Swimmers are being strongly discouraged from taking a dip in the Rhone until further notice.
The river has an exceptionally strong flow at the moment and water levels are very high.
Cantonal authorities say the dams at Seujet and Verbois are currently fully open to bring down the levels in the lake – which is 15 cm higher than the maximum allowed.
Experts say this is creating very dangerous conditions and unpredictable currents.