Hydropower dams

Reservoirs that store water behind a dam and release it through turbines.

Capture · Water and ocean · No. 29

• Capture
• Output: Electricity

Large dams have uprooted 40-80 million people, yet their turbines turn over 90% of falling water's energy into power.

Chart: Illustration: Hydropower dams

Where it comes from

Chart: Map of where it comes from

Sunlight lifts water into clouds, rain fills rivers, and a dam turns that height into stored energy. The power on offer is the water's weight times the height it falls.

Three things to know

1. New dams run at only about 43% of rated output on average (2025), because water is saved for dry seasons and peak demand.
2. Life-cycle emissions swing from 1 to 2,200 g CO2e/kWh, mostly from methane bubbling off drowned vegetation in the reservoir.
3. Only 37% of the world's rivers longer than 1,000 km still flow freely end to end, with dams the leading cause.

The journey

1. 1 Water cycleSun lifts water; rain fills rivers
2. 2 ReservoirDam stores months of river flow
3. 3 PenstockLarge pipe drops water to the powerhouse
4. 4 TurbineFrancis, Kaplan or Pelton, over 90% efficient
5. 5 GeneratorSpinning shaft becomes electricity
6. 6 GridAC lines carry power to cities

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