The utility there is now testing underwater power saws to remove the trees. A similar problem has occurred in Canada, in hydro projects built by Hydro Quebec. The stones and soil in the flooded area contain naturally occurring mercury and other metals. When the land was flooded, the mercury dissolved into the water, and then into the fish in the water. The creatures that eat the fish, from bears and eagles to the native Cree people, are suffering from mercury poisoning.
Mercury poisoning can cause brain damage, birth defects, liver disorders, and other ailments.
Hydropower can cause many other effects on water quality. Rivers and lakes can be filled with sediment from erosion. Water falling over spillways can force air bubbles into the water, which can be absorbed into fish tissue, ultimately killing the fish. By slowing down rivers, the water can become stratified, with warm water on top and cold water on the bottom. Since the cold water is not exposed to the surface, it loses its oxygen and becomes uninhabitable for fish.