Material Insights · Understanding the Material World

Natural reservoirs: Impact and additionality

Firming renewables at a heavy social and ecological cost

What this tab covers The impact and additionality lens: what it displaces on the grid, who gains and who bears the burden along the supply chain, and what happens at end of life.

Reservoir hydro already firms more renewable power than any other store. Its social and ecological costs are large and site-specific.

Grid decarbonisation versus fossil displacement

Mixed

Seasonal reservoirs can carry energy from wet months to dry months and cover long wind droughts, which displaces gas and coal more than a few hours of batteries can. Hydropower generated about 4,495 TWh in 2025. Against this, reservoirs worldwide emit about 0.8 Gt CO2e a year, much of it methane, and shallow tropical reservoirs can rival fossil plants.1312

Hydropower generation, 2025 4,495 TWh1 Reservoir emissions, all uses about 0.8 Gt CO2e a year3 Life-cycle emissions, hydropower median 24 g CO2e/kWh (range 1-2,200)12

Just transition and local community impact

Unfavourable

Large dams have displaced an estimated 40 to 80 million people and flood farmland and cultural sites. Benefit-sharing and resettlement are the decisive issues. Ground heat and aquifer storage have small footprints by comparison.14

People displaced by large dams 40-80 million14

Circular economy and end of life

Mixed

Largely not applicable: no materials are consumed. Dam removal at end of life is costly, and trapped sediment must be managed.10

Sources

  1. Year of the water battery: global pumped storage capacity surpasses 200 GW (2026 World Hydropower Outlook) International Hydropower Association, 2026
  2. Greenhouse gas emissions from reservoir water surfaces: a new global synthesis (Deemer et al., BioScience 66) University of Amsterdam repository, 2016
  3. Mapping the world's free-flowing rivers (Grill et al., Nature 569) Michigan Technological University repository, 2019
  4. Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters IPCC, 2014
  5. Fact sheet: The World Commission on Dams International Rivers / River Resource Hub, 2007