a trace element present in alkaline desert soils.
But impacts of it were not observed in wildlife until 1983. Investigators learned that, as the water evaporated, the selenium and other minerals and salts were being left behind, accumulating in the ponds and entering the food chain at toxic levels. The tragic results caught the attention of the public. BOR and refuge personnel tried to frighten birds away by using exploders and fireworks. Nearby habitat was flooded with fresh water in an effort to lure them from the poisoned evaporation ponds.