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Dams: Facts

What is a Dam?


A dam, whether it is big or small, is a structure that holds back water. It is a barrier built across a stream or river to stop or redirect the flow of water. Some dams form by chance, through natural processes like earthquakes, volcanoes, landslides or glacial ice. But the 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.

Humans have been building dams for thousands of years. The ancient Egyptians and Romans built dams to control water, some of which are still in use today. Modern dams are more complex to build and take a lot of work, power, time and money. A dam can be made of concrete, rocks, wood, steel or earth. Dam engineers design, build and maintain large dams that can range from 50 to 1,000 feet (15 to 300 meters) in height. Some examples of large dams are the Grand Coulee Dam in Washington (550 feet tall), Dworshak Dam in Idaho (717 feet tall), Glen Canyon Dam in Arizona (710 feet tall) and Hoover Dam in Nevada (726 feet tall.) The tallest man-made dams in the world are Nurek Dam in Tajikistan (984 feet) and Jinping-I Dam in China (1,001 feet.)

Hoover Dam
Hoover Dam — US Geological Survey

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, thick 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.

Why Are Dams Built?

Water Skiing

Millions of people all over the world depend on dams to help provide needed water at the right time and place. When a dam is built, it creates a reservoir of water, resulting in a man-made lake. This water is used to irrigate farm crops, to store water for cities and industries, and to provide recreation for boating, swimming and fishing. Dams also provide hydroelectric power, allow navigation for ships, and help control flooding. Without dams, modern life as we know it would be very different.

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 several different types of dams. To decide what kind of dam is to be built, engineers consider the location, materials to be used, weather conditions, the kinds of rocks and soil at the site, how much water a dam will have to support and how big it must be. There are four main types of dams:

Gravity Dam
Gravity dam: Grand Coulee Dam (US Bureau of Reclamation)

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 Dam
Arch dam: Glen Canyon Dam (USBR)

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 Dam
Buttress dam: Bartlett Dam (USBR)

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 Dam
Embankment dam: Pineview Dam (USBR)

Embankment Dams are usually earthen or compacted rock 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. Embankment dams are the most common type in the United States.

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

Dams and Hydroelectric Power

Hydroelectric Power

Some dams are built specifically to produce hydroelectric power, which is electricity generated from the movement of water. This sort of power is efficient, renewable and non-polluting. Dammed water from the reservoir is carried by a huge pipe, called the penstock, to a powerhouse which is usually located by the dam. At the powerhouse, the power of the water turns the blades of a turbine around and around, which spins a generator that makes electricity.

Three Gorges Dam, USGA

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 carried by power lines to homes where you may use it to watch TV, cook food, or work on the computer. Dams supply about one-sixth of the world's electricity. Some countries, such as Norway, Brazil and Canada, get the majority of their electricity from hydropower. The world's largest hydroelectric dam in terms of power output is Three Gorges Dam in China.

Pros and Cons of Dams

Dam 9

Dams have enabled people to control water and to make moving water useful for many purposes. They have allowed farms and cities to thrive in places previously unsuitable. They have saved lives and property from destructive flooding. Hydroelectric power reduces the need for energy from fossil fuels such as coal and oil, which cause serious pollution and greenhouse gases. Many large dams are viewed as marvels of engineering and human ingenuity; in fact, some dams are considered Wonders of the Modern World or National Historic Landmarks.

However, dams can also cause some problems. When a dam is built, the resulting reservoir of stored water replaces the original river environment with a manmade lake. Often, people living in the area are displaced and forced to move to a new location. Sometimes, archeological and historic sites are flooded by the rising water. Wildlife habitat is impacted, as conditions change for native plants and animals in the waterway. Sediments build up behind dams and water temperatures change, making survival difficult for some species.

fish ladder

One particular concern is migratory fish such as salmon, who must get past the dams to swim upstream where they will spawn. Dams change the ways rivers function, creating slow-moving pools of warm water that prolong the fish's journey and attract predators of salmon. Fish ladders have been installed next to dams to make it easier for fish to avoid turbines as they migrate upstream. To help young fish traveling downstream, barges are sometimes used to transport them around dams, and spillways direct water over the tops of dams to imitate a river's natural flow. But even with these efforts, the numbers of salmon in the Pacific Northwest have greatly decreased since the rivers were dammed. In recent years, some smaller dams, such as Elwha Dam in Washington and Edwards Dam in Maine, have been removed and the ecosystem restored. Learn more about the challenges that dams present to fish survival on Science Trek's Salmon page.

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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