Hydropower dams: Impact on the biosphere

Displaced towns, blocked fish and methane from drowned forests

What this tab covers Effects on human, animal, plant, fungal and microbial life and on whole ecosystems: in the short term (construction and early years), medium term (operating life) and long term (decommissioning and legacy).

Large dams have the largest human and ecological footprint of any renewable source: they have displaced 40-80 million people, fragment rivers, and one failure (Banqiao, 1975) killed about 171,000 people. Their life-cycle emissions range from near zero to high.

Deaths per TWh, 1965-2021, including Banqiao 1.3¹¹ Deaths per TWh, excluding Banqiao 0.04¹¹ People displaced by large dams 40-80 million¹² Reservoir greenhouse gas emissions, all reservoirs worldwide 0.8 Gt CO2e per year (0.5-1.2)¹⁴ Life-cycle emissions, g CO2e/kWh 24 (median; range 1-2,200)⁴

Effects over time

Life affected	Short term	Medium term	Long term
Humans¹²¹¹	Displacement of people from land to be flooded: 40-80 million people moved by large dams worldwide.	Reliable power, irrigation and flood control; loss of fisheries and farmland downstream; dam-failure risk.	Dam failure risk persists for the life of the structure; the 1975 Banqiao failure killed about 171,000 people.
Animals¹²¹³	Construction disturbance and river diversion.	Dams block fish migration and change river flow and temperature; the World Commission on Dams linked them to the extinction of many fish and other aquatic species.	Lost connectivity: only 37% of rivers longer than 1,000 km remain free-flowing over their whole length, with dams and reservoirs the leading cause.
Plants¹⁴¹⁰	Forest and farmland cleared or flooded.	Flooded vegetation decays, releasing carbon; riverbank vegetation downstream changes with altered floods.	Flooded land stays lost as habitat for the life of the reservoir (on average 0.027 m2 occupied for a year per kWh in a Norwegian study).
Fungi and soil life¹⁴	Flooded soils become oxygen-free.	Microbes in reservoir sediments produce methane from organic matter.	Little evidence of effect beyond the reservoir.
Ecosystems¹⁴	River turned into a lake.	Reservoir surfaces worldwide (for all purposes, not only hydropower) emit an estimated 0.8 billion t CO2e a year (range 0.5-1.2), mostly as methane.	Decommissioning releases trapped sediment; removing dams can restore rivers.

How the effects are reduced

The most important choice is the site: deep, cold reservoirs with a small flooded area per MW have far lower emissions and land use than shallow tropical ones. Fish ladders and bypasses, environmental flow releases that mimic natural floods, sediment flushing, clearing vegetation before flooding, and fair resettlement and benefit-sharing (as recommended by the World Commission on Dams) reduce harm. Dam safety regulation and monitoring address failure risk.

Sources

Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters, Table A.III.2 (life-cycle emissions, min/median/max) IPCC AR5 Working Group III, 2014
Land under water: estimating hydropower's land use impacts (on Dorber, May and Verones, Environmental Science and Technology 2018) NASA Landsat Science, 2018
What are the safest and cleanest sources of energy? Our World in Data, 2024
Fact sheet: The World Commission on Dams International Rivers (River Resource Hub), 2007
Mapping the world's free-flowing rivers (Grill et al., Nature 569) Michigan Technological University Digital Commons, 2019
Greenhouse gas emissions from reservoir water surfaces: a new global synthesis (Deemer et al., BioScience 66) University of Amsterdam (UvA-DARE), 2016