The dam removal and the rapid restoration of the Holstenkoski will facilitate fish migration upstream by up to 43 kilometers, enhancing the ecological condition of the Aneriojoki River.
The river Aneriojoki has a native and genetically specified Brown trout (Salmo trutta) population. One of the main targets is to improve the spawning possibilities of the native trout population and other fish species and to improve fish migration between spawning and feeding areas in The Baltic Sea.
River Kiskonjoki-Perniönjoki watershed has been selected as one of the Finnish watersheds to be specially protected by the Finnish Ministry of the Environment (1992) and by the Nordic Council (1990). River Anerionjoki and Holstenkoski area belongs to the Natura 2000 network. Protected species by EU habitat directive Otter (Lutra lutra) and Dipper (Cinclus cinclus) live in the river Aneriojoki. Many valuable and protected headwaters, brooks, and riparian forests are also upstream of the Holstenkoski Dam. The river Varesjoki above the Holstenkoski dam is classified as having good ecological status (WFD).
| Project status | Completed |
|---|---|
| Project type | Dam removal: demolition |
| Demolition Date | Spring 2024 |
| Country | Finland |
| River | Anerionjoki |
| Km opened up | 43 km |
| Focal species | Native trout population (Salmo trutta, endangered at inland waters of Southern Finland) and salmon (Salmo salar, vulnerable at Southern Finland) |
| Organisation | The Regional Council of Southwest Finland / Valonia |
| Website | Valonia |
After the partial dam removal (12 metres of the 45-metre wide dam) and restoration, Holstenkoski will be an important example site for educational purposes. The Holstenkoski project could support upcoming political decision-making for biodiversity and ecosystem restoration projects and other future dam removals and river restorations. The Holstenkoski dam removal is a regionally unique project because similar removals have not been made in southwest Finland rivers, which are important for threatened migratory fishes.
The Holstenkoski dam removal will benefit fishing tourism in the Baltic Sea due to increasing sea trout smolt production. The dam removal area will also become a local nature attraction site. The ecological status of the Anerionjoki River will be enhanced after dam removal and rapid restoration. Dam removal will improve significantly, especially migratory fishes (ie. trout and salmon, both threatened species in Southern Finland), spawning possibilities and strengthening the genetic mixing of fish populations.
The removal of the Holstenkoski dam marked a significant step in restoring natural river dynamics and improving aquatic biodiversity. Following the approval of the Water Act permit in April 2023, a 12-metre section of the dam was successfully removed in early March 2024. Immediate restoration efforts included dredging 150 metres upstream of the former dam site, enhancing water flow and preparing the habitat for ecological recovery. Stones were strategically placed along the riverbanks to stabilise the shoreline, all completed ahead of the bird nesting period to minimise ecological disruption.
Further restoration work, undertaken in August, focused on re-establishing natural rapids. Gravel, large stones, and boulders were added to the riverbed below the dam, mimicking natural stream conditions essential for fish spawning and invertebrate habitats. This phase, completed by the end of August, has significantly improved riverbed complexity, benefiting a wide range of aquatic species and restoring a more natural hydrological regime.
To enhance riparian vegetation and prevent erosion, 1,000 tree seedlings—predominantly silver birch and tar alder—were planted along the riverbanks above the flood level in early autumn. Additionally, 2,000 square metres of bare land left by dredging were sown with a mix of grass and flowering plant seeds, supporting pollinators and improving habitat diversity. These efforts have contributed to the long-term ecological resilience of the river system.
The Holstenkoski dam removal has opened over 40 kilometres of stream and river habitats, allowing fish and other aquatic species to migrate freely for the first time in decades. This has direct benefits for fish populations, including species crucial to the river’s ecosystem, as well as for otters and freshwater mussels that rely on unimpeded river flow. Beyond ecological gains, the restored river enhances local biodiversity, strengthens ecosystem services, and boosts recreational opportunities, underlining the wider social and environmental value of dam removal projects.
The success of this initiative underscores the importance of collaboration and funding in large-scale river restoration. Dam removal in Finland is a complex process requiring multi-stakeholder cooperation, balancing ecological objectives with cultural, legislative, and recreational considerations. Moving forward, corporate funding could play a key role in accelerating similar projects, ensuring that more rivers are restored to their natural state for future generations.
The project was officially completed in February 2025.
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