The unnamed woman was canyoning near the Swiss town of St Gallen, the Bild daily reported. The sport has gained in popularity over recent years and involves abseiling, climbing, jumping into water and swimming in mountain rivers.
Despite heavy rainfall, a group of tourists set off from the town for a day in the mountains. But water levels in the river they were to hike along had risen dramatically and was running extremely fast.
When the group were deep in a ravine a surge of water suddenly filled the valley, sweeping two women to their deaths. One of whom was the 24-year-old group leader, from Belgium.
Others managed to climb to safety and were rescued by a helicopter. Three of the five saved were taken to hospital and treated for injuries.
Mountain rescue teams began searching for the two missing women and continued into the night. Bad weather made the hunt difficult, but at around 11pm they found the bodies of the women.
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“It had rained a lot on Sunday afternoon in the area, but how exactly the incident happened is still being investigated,” a police spokesman from the St Gallen area told Swiss paper Blick.