Lake Thun is fed from the east by the glaciers and smaller lakes in the Bernese Alps. The water passes from the higher peaks through the river Aare, which itself passes through Lake Brienz on its way to us.
The water in the mountains is extensively used for hydro-electric power before making its way down to the valley, and at this time of year, when a lot of the mountain reserves are locked up in the form of winter snow and ice, the power company pumps the water it can gain through a huge network of underground tunnels and pipes. Once the water has used gravity to turn giant turbines buried in the mountain, the company pumps the water back to the top of the mountain again.
Because of the frozen sources and the volume of water being held back from its usual descent, the lakes on either side of Interlaken are at a lower ebb than usual, causing the water level to drop and for shallower parts of the shoreline to dry up. The surface of the lake is around half a metre lower than its summer level in winter – this means that there are many areas of rocky foreshore to explore, which are under water at other times of the year.