CogKnoSys

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Physical

Hydroelectric Dams

Hydroelectric dams hold back rivers and use the falling water to spin turbines and generators that make electricity.

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The idea

A hydroelectric dam blocks a river and stores water in a lake behind it, called a reservoir. Most hydroelectric power comes from the potential energy of this stored water. The water rushes down a large pipe called a penstock and spins a turbine, which drives a generator. The power depends on how much water flows and on how far it drops, a height difference called the head.

In 2023, hydropower supplied 15 percent of the world's electricity, almost 4,210 terawatt-hours. A dam with a reservoir is a flexible source, because its output can be raised or lowered in seconds or minutes as demand changes. Because hydroelectric dams use no fuel, generating power this way does not produce carbon dioxide. Run-of-the-river stations have small reservoirs or none, so they use only the water flowing past at that moment.

Dams also bring real costs. Building large dams and reservoirs often floods valleys and displaces people and wildlife. Damming interrupts a river's flow and can harm local ecosystems. Drought and seasonal changes in rainfall can also severely limit how much power a dam produces.

Check yourself

Question 1 of 3

What share of the world's electricity did hydropower supply in 2023?

Pick one. The first pick counts.

Composition

what it's made of

  • Dam and reservoir — A barrier that holds back a river, storing water and its potential energy.
  • Penstock — A large pipe that carries water from the reservoir down to the turbine.
  • Turbine — A bladed wheel that the moving water spins.
  • Generator — A machine, turned by the turbine, that changes motion into electricity.
  • Head — The height the water drops; a bigger drop gives more power.

Connection

how it relates

Under Electrical Energy

Related

  1. Columbia Riverstrength 80 of 100

    In 2012, hydroelectric dams on the Columbia River and its tributaries produced 44 percent of all U.S. hydroelectric generation.

  2. Bonneville Damstrength 70 of 100

    Bonneville Dam on the Columbia River is made of run-of-the-river dam structures that generate electricity without a huge reservoir.

  3. Potential Energystrength 60 of 100

    Hydroelectric dams turn the potential energy of water stored up high into electricity as the water falls.

Cause and effect

  1. Columbia River Salmon Runsstrength 70 of 100

    Hydroelectric dams can block migrating salmon from reaching their spawning streams upstream.

Change

how and why it evolves

Past
In 1878, William Armstrong developed the world's first hydroelectric power scheme at Cragside in England. Hoover Dam's power station became the world's largest in 1936, until Grand Coulee Dam overtook it in 1942.
Now
In 2025, global installed hydropower capacity reached almost 1,456 gigawatts.
Next
New hydropower will likely grow only slowly, because environmental permits and construction take a long time.

Knowledge you can walk. Read the idea, then step up, across or down.

Hydroelectric Dams

Hydroelectric dams hold back rivers and use the falling water to spin turbines and generators that make electricity.

Learn, discuss, create, buy or invest

The idea

A hydroelectric dam blocks a river and stores water in a lake behind it, called a reservoir. Most hydroelectric power comes from the potential energy of this stored water. The water rushes down a large pipe called a penstock and spins a turbine, which drives a generator. The power depends on how much water flows and on how far it drops, a height difference called the head.

In 2023, hydropower supplied 15 percent of the world's electricity, almost 4,210 terawatt-hours. A dam with a reservoir is a flexible source, because its output can be raised or lowered in seconds or minutes as demand changes. Because hydroelectric dams use no fuel, generating power this way does not produce carbon dioxide. Run-of-the-river stations have small reservoirs or none, so they use only the water flowing past at that moment.

Dams also bring real costs. Building large dams and reservoirs often floods valleys and displaces people and wildlife. Damming interrupts a river's flow and can harm local ecosystems. Drought and seasonal changes in rainfall can also severely limit how much power a dam produces.

Check yourself

3 quick questions, instant feedback.