There are over 45,000 large dams in the world. Hundreds more are planned or under construction. WWF 2013
Many different types of fish need to pass barriers in the river and need suitable habitat for their whole life cycle. It is important to understand what types of fish the river would naturally support. These might be purely fresh water fish trying to move around a river reach, or migratory fish such as sea trout, salmon, eels and sturgeon.
Options for river continuity
To support a natural fish population the complete removal of obstacles like dams is normally the most preferable option because it removes the obstruction and the impounding effects on habitat upstream. If the water levels above dams have to be preserved, dams can be replaced by nature-like submerged weirs. Rock ramps can be constructed as a part of a dam. Fish passes and bypass channels are constructed around obstacles.
For good efficiency and effectiveness, the entrance location must be in a position where fish can find it, normally near to the dam or the turbines of a power plant. The attraction flow must be sufficient. The technical performance of the fish pass must enable fish to ascend the fish pass. Safe downstream migration must also be considered including physical screens, angled bar racks and louvers associated with surface bypasses.
Effects of river continuity measures
Long bypass channels, resembling natural brooks or rapids, enable migration of all kinds of fish and invertebrates. They also enable migration of mammals and waterbird families migrating along rivers and banks. Channels with low gradient enable spawning and rearing of juveniles which can go some way to compensate for the loss of original reproduction areas in rivers.
If space is not available for a bypass channel, technical fish passes can be constructed. In vertical slot fish passes natural bottom substrate is possible like in nature-like fish passes.
Continue to the next page: Restoration of Habitats
Featured case - Vantaanjoki river
The restored Sahakoski at Vantaanjoki river (K.Samanen)
River Vantaanjoki flows to the Baltic Sea
in Helsinki city. The river was famous for its salmon from medieval times but through dam construction and later through bad water quality the original stock became exstinct.
Through new sewage treatment systems the water was purified in the1980's and the first technical fish pass was constructed in one the river arms in the estuaries in 1986. Rapids along the river were restored by stone material and by spawning gravel. Dams were removed and nature-like fish passes were constructed. Through stockings, salmon and sea trout begun to reproduce in the river.
The first fish pass was replaced by a rocky ramp in 2002 to enable more species to manage the rather steep rapid. Salmon and sea trout, both highly threatened species in the Baltic sea, begun to migrate and reproduce to the river.