Dams and weirs in rivers are normally an insurmountable obstacle for fish and invertebrates. Since many of these creatures migrate upstream and downstream, fish corridors have to be installed on such structures so that the fish can make their way upstream. There are many different types of fish corridors, and the choice has to be made on the basis of the local circumstances and the species of fish in the area.
Migration downstream is a problem that has yet to be solved at many locations. It is often the case that fish swim through the turbines, which injures the fish and causes the turbines to malfunction. However, research is being carried out in order to find a solution to downstream migration, and a breakthrough is expected to be achieved within the next few years.
It is not permitted to take all the water out of a river in order to operate a hydropower plant. A certain amount (residual water) must be left in the river. The specified volume of residual water must always be present in the river below the intake point, depending on the local conditions. In very dry years, for example, it is permissible for less water to be present than specified in the regulations governing residual water.
The minimum volume of residual water is specified in Article 31 of the Swiss Federal Act on the Protection of Bodies of Water. In justified exceptional cases, deviations may be made from the specified volume in both directions. For fluvial power plants, a volume of residual water does not normally apply, since after exiting the turbine the water is fed back into the natural body of water at the foot of the dam or weir.
Reservoirs usually have a considerable impact on the body of water concerned, and in the past it was often the case that insufficient attention was paid to the problem of flooding. Particularly with very old facilities, when flooding occurs there is a high risk that build-ups of floating debris (log or driftwood jams) at the dam or weir can partially or completely block the passage of the river, which can result in very severe damage. The restoration of old power plant sites represents an opportunity to lessen this potential threat in an economical manner.