Development of the Merikoski area
The construction of the Merikoski power plant in the 1940s closed the lower part of the Oulujoki river catchment area with a dam. The construction of the hydroelectric power plant had a significant impact on aquatic organisms and especially on migratory fish. The Merikoski dam obstructed fish migration to the Oulujoki river and resulted in the destruction of the breeding grounds upstream during its construction. Flow regulation and alterations affect ecosystems by limiting the natural fluctuations in water levels.
The Merikoski area has been actively developed for more than 20 years. The fish pass bypassing the Merikoski power plant was completed in 2003. Over the years, over 18,000 salmon and almost 4,000 trout have migrated to the Oulujoki river through the fish pass.
In 2021, a preliminary assessment was completed on the technical feasibility and costs of the fisheries restoration proposals for the Oulujoki river (Sweco 2021). This assessment evaluated the potential for reintroducing water to the old Merikoski river channel and examined the prospects for fish reproduction in the area.
The Oulujoki waterways vision and areas identified for further study
The Oulujoki watercourse vision, completed in 2023, identified several key areas for further study, including developing the existing fish pass, examining the natural fish pass, implementing year-round flow in the old river channel and the related construction of a small-scale hydroelectric plant, developing the Hupisaaret area, and improving breeding opportunities for Hartaanselkä whitefish.
The City of Oulu, in collaboration with Oulun Energia, Fortum and the North Ostrobothnia Centre for Economic Development, Transport and the Environment, started a study in 2024 to evaluate the feasibility of the various objectives concerning the Merikoski area.
In May 2024, Oulun Energia conducted a flow test on the Merikoski dam channel as part of the development work. During the flow test, the Merikoski dam bridge gates were opened on three days, and water flow in the area was monitored at three different flow rates. At the same time, the optimal water flow levels for environmental purposes to determine their potential positive impacts on local aquatic organisms. Water flows were monitored using an aerial drone and by direct observation from the shore.
The project completed at the end of 2025 when the the final report was published (only in finnish).
Restoration plans build on the survey's findings
Following on from the survey and its final report, the City of Oulu and Oulun Energia launched a more detailed study at the end of 2025.
The City of Oulu's part of the study focused on developing the area below the dam structures. Oulun Energia's own study looked at different options for building discharge structures. The work also included a preliminary hydraulic design for the water intake opening, and a review of the structures and equipment to be removed or added, along with the permits needed. Identifying risks related to technical feasibility, dam safety and gate operation, and drawing up preliminary cost estimates, were also part of the work. As a second option, the study looked at improving the fish pass, to increase the number of fish passing over the dam through it. Oulun Energia has also already taken action to stop fish getting stuck in the deep pools below the dam.
The more detailed study was completed in spring 2026. The City of Oulu has published a release on the results of the feasibility and risk assessment for the restoration measures (only in finnish).