Good And Bad Dams

All dams have some environmental impact but so do most of their alternatives. However, some dams are definitely better than others.

What is considered good?
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.

The Owen 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).

What is considered bad?
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.

Back to Top