Environmental Impacts of Dams

Low flows below dams killed thousands of salmon on the Klamath in 2002

In addition to huge social impacts, large dams directly impact rivers in a variety of physical and biological ways. The most significant is the alteration of a river's flow, which affects downstream ecosystems and the landscape through which the river flows.

A dam also holds back sediments that would naturally replenish downstream ecosytems. When a river is deprived of its sediment load, it seeks to recapture it by eroding the downstream river bed and banks, undermining bridges and other riverbank structures. Riverbeds downstream of dams are typically eroded by several meters within the decade of first closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam.

Riverbed deepening will also lower groundwater tables along a river, threatening vegetation and local wells in the floodplain and requiring crop irrigation in places where there was previously no need. Altering the riverbed reduces habitat for fish that spawn in river bottoms, and for invertebrates.

Large dams have led to the extinction of many fish and other aquatic species, the disappearance of birds in floodplains, huge losses of forest, wetland and farmland, erosion of coastal deltas, and many other unmitigable impacts. And contrary to the dam industry’s claims that dams offer “green” energy, most reservoirs emit greenhouse gases, some in fairly high quantities.