All dams have environmental and social impacts, but some dams are better than others. Generally, projects that are placed on the main stem of a river, have large reservoirs and are upstream of important habitats such as wetlands tend to have the worst impacts, as do projects that require the displacement of large numbers of people.
Given these constraints it is not easy to find suitable sites for dams, but not impossible. The Owen Falls Dam on the upper Nile River in Uganda, for example, is a low impact Run-of-River installation in which the dam and turbines are placed in a narrow canyon, where 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.