A dam is a structure built across a stream or river to hold water back. Dams can be used to store water, control flooding, and generate electricity.


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Conservation, Earth Science, Geology, Meteorology


Aerial view of the Hoover Dam

An aerial view of the Hoover Dam. This dam has been harvesting hydroelectric power for over 80 years now.

Photograph by Andrew Zarivny
An aerial view of the Hoover Dam. This dam has been harvesting hydroelectric power for over 80 years now.
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Morgan Stanley

A dam is a structure built across a river or stream to hold back water. People have used different materials to build dams over the centuries. Ancient dam builders used natural materials such as rocks or clay. Modern-day dam builders often use concrete.

Manmade dams create artificial lakes called reservoirs. Reservoirs can be used to store water for farming, industry, and household use. They also can be used for fishing, boating, and other leisure activities. People have used dams for many centuries to help prevent flooding.

The ancient Mesopotamians may have been some of the first humans to build dams. The oldest known dam is the Jawa Dam, located in present-day Jordan. It was built in the fourth century B.C.E. Dams provided farmers with a steady source of water to irrigate crops. This allowed ancient Mesopotamians to feed a growing population.

The Romans were master-dam builders too. They used dams to divert water for drinking, bathing, and irrigation. One of the oldest dams still in use is the Cornalvo Dam in Spain. The ancient Romans built it in the first or second century C.E.

The force of flowing water creates mechanical power. People have harnessed this power for centuries with the use of dams. Small dams powered paddle wheels in pre-industrial Europe and America. These were used to help saw logs or grind corn and other grains.

During the Industrial Revolution, engineers began to build bigger dams. These industrial-sized dams could hold back more water to power the big machinery of factories and mines. They also could turn giant turbines to generate electricity.

The early 1900s ushered in an era of “big dam” building in America as demands for electricity increased. During the Great Depression, President Franklin D. Roosevelt put Americans back to work building massive dam projects. The most famous of these is the Hoover Dam.

The Hoover Dam sits on the border between Nevada and Arizona. It was completed in 1936. The Hoover Dam is regarded as an engineering marvel. It was the tallest dam ever built at the time—222 meters (727 feet). The dam helped to control the flow of water on the Colorado River by creating Lake Mead, one of the largest reservoirs in the United States. Lake Mead provides drinking water for the city of Las Vegas.

Dams have long been viewed as a symbol of human ingenuity. However, ecologists who study rivers and lakes have uncovered some environmental downsides to dam construction. Dams change the way rivers function, and in some cases, this can harm local fish populations.

Flooding landscapes to create reservoirs can have consequences for biodiversity as well. Brazilian biologist Raffaello Di Ponzio studies the impact of big dam projects on the plants and animals of the Amazon Rainforest. More than 200 hydroelectric dams have been proposed in Brazil. While these dams could help satisfy growing South American energy demands, they would also flood more than 10 million hectares (25 million acres) of the Amazon Rainforest.

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Tyson Brown, National Geographic Society
National Geographic Society
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Sarah Appleton, National Geographic Society, National Geographic Society
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André Gabrielli, National Geographic Society
Last Updated

June 21, 2024

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