All dams have some environmental
impact but so do most of their alternatives. However, some dams
are definitely better than others.
Building a dam inevitably has some impacts on a river and its
ecosystem. However, careful siting, design and operational
procedures can minimize these impacts. Ideally, dams should be
sited where no local people need to be relocated. However, where
this is not possible, consent from affected people and
satisfactory compensation have to be a priority.
In the case of hydropower, a good
dam is generally a project with virtually no reservoir, or with
small areas that store only daily or weekly river flows.
These are called "Run-of-River" installations and
usually become economic when there is a large change in land
height such as that found at a large waterfall.
Compared to dams with large storage reservoirs, such
Run-of-River projects tend to have small water storage areas,
allow sediment to pass through, seldom displace many people, and
have small effects on seasonal river flow.
Falls Dam on the upper Nile River in Uganda ranks as a good
dam. It exemplifies the advantages of a low impact Run-of-River
installation in which the dam and turbines are placed in a
narrow canyon, and the river flow is almost constant throughout
the year. The dam has the advantage of the huge storage capacity
of Lake Victoria. This simple dam has been Uganda’s principal
electricity source for half a century.
However, not all run-of the river
projects are low impact and some have sizeable reservoirs. The
impacts of proposed dams thus have to be assessed on an
individual basis. To protect river ecosystems, all dams need
mitigation measures and special management procedures. Managed
flood releases, fish ladders and habitat creation and protection
are some examples. In some countries, such as the US
and Switzerland, a labelling system awards certificates to
hydropower plants that meet environmental criteria. The Swiss
naturemade star label is supported by WWF
Switzerland (site in german).
The impact caused by the Kariba
Dam in Zimbabwe ranks it as a bad dam.
Destruction of wildlife caused by the flooding of Lake Kariba
and the thousands of people displaced by its construction are
part of its history.
The enormous amounts of water now lost by the Zambezi River due
to evaporation from the large reservoirs at Kariba and
downstream at Cahora
Bassa Dam in Mozambique has resulted in a severely
diminished flow of water reaching the river delta.
Before the dams were built, the normal flow used to flood
wetlands along the coastal area and provide large surges of
freshwater to the estuary during the annual floods making it
very productive for fish and prawns. The changes in the river
have greatly reduced the productivity and extent of habitat for
species living in the lower portions of the Zambezi River.
In the case of the Yacyretá
dam in Argentina, the problem is not one of too little water
but of raised water levels in the Iberá marshes, an
internationally important wetland.
WWF believes that if the World
Commission on Dams guidelines are applied in the planning of
dams, such bad dams would not be built.
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