Chapfensee, reservoir at Mels, Canton of St. Gallen, Switzerland

Chapfensee is a reservoir on the Chapfen high plateau above Mels in the canton of St. Gallen, held at 1029 m above sea level by a northern dam wall about 20 metres high at the original outflow into the Schmelzibach and a somewhat longer eastern wall only 10 metres high. Its catchment covers 17.6 square kilometres, fed among others by the Cholschlagerbach, and the water serves the Chapfensee-Plons power plant.

The power plant was built in 1947/1948 and entered service in 1948, using a fall of 550 metres down to Plons; it runs about 5,600 hours a year and produces roughly 20 million kWh, making it the fourth-largest plant in the canton. The dam consists of a northern wall about 20 metres high at the original outflow into the Schmelzibach and a somewhat longer eastern wall only 10 metres high. In November 2015 voters approved a necessary refurbishment; on 29 September 2017 the lake was drained and stayed empty until spring 2018. After a defect in the pressure pipe the plant was taken out of service in December 2021, and on 3 May 2026 the voters of Mels approved a construction credit of 6.9 million francs, with work starting that summer aiming at recommissioning in summer 2027.

The name Chapfensee comes from the Chapfen high plateau above Mels where the reservoir lies; the lake and the wet meadows stretching almost to Vermol are protected. The water is fed from a 17.6 km² catchment that includes the Cholschlagerbach, and the lake belongs to the Chapfensee-Plons power plant.

The reservoir sits on the Chapfen high plateau above Mels at 1029 m. Since the 2017 drawdown the water has often stood near the level of the bottom outlet, so the shoreline can look bare. The dam walls are about 20 m high to the north and 10 m to the east.

The reservoir and the wet meadows around it, stretching almost to Vermol, are protected. From 29 September 2017 the water was drained for repairs to the pressure pipes and dam walls, and the lake stayed empty until spring 2018. After a defect was found in the pressure pipe, the plant was taken out of service in December 2021 and the lake remained near the level of the emergency outlet in the following summers.