November 27, 2012 11:34 am

Beavers build them from sticks. Landslides create them from trees, mud, and debris. Humans make them from earth and concrete. These structures are dams. Dams hamper the flow of water in a river or stream.
Landslides don’t mean to create dams. They do so by dumping a lot of earth and other stuff in a river. Scientists think beavers build dams for protection. Beaver dams capture water in front of the lodges in which beavers live. Beavers can hide from their enemies in this deeper water. A dam also protects the beaver lodge by slowing the river’s speed.
We build dams to control water. A dam built across a river or stream stops the water’s flow. Water collects in a lake behind the dam. The lake stores water for people to use later. The lake, or water storage area, is called a reservoir.
The water in reservoirs travels in pipes to people’s homes for drinking water. It can flow through canals for farmers to use in watering their crops. People also sail boats and swim in reservoirs.
Many dams use reservoir water to produce electricity. Water flows into large machines called turbines inside the dams. The turbines power other machines that generate electricity. Electricity produced in this way is called hydroelectric power.
Some dams are built to prevent flooding. During the rainy season, the reservoir stores the river’s extra water. During the dry season, the dam sends the reservoir water back into the river.
If you’ve ever visited a large dam, you know it is an amazing sight. Dams are some of the biggest structures ever built.
The Hoover Dam on the border of Nevada and Arizona is as tall as a 72-story building. The Grand Coulee Dam in the state of Washington contains enough concrete to build a sidewalk all the way around the Earth.
Few dams are this big, however. Most dams are small structures less than 10 feet (3 meters) tall.
Many large dams are made of concrete. Some are made of packed earth or rocks. Because these materials are not as strong as concrete, dams made of earth or rocks must be very thick. The Tarbela Dam in Pakistan is made of earth and rock. It contains more than 15 times as much material as the Grand Coulee Dam.
Dams must be strong enough to withstand the pressure of water against them. Dams also must be cared for and repaired. A dam that breaks can cause disaster. In 1889, a dam in Pennsylvania broke and let loose a wall of water. The water submerged the town of Johnstown, knocking down houses and killing more than 2,000 people.
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