Demolition of old dams revives biodiversity – the Finnish city of Lahti aims to re-establish endangered trout in the river Kumiankoski - Lahti

Demolition of old dams revives biodiversity – the Finnish city of Lahti aims to re-establish endangered trout in the river Kumiankoski

The European Green Capital city of Lahti in Finland is restoring the river Kumiankoski rapids during the Green Capital year. Demolishing obsolete dams is one of the best and quickest measures to protect and restore natural biodiversity in the area.

Obsolete dams are being demolished throughout Finland and the whole world. In Lahti, southern Finland, the Kumiankoski stream restoration is starting in March 2021.

The dam, originally built to serve an old water mill and a hydropower plant, will be demolished and the rapids brought back to their natural state.
“Demolishing a dam is a fast, one-time and permanent measure to restore a crucial habitat for endangered migratory fish and other stream water species,” says Matti Kotakorpi, a water protection specialist in the City of Lahti.

Recent studies have revealed that there are over 1.2 million migratory obstacles in European streams and rivers. The need to protect and restore river ecosystems is also urgent in Finland. Almost all river water habitat types in southern Finland are endangered at some level.

Stream restoration enables the return of trout stock

Fragmentation of aquatic environments due to dams and other obstacles to migration is a major cause of the poor status of aquatic biodiversity worldwide. Getting rid of the obstacles to migration will improve the ecological status of watercourses. The connectivity of rivers is one of the major goals of the EU’s water framework directive.

Streams and rapids are productive aquatic environments and hot spots of biodiversity. Therefore, increasing the length of rapids will have a positive impact on the natural environment and the landscape.

In Lahti, the culture-historically valuable surroundings will regain some of their lost natural values when the length of the rapids doubles as a result of the restoration. The restoration measures will have no negative effects on the culture-historically valuable mill area.

“The goal of the restoration project is to build a natural rapids stretch that enables fish migration. Fish stocking to return trout to the river will start immediately after the restoration,” says Matti Kotakorpi.

Water levels in the lake can still be maintained

The dam of the Kumiankoski mill was used to regulate the lakes upstream of the river. The mill and the micro-hydropower plant have not been used for decades. In the future, the current water levels will still be maintained by a solid bottom dam that will be constructed during the restoration project.

Cooperation with local land- and water owners is crucial for the project, as the city of Lahti does not own the land and water areas in question. However, the City of Lahti has an obligation to regulate water levels in the lake and river area. This obligation will end when the restoration is completed.

More information

Matti Kotakorpi
water protection specialist
Tel. +358 50 5391696