Holstenkoski Dam, Finland

Partial removal in southwest Finland opened more than 40 km

The River Aneriojoki
The River Aneriojoki is located in southwest Finland (Figure 1). It springs from Lake Aneriojärvi and flows into River Kiskonjoki, which in turn runs into the Baltic Sea. The Kiskonjoki-Perniönjoki watershed is of particular ecological importance and has been chosen to be especially protected by the Rapids Protection Act. River Anerionjoki in particular is part of the Natura 2000 network and hosts species included in the EU habitat directive, like otter (Lutra lutra) and dipper (Cinclus cinclus). River Aneriojoki also hosts a native and genetically specific Brown trout (Salmo trutta) population which is characterized as endangered at inland waters of southern Finland, and salmon (Salmo salar) which is vulnerable at this part of the country.

Figure 1. Location of River Aneriojoki in southwest Finland

Dam removal
The Holstenkoski Dam (Figure 2) was a 2-m-high and 45-m-long structure made out of concrete and stone. The dam was fragmenting River Aneriojoki for more than 100 years and upstream of that barrier there are many valuable and protected headwaters, brooks, and riparian forest. The dam was built in the beginning of 1900 and took its current form in the 1920s to provide water to sawmill, mill and a hydroelectric plant.

Figure 2. The Holstenkoski Dam © Janne Tolonen

The project was executed by Centre for Economic Development, Transport and the Environment of Southwest Finland, WWF Finland, Valonia and the City of Salo which ran the permit application process according to the Water Act. For the project to be implemented, a permit in accordance with the Water Act was needed and thus applied from the Regional Administration Office. The application included permits for the partial dismantling of the Holstenkoski Dam, the cancellation of the water management permits, the dredging of the reservoir and the restoration of the rapids. The permit was granted in May 2023 and the removal works began almost a year later in March 2024 (Figure 3). The removal works of the dam and the dredging of the former reservoir took place in March 2024 but were suspended during the birds’ nesting season that lasts from April till July. The works continued in August and the restoration of the rapids upstream and downstream of the dam lasted till September of 2024 (Figure 4).

Figure 3. The removal works of the Holstenkoski Dam on March 4, 2024 © Vesa-Matti Väära

Figure 4. Restored rapids on June 10, 2024, upstream of the removal site of the Holstenkoski Dam © Anni Himberg – Valonia

The Holstenkoski Dam was partially removed, as only 12 of the 45-m-long structure were removed (Figure 5). Part of the structure was preserved as a historic relic. Archaeological surveys were carried out, so that any cultural and historical sites were taken into account prior to the onset of the restoration works. Prior to the removal works, the water had to be redirected, so the part of the dam that was going to be demolished was left to dry out. The removal was done in two stages, so that the drop in water level would occur in a controlled manner.

Figure 5. The removal site on June 10, 2024, after the partial removal of the Holstenkoski Dam © Anni Himberg – Valonia

The landowners were very supportive of the project which was funded by the Ministry of Agriculture and Forestry’s Migratory Fish Stocks Revitalization Program NOUSU, the Open Rivers Program, City of Salo, WWF Finland, Raija and Ossi Tuuliainen Foundation, Valonia, Kisko Churchkake Treatment association, the Southwest Finland Fisheries Region and the Southwest Finland Nature Conservation District. During the preparation phase of the project, the project managers formed a long-term cooperation with the fisheries services of Centre for Economic Development, Transport and the Environment of Southwest Finland and the regional museum of Southwest Finland.

Ecological and community benefits
With the partial removal of the Holtenkoski Dam, more than 40 km were opened for fish and other aquatic species. This is expected to enable fish migration between the Baltic Sea and the spawning and feeding areas up in the rivers, to improve the spawning conditions and enhance their probability for the native trout population and other fish species. The ecological status of the River Anerionjoki will also be enhanced after the dam removal and the rapid restoration.

The project will be used as an important example site for educational purposes. The outcome of the project is also expected to provide insight and support the upcoming political decision-making processes for biodiversity and ecosystem restoration projects and future dam removals. The importance of the removal of the Holstenkoski Dam amplifies by the fact that this project is regionally unique, since similar removals have not been made in southwest Finland rivers so far. The restoration of the free-flow conditions and of the rapids at the former Holstenkoski Dam are also expected to boost the recreational and touristic activities in the area, like fishing tourism. The restored site will also become a local nature attraction site. On September 12, 2024 a live interview took place at the restored site, which was organized by the World Fish Migration Foundation, the WWF Netherlands and Valonia and was supported by the Open Rivers Programme (Video 1). The following day, on September 13, an opening ceremony was organized at the site of the former Holstenkoski Dam to celebrate the completion of the project and the successful restoration of the site (Figure 6).

Video 1. Live interview with Jarkko Leka from Valonia on September 12, 2024 from the site of the partially removed Holstenkoski Dam

Figure 6. Opening ceremony on September 13, 2024, to celebrate the partial removal of the Holstenkoski Dam © Anni Himberg

Before & After photo pair

The removal site at: (left) before and (right) after the demolition of the Holstenkoski Dam © (left) Janne Tolonen and (right) Anni Himberg – Valonia

With many thanks to Jarkko Leka and Valonia for providing the information and the visuals presented herein.

Written by Foivos A. Mouchlianitis