What's Wrong with Dams?

A dam can disrupt a river's entire ecosystem, from headwaters to delta. Even smaller dams have detrimental effects, especially as many rivers suffer the cumulative effects of numerous dams throughout their watersheds.
When a dam impounds a river, it eliminates the ever-changing channels needed to nurture aquatic and river species. Dams change water temperatures and degrade water quality in ways harmful to native vegetation and wildlife. They trap materials and nutrients essential to river species‚ habitat, allow pollutants to accumulate, and prevent sediment and water from reaching river mouths, disrupting the ecology of deltas and estuaries.
Dams obstruct fish migration. Many dams were built without provision for fish passage. This is especially harmful to fish that must migrate over the course of their life cycle between rivers and oceans. In the Pacific Northwest, for example, the cumulative impacts of dams have helped push wild salmon to the brink of extinction.
Dams also disrupt the interaction between a river and its banks, changing the natural pattern of floodplains. Relying on dams for large-scale water storage, and to deliver water to places it does not naturally occur, disturbs the balance between groundwater and surface water and can harm the quality of surrounding soil.

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