Dams and stream environments

Any dam built in a river changes the natural conditions of the stream The resulting harms to ecosystems can be mitigated in a number of ways, however.

In its natural state a river descends downstream, sometimes more and sometimes less steeply. When a dam is built for a hydropower plant, it levels out the stream gradient over a long distance. This affects the hydrological conditions of the river and, consequently, the living conditions of many plant and animal species.

The greatest temporal changes occur close to the power plant, where the water level can rise or drop by up to one metre an hour when water is run through the plant according to fluctuations in electricity demand.

These impacts can be mitigated by applying the principle of environmental flow to hydropower production, in other words adjusting the flows so that they are as favourable as possible for the ecosystem.

The dam also forms a concrete barrier in the river that impedes the passage of migratory fish and the spread of many other species. The passage of fish over the dam can be assisted by building fishways. A fishway that imitates a natural brook can enrich the stream environment and partly make up for the loss of ecological values resulting from the dam.