New weir improves water quality from Juuka to Pielinen

A new bottom weir on the Vuokonjoki river in Juuka has replaced an ageing gate dam, improving conditions for nature and reducing shoreline erosion, Yle reports. The river flows into Pielinen, Finland’s fourth-largest lake.

The new weir, completed in July, allows water to flow freely through a rock-lined channel. Its operation requires no manual adjustment, and fish and other aquatic organisms can move through it without obstruction.

The project has its roots in more than 200 years of changes to the Vuokonjärvi lake. The lake level was lowered several times from the 1830s onwards, including through a roughly 200-metre channel blasted into rock. The work created about 500 hectares of additional farmland for farms in Vuokko village.

Residents later built a regulation dam to reduce flood damage. The gate dam was completed in the early 1970s, but its steel gates required frequent adjustment according to water conditions. Rapid and repeated changes in the water level also caused erosion and runoff along the lake’s marsh-like shores.

The new bottom weir is about 20 metres wide and 60 metres long. It reduces fluctuations in the water level, meaning floods at Vuokonjärvi should not rise as high as before. This is expected to benefit both agriculture and recreational use of the shoreline.

Mikko Vauhkonen, secretary of the Vuokonjärvi water management company, said the bottom weir was considerably cheaper to build than a new gate dam. He also said it would improve the load-bearing capacity of fields near the shore and benefit natural conditions.

“A significant factor was also that the bottom weir operates by itself, so no one has to keep watching the weather forecasts,” Vauhkonen said, according to Yle.

The lake’s water quality is currently satisfactory, while the Vuokonjoki river’s water quality is poor. Fishermen have noticed dirty water as far away as Vuokonlahti in Pielinen. Reducing erosion is expected to gradually improve water quality in Vuokonjärvi, the Vuokonjoki and the part of Pielinen near the village, although the change will take years.

A faster change has already been observed in the fish channel built at the bottom of the weir, where shoals of small fish have been seen after the barrier to their movement was removed.

Ari Heiskanen, a water management expert in the eastern water unit of the Southeast Finland Centre for Economic Development, said the project would provide significant benefits for nature. He described the bottom weir as a more natural structure that allows fish and other aquatic organisms to move freely between Pielinen and Vuokonjärvi.

The project cost 130,000 euros, of which the state covered 70 percent. Heiskanen said funding from the Ministry of Agriculture and Forestry is currently readily available for similar projects because bottom-weir projects have been designated as highly important.

Other recent bottom-weir projects in eastern Finland include one at Ylä-Pieksä in Muurame and another at Pieksänjärvi in Pieksämäki. Several dozen similar projects are under way across Finland.