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Dams: Facts

What is a Dam?


A dam, whether it is big or small, is a plug in a stream. It is a barrier built across a stream or river to stop or check the flow of water. Some dams form by chance, such as by natural processes like landslides> or glacial ice. But dams we're most familiar with are built on purpose, by beavers or humans.

A beaver dam is an example of a small dam; it is made by using sticks and mud to slow down the flow of a stream or a river. This causes water to pool behind the jam of sticks and mud and results in a new pond being built.

Large dams, on the other hand, are more complex to build and take a lot of work, power, time and money. A dam can be made of concrete, rocks, wood, or plain old earth. An example of a large dam is the Grand Coulee Dam in Washington. It is about 550 feet tall and is made of concrete.

The most important load that a dam must support is the water behind it. How much the water pushes on the dam is called water pressure. Water pressure increases with the depth of the water.

In deep water, there is more water “piled up,” which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand greater pressure at the bottom than at the top.

As a result, many dams are built in a triangular shape. A wide bottom withstands the great load of the water deep below the surface, while the top of the dam can be built thinner so as not to use unnecessary costly materials.

Why Are Dams Built?

Water Skiing

Dams store water for irrigation, drinking water, making electricity, and for recreation. Dams can also help control flooding.

You may be receiving drinking water from a dam's reservoir. You might be eating food grown on a farm that was irrigated by a dam's reservoir water. Or you might be receiving power generated from a hydroelectric plant using water stored behind a dam.

Types of Dams


There are different types of dams based on what materials are used and dam design. To decide what kind of dam to build, engineers think about location, materials, temperatures, weather conditions, the kinds of soil and rock at the site, and the size of dam to be built.

Gravity Dams use the downward force of the weight of the construction materials to resist the horizontal force of the water. These are the largest and heaviest of the concrete dams. These kinds of dams are built with a large base and rely on their weight to prevent the water from tipping them over. Gravity dams can only be built on a solid rock foundation.

Arch Dams are supported by the walls of the canyon in which they are built. The arch dam is built in a curved arch facing the water. Arch dams can only be built in narrow canyons where the rock walls are solid and steep. Notches, called keyways, are cut into the canyon walls on both sides, and then the dam is built so that it fits into these notches. The water pushing on the dam helps secure the dam in place.

Buttress Dams have a sloping slab that is supported by buttresses or walls. Buttress dams use less concrete than the other two types of dams. Buttress dams have less material in the wall itself but use support buttresses around the outside for support. Water pushes against the buttress dam, but the buttresses (supports) push back and prevent the dam from tipping over.

Embankment Dams are usually earthen dams. Because earth is not as strong as concrete, earthen dams are very thick. When building an earthen dam, engineers use soils that do not let water seep through.

Here's a diagram that illustrates these four dam types.

Dams and Hydroelectric Power


Some dams are built specifically to produce hydroelectric power, which is electricity generated from water. This sort of power is very efficient and pollutant-free. Water is carried by huge pipes to a powerhouse which is usually located by the dam. At the powerhouse, the power of the water pushes turbines around and around, and this continual motion creates a force that produces electrical energy.

The electrical energy that is generated at the power plant is a result of converting the potential energy of the water behind the dam into electrical energy. This hydroelectric power is then collected and distributed to homes where it is used to watch TV, play on the computer, cook food, etc. An example of a dam that produces hydroelectric power is the Bonneville Dam on the Columbia River.

Idaho Dams


In Idaho, dams that are 10 feet or higher or store more than 50 acre feet of water are regulated by the Idaho Department of Water Resources. Idaho has nearly 600 water storage dams that are regulated by this state agency. Learn about some of the bigger ones around the state.

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