Dam problems - Environmental impacts

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Recovery of dead wood obstructing the Petit Saut dam, French Guyana.
© Michel GUNTHER / WWF-Canon

Dams can cause significant environmental damage.

When a dam is constructed, be it for hydropower or water supply, the destruction is highly visible. But the environmental impacts of a dam stretch much further downstream than the location of the actual dam site.

Dams block migratory fish species from their spawning and feeding sites.
Fish ladders have been successful to some extent for species such as salmon. But they are not always effective and are not a viable solution for many tropical rivers.

Dams disturb natural fluctuations in water flow.

This can be particularly damaging in seasonal floodplains, affecting deposits of nutrients as well as the lifecycles of species that depend on these fluctions for feeding and breeding grounds. Dams change daily flows by releasing water as a reaction to human demands, like energy and irrigation, instead of as nature intended.


Water quality can be degraded.

Reductions in water quantities can increase salinity and make the water unusable for drinking and irrigation. Decomposition of organic matter and the leaching of mercury from the soil can introduce toxins.

The transport of sediment along the river is disrupted.

This affects the morphology of the riverbed, downstream floodplains, and even coastal deltas. In turn, this can:
  • increase flood risk
  • hamper navigation
  • lower groundwater tables
  • cause an accumulation of toxic materials
  • affect entire ecosystems.
The build up of sedimentation in the reservoir also reduces the dam's capacity and operational lifetime.

Reservoirs can emit climate-changing greenhouse gases.

Particularly in tropical areas, the release of carbon dioxide (CO2) and methane as flooded trees and plants decompose is a serious cause for concern that needs more investigation.

Pink salmon, also known as Humback salmon, (Oncorhynchus gorbuscha) migrating upstream to spawn. British Colombia, Canada
 / ©: WWF-Canon/Elizabeth Kemf
The abundance of life found in the Mekong Basin, depends upon the seasonal variations of the river.
© WWF-Canon/Elizabeth Kemf

Quick facts

  • Worldwide, the amount of water stored in reservoirs behind dams is 3 to 6 times the quantity contained in rivers.
  • It is estimated that wetland areas decreased in extent by 50% during the 20th century.
  • More than 20% of the world’s 10,000 recorded freshwater fish species have become extinct, threatened, or endangered in recent decades.
Silted river below Mettur Dam, India. / ©: WWF-Canon / Mauri RAUTKARI
River silted and polluted downstream of the Mettur Dam, India.
© WWF-Canon / Mauri RAUTKARI
 / ©: Michel GUNTHER / WWF-Canon
Dead trees drowned by Itaipu lake created by the Itaipu dam in the Atlantic rainforest. Brazil - Paraguay
© Michel GUNTHER / WWF-Canon
Dams are not always bad news for all species.
Once reservoirs become established they can become important sites for birdlife. Out of 1,345 wetlands of international importance under the Ramsar Convention, about 100 are artificial, many of these being water storage areas.

Some dam projects have implemented specific habitat restoration measures that can to some extent compensate for the their negative impacts.

Dams also affect another species — humans.

  • Dams are often promoted as a way for economic development. Yet their harmful social impacts are often overlooked.

    Learn about solutions that make sure that the environmental and socio-economic costs of dams don’t outweigh their benefits.

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