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American Falls Dam
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DAMS
The first record of a dam was from the Greek historian Herodotus. The dam was built across the Nile River in about 2900 B.C. to protect the city of Memphis from flooding. Dams were popular with the Romans, and two of their dams are still in use today, almost 2000 years later. Dam construction was mostly lost after the Romans until the 1800's.

Dams can serve many different purposes. A dam can be either a storage, diversion, or detention dam. Storage dams are built to trap water so that it can be used later, for example, an artificial reservoir to provide a city with drinking water. Diversion dams are often used for irrigation. Detention dams are constructed to either stop or slow the amount of water in a river. This is mostly to prevent flooding.

The most important load that a dam must be able to support is the water behind it. How much the water pushes on the dam is called water pressure. A dam's shape can help it to withstand the pressure. How does a dam withstand the crushing pressure of a lakeful of water? Water pressure increases with the depth of the water. In deep water, there is more water backed up, which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand the pressure at the bottom. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the top of the dam can be thinner so as not to use unnecessary costly materials.

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