Dams Alter a River's Natural Flow Cycle
River systems have natural patterns of water movement, which often vary greatly according to the season. These patterns are crucial to a number of other ecological processes in and around the river. The purpose of dams is to stabilize or capture any large annual or seasonal changes in water flow. The captured water serves two purposes: it makes water available year-round to meet human needs and it serves to help control flooding or drought. However, the natural flow cycles and disturbance patterns of a river are key elements shaping the ecosystem and the life forms that occupy it. These natural flows are crucial to maintaining the health of a river, the species that live in or near it, and the seacoast into which it flows.
Dams Reduce Native Species Diversity
Dams alter the conditions and rhythms of evolution of local species. If they cannot adapt to the post-dam environment, these species die. "Dams have been the dominant factor in the decline of aquatic fauna in this country," declares Larry Master, chief zoologist of the Nature Conservancy. Master has conducted the most comprehensive study to date on the status of the nation's freshwater animals. His analysis of four broad freshwater groups is alarming: about 20% of dragonfly species, 35% of fishes, 64% of crayfish, and 67% of freshwater mussels are either extinct, imperiled, or vulnerable.
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