The British Dam Society
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About Dams

Back to people & env.People & Environment - Plants & animals

Dams are often constructed across rivers to store water that would naturally find its way to the lower reaches of the river and into the sea. The presence of the dam upsets the natural balance of the river, affecting the animal and plant life in and around it. These are some of the reasons.

  • Upstream of the dam, the river is flooded and becomes a reservoir.
  • The nature of the river flow downstream is changed.
  • The dam can hold back sediment that normally finds its way downstream.

When the river valley is inundated with water, animals are forced to leave the area and plants and trees are killed. Sometimes, rare species can be affected.

Fish ladder

The fish ladder at Pitlochry
Dam in Scotland

For some large reservoir projects, nature reserves have been created. Plants and trees have been replanted in them and some of the affected animals have been moved there. However, the reserves can only be really successful when careful thought has been given to the way that the plants and animals depend on each other and their environment.

A dam across a river can form a barrier to fish that migrate, such as salmon. Fish passes can be included in the design of a dam to allow adult fish to swim upstream to spawn, and back downstream later with their young. Fish passes usually take the form of a fish ladder or a fish lock. These fish passes have to be designed very carefully to make sure that the conditions are right for the fish to use them.