# Water cycle: Impact on nature

Low carbon on average, but rivers pay a heavy price

Source page: https://www.materialinsights.org/energy/e_hydro/nature/

What this tab covers What taking energy from this source does to land, water, air and climate, wildlife and ecosystems, with life-cycle emissions and land use where measured.

Hydropower has a low median carbon footprint (24 g CO2e/kWh, IPCC AR5) but by far the widest range of any source, because some reservoirs emit large amounts of methane. Its biggest impacts are on rivers: fragmentation, flooded land and the displacement of people.
 Life-cycle emissions 24 (median) g CO2e/kWh^[11](https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7) Life-cycle, IPCC AR5; range 1 to 2,200. The extreme upper end comes from a few tropical reservoirs with high methane emissions; UNECE 2021 gives 6 to 147 for its modelled plants, mostly excluding biogenic reservoir emissions. NREL's harmonisation project did not publish a harmonised hydropower median. Land use 4 to 27 (Norway study average 27; adjusted 7; Swiss database value 4) m2-years of land per MWh^[16](https://science.nasa.gov/missions/landsat/land-under-water-estimating-hydropowers-land-use-impacts/)

#### Effects on nature

| Area | Effect | Size | How it is reduced |
|---|---|---|---|
| **Rivers and ecosystems** | Dams block fish migration, trap sediment and change flow and temperature downstream.^[13](https://digitalcommons.mtu.edu/cee-fp/106) | Only 37% of rivers longer than 1,000 km remain free-flowing over their whole length; 23% flow uninterrupted to the ocean. | Fish passes, environmental flow releases, sediment bypass, run-of-river designs, keeping key rivers free-flowing. |
| **Climate (reservoirs)** | Flooded vegetation and soils decompose, releasing methane and CO2.^[14](https://dare.uva.nl/id/6fe8a425-ca11-48ae-a045-9e913f8198b6) | Reservoir water surfaces (all uses, not only hydro) emit about 0.8 (0.5-1.2) Pg CO2e a year, mostly as methane. | Avoid shallow tropical reservoirs with large flooded areas per MW; clear vegetation before flooding. |
| **People** | Reservoirs flood homes, farmland and cultural sites.^[15](https://riverresourcehub.org/wp-content/uploads/2007/11/01.wcdfactsheet.pdf) | Large dams have displaced an estimated 40 to 80 million people. | Consent, fair resettlement and benefit sharing, as set out by the World Commission on Dams. |
| **Land** | Reservoirs flood valleys; the area per unit of electricity depends on terrain.^[16](https://science.nasa.gov/missions/landsat/land-under-water-estimating-hydropowers-land-use-impacts/) | 0.004 to 0.027 m2-years per kWh (4 to 27 m2-years per MWh) in Swiss and Norwegian studies. | Use natural lakes and steep valleys; favour high-head sites. |
| **Climate (life cycle)** | Emissions are site-specific and highest in warm, shallow reservoirs.^[12](https://digitallibrary.un.org/record/4020227) | UNECE 2021: 6 to 147 g CO2e/kWh, mostly excluding biogenic reservoir emissions, which can be very high in the tropics. | Site selection. |

Hydropower burns nothing, but a dam converts a river into a lake. That changes everything downstream: sediment that once built deltas settles in the reservoir, fish cannot reach spawning grounds, and flows follow electricity demand rather than seasons. The World Commission on Dams linked large dams to the extinction of many fish and other aquatic species.

Land and climate effects vary more than for any other source. Norwegian mountain reservoirs, often enlarged natural lakes, flood little new land per kWh. Shallow lowland tropical reservoirs can flood large forests for modest output and emit enough methane to rival fossil power. Run-of-river plants avoid most reservoir impacts but give up storage.

#### Sources

1. [Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters (Schlömer et al.), Table A.III.2, p. 1335](https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7) IPCC AR5 Working Group III, 2014
2. [Carbon Neutrality in the UNECE Region: Integrated Life-cycle Assessment of Electricity Sources (ECE Energy Series 73)](https://digitallibrary.un.org/record/4020227) UNECE, 2022
3. [Mapping the world's free-flowing rivers (Grill et al., Nature 569)](https://digitalcommons.mtu.edu/cee-fp/106) Michigan Technological University Digital Commons, 2019
4. [Greenhouse gas emissions from reservoir water surfaces: a new global synthesis (Deemer et al., BioScience 66)](https://dare.uva.nl/id/6fe8a425-ca11-48ae-a045-9e913f8198b6) University of Amsterdam (UvA-DARE), 2016
5. [Fact sheet: The World Commission on Dams](https://riverresourcehub.org/wp-content/uploads/2007/11/01.wcdfactsheet.pdf) International Rivers (River Resource Hub), 2007
6. [Land under water: estimating hydropower's land use impacts (on Dorber, May and Verones, Environmental Science and Technology 2018)](https://science.nasa.gov/missions/landsat/land-under-water-estimating-hydropowers-land-use-impacts/) NASA Landsat Science, 2018

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_hydro/index.md)
- [Overview](https://www.materialinsights.org/energy/e_hydro/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_hydro/correlation/index.md)
- [How energy resides](https://www.materialinsights.org/energy/e_hydro/resides/index.md)
- [Abundance](https://www.materialinsights.org/energy/e_hydro/abundance/index.md)
- [Fluctuation in time](https://www.materialinsights.org/energy/e_hydro/time/index.md)
- [Across the Earth](https://www.materialinsights.org/energy/e_hydro/geography/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_hydro/links/index.md)