At the Flumenthal hydropower plant in the canton of Solothurn, fish can now once again swim freely upstream along the Aare River. Alpiq Hydro Aare AG has completed a 480m-long fish ladder that allows aquatic species to bypass the dam, reconnecting habitats and creating new spawning grounds.
The new facility replaces an older structure from 1970 that no longer met ecological requirements. Built over two years, the project cost approximately CHF 20 million and was funded through Switzerland’s “Netzzuschlagsfonds,” part of a national programme for the ecological upgrading of hydropower plants.
Thomas Fürst, managing director of Alpiq Hydro Aare AG, said: “We are delighted to officially open this completely new and near-natural fish ladder. It will provide the Flumenthal power plant with a state-of-the-art facility that meets today’s fish migration requirements and promotes a diverse habitat for aquatic fauna.”
Located on the left bank of the Aare in Riedholz, the fish ladder spans a vertical drop of around eight metres. It was developed in collaboration with the federal government, the canton of Solothurn, and environmental organisations, with a focus on supporting native species such as barb, perch, and chub.
The system includes several interconnected parts:
- A rock ramp with rapids and gravel banks
- A ramp with block structures and transverse bars
- 37 basins forming a tailwater rock ramp
- A vertical slot pass at the mouth for precise water level management
A fish counting chamber was also installed to monitor the system’s effectiveness. Depending on river conditions, the ladder carries 3 to 5 cubic metres of water per second, automatically adjusting to changes in the Aare’s flow.
Design features such as tree stumps, a restructured riverbed, and a doping turbine support both migration and biodiversity. The lowest section, near the mouth of the river, includes a setback entrance and additional lock flow to help ground-oriented migratory fish locate the passage. Two more entrances near the river weir guide fish through a collecting channel that rejoins the main route.
This project is part of a broader national commitment. Under Switzerland’s revised Waters Protection Act, adopted in 2011, all hydropower plants must restore fish migration routes by 2030. The new fish ladder at Flumenthal marks a significant step toward meeting that goal.
