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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.
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.
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