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How does a dam work?

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Quick answer

A dam blocks a river to store water in a reservoir. The wall resists the water's pressure, which increases with depth. Many dams also generate electricity by releasing water through turbines that spin generators. This creates hydropower, a renewable energy source.

A dam works by blocking a river to hold back a large reservoir of water, using the wall's strength to resist the immense pressure. Many dams also generate electricity by letting the stored water rush down through turbines on its way out.

Step by step

  1. A river is blocked by a large wall, causing water to collect behind it and form a reservoir.
  2. The dam's weight and shape resist the water's pressure, which increases with depth, so the base is thickest.
  3. Gates or spillways control the release of water to prevent overflow and manage downstream flow.
  4. In hydroelectric dams, water released from the reservoir flows through turbines, spinning them.
  5. The spinning turbines drive generators, converting mechanical energy into electrical energy.
  6. The water then continues downstream, having been used for power generation or other purposes.

Key things to understand

  1. A thick, strong wall holds back the river, forming a reservoir behind it.
  2. Water pressure rises with depth, so dams are built much thicker at the base.
  3. Controlled gates and spillways release water and prevent dangerous overflow.
  4. In a hydroelectric dam, falling water spins turbines that drive generators.
  5. Dams also store water for drinking and irrigation and help control floods.

Labelled diagram

  1. Reservoir

    Stored water behind the dam; its depth creates pressure that the dam must withstand.

  2. Dam wall

    Thick at the base to resist the increasing water pressure with depth.

  3. Intake

    Opening that lets water flow from the reservoir into the penstock.

  4. Turbine

    Blades spun by the rushing water, converting its energy into rotation.

  5. Generator

    Connected to the turbine; converts rotation into electricity.

  6. Spillway

    Channel that safely releases excess water to prevent overtopping.

Cross-section of a hydroelectric dam

Real-world examples

  • The Hoover Dam on the Colorado River stores water and generates electricity for millions of people.
  • The Three Gorges Dam in China is the world's largest power station by installed capacity.
  • The Aswan High Dam in Egypt controls the Nile's floods and provides irrigation and hydropower.
  • Small run-of-river dams divert part of a river's flow through turbines without a large reservoir.

Common mistake

✗ Myth: Dams are solid barriers that completely stop the river's flow.

✓ Fact: Most dams release water downstream through controlled outlets or spillways to maintain flow and generate power. The water is stored and then released as needed.

People also ask

How do dams generate electricity?
Water stored in a reservoir is released through tunnels or penstocks. It flows through turbines, spinning their blades. The turbines are connected to generators, which convert the mechanical energy into electrical energy. This process is called hydroelectric power generation.
Why are dams thicker at the bottom?
Water pressure increases with depth, so the deepest part of the dam faces the greatest force. A thicker base distributes that force over a larger area and provides the strength needed to resist it. This prevents the dam from tipping or cracking.
What happens when a dam breaks?
A sudden release of stored water rushes downstream as a powerful flood. It can destroy homes, roads, and bridges, and cause loss of life. The flood's speed and volume depend on the reservoir's size and the breach's size.
How does a dam generate electricity?
Water released from the reservoir falls through turbines, spinning them; the turbines drive generators that produce electricity — this is hydroelectric power.
What is a spillway?
A channel that safely releases excess water during heavy rain or floods, preventing water from overtopping and damaging the dam.

Part of the guide: Water & Weather Phenomena →