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Twice as much water into the old river Aare in Winznau


Since 1 January 2022, Alpiq has been discharging much more water into the old course of the Aare in Winznau than before. The annual average is twice as much water, i.e. 20 m3 per second. The quantity has been doubled as a result of the updated environmental regulations in the new concession for the Gösgen hydropower plant.


The quantity of water in the old Aare below Olten varies depending on the season and the amount of precipitation. Since 1 January 2022, Alpiq has been obliged to discharge between 15 and 25 m3 of water per second into the old course of the Aare at the Winznau weir. That is twice as much as in previous years. The reason for the significantly higher water quantities is the new concession for the Gösgen hydropower plant that came into effect in 2020 along with the environmental regulations it contains. The greater quantity of water upgrades the habitat along the entire stretch of the river Aare between Winznau and Niedergösgen. Alpiq roughly reproduces the Aare's natural flow conditions and allows less water to flow over the weir in winter and more in summer. From May 2022, the flow rate has been 25 m3 per second, i.e. 25,000 litres of water per second. This amount of water could fill the 50-metre swimming pool in Olten in just over one minute.

Gösgen power plant produces less electricity

The more water that flows over the weir into the old Aare, the lower the electricity production at the Gösgen power plant. That is because the water in the old Aare is missing in the canal and the powerhouse in Niedergösgen. Doubling the amount of water in the old course of the river reduces the Gösgen hydropower plant's annual power production by about 5 GWh. This quantity is the equivalent of the annual electricity consumption of around 1,100 households or the production of a medium-sized small-scale hydropower plant. Part of this loss can be compensated by the small-scale hydropower plant built into the weir. In the next few years Alpiq is planning to construct a new weir to replace the existing structure at Winznau. This project will also include a new and larger weir power plant.

Beware of floods

The higher quantities of water in the old course of the Aare below Winznau not only enrich the habitat of animals living in and around the water. They also upgrade the river banks for leisure activities and those seeking relaxation. However, there are certain risks for those spending time near the water below the weir. That is because floods can occur even if there are no storms or heavy rainfall. If the machines at the Gösgen power plant fail due to damage, very large amounts of water would immediately have to be discharged via the old Aare. Each of the five machines processes up to 70 m3 of water per second. Fortunately, such failures rarely occur. Nevertheless, anyone enjoying the river landscape on the banks of the old Aare should always remain alert. The water level could rise significantly at any time, and the quantities of water could increase to well over 15 to 25 m3 per second even when the sun is shining and there's not a cloud in the sky.