Dam problems - Environmental impacts

River silted and polluted Mettur Dam India
© WWF-Canon / Mauri RAUTKARI

More damage than benefits

Lots of environmental damage ... When a dam is constructed, the scars on the landscape are highly visible and when the reservoir starts filling up the damage to the inundated land, sometimes farmlands, sometimes pristine ecosystems, is obvious. But the environmental impacts of a dam stretch much further than the location of the actual dam site.

• Migratory fish species are particularly vulnerable to dams, which block access to their spawning or feeding sites. Fish ladders have been successful to some extent for species such as salmon, but are not always effective, and are not a viable solution for many tropical rivers. Fish populations are also susceptible to changes in water quality and water temperatures.
• Dams change the hydrology of the river and disturb the seasonal fluctuations. This can be particularly damaging in seasonal floodplains, affecting deposits of nutrients as well as the lifecycles of many fish. Dams also change daily flows by releasing water as a reaction to demands for energy, irrigation or even for navigation and recreation.
  
  
• Water quality can be degraded. Reductions in water quantities can increase salinity and make the water unusable for drinking and for irrigation, as there is no longer enough water travelling downstream to flush the ecosystem. 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 flood plains and even coastal deltas, and in turn impacts on ecosystems in these areas. The build up of sedimentation in the reservoir also reduces the capacity of the dam and there is the danger of accumulation of toxic materials.
  
  
• In tropical areas, the emissions of greenhouse gases from reservoirs pose a serious concern that needs more investigation.

... and a bit of good news
However, dams are not always bad news for all species. Once reservoirs become established they can become important sites for birdlife. Out of 1345 wetlands of international importance under the Ramsar Convention, about 100 are artificial, many of these being water storage areas.

In South Africa in particular, where natural lakes are practically absent, dams have provided habitats for birds such as pelicans, darters and cormorants. Some dam projects have implemented specific habitat restoration measures that can to some extent compensate for the negative impacts of dams.