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. And as illustrated by the Colorado River in the Grand Canyon, fast-moving rivers can be filled up with sediment when they are slowed down. The Department of Interior, overseen by Secretary Bruce Babbitt, recently flushed huge amounts of water out of dams in an attempt to clear away the sediment. Another important habitat disruption comes from the operation of the dam to meet electric demand.
Water is stored up behind the dam and released through the turbines when power demand is greatest.
This causes water levels to fluctuate widely on both sides of the dam, stranding fish in shallow waters and drying out the habitat. In most cases, nature loses out to boaters and electric customers. The risk of a dam breaking should not be ignored. In northern India and Nepal, in the Himalayas, huge hydroelectric projects are planned that would create large reservoirs in a geographically unstable region. Frequent earthquakes make the dam a risky venture for heavily populated areas downstream.