# Hydropower dams

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

Source page: https://www.materialinsights.org/energy/e_hydro_dams/

Capture · Water and ocean · No. 29

- Capture
- Output: Electricity
 Capture · Water and ocean · Foundation · Energy

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 cycle**Sun lifts water; rain fills rivers
2. 2 **Reservoir**Dam stores months of river flow
3. 3 **Penstock**Large pipe drops water to the powerhouse
4. 4 **Turbine**Francis, Kaplan or Pelton, over 90% efficient
5. 5 **Generator**Spinning shaft becomes electricity
6. 6 **Grid**AC lines carry power to cities
 Covered in [Earth's energy flows: sun, wind, water, heat and lifeEnergy](https://www.materialinsights.org/learn/energy/earths-energy-flows/)

##### Go deeper pick a tab to open it in full

## Other sections

- [Overview](https://www.materialinsights.org/energy/e_hydro_dams/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_hydro_dams/correlation/index.md)
- [Efficiency](https://www.materialinsights.org/energy/e_hydro_dams/efficiency/index.md)
- [Techniques](https://www.materialinsights.org/energy/e_hydro_dams/techniques/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_hydro_dams/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_hydro_dams/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_hydro_dams/links/index.md)