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Dams are impoundments of rivers and streams. They block the free flow or water, or they may direct or retard the flow of water. Usually a dam impounds water behind it, creating a lake or reservoir. Dams have an outlet for the water that they impound. The spillway or weir is usually a trough or pipe or some outlet that allows the build up of water behind the dam to flow out at the rate that it is flowing in once the reservoir had been filled. The water may flow out continuously or be released intermittently.

Dams may be the results of acts of nature. Landslides or earthquakes that block the flow of streams or rivers have formed numerous lakes. Animals, especially beavers, also build dams ...

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