Lac de Pérolles, Reservoir on the Sarine in Fribourg, Switzerland

Lac de Pérolles is the reservoir held back by the Maigrauge gravity dam on the Sarine, completed in 1872 and raised by 2.5 metres in 1910. The dam is 18 metres thick at its base and 4.5 metres at the crest. Sediment carried by the river over more than 130 years has partly filled the lake and formed small shrub-covered islands visible from the dam.

Engineer Guillaume Ritter dammed the Sarine at Maigrauge to supply Fribourg with drinking water and power; the gravity dam was completed in 1872, the oldest concrete dam in Europe. Water reached the sawmills in spring 1874 and homes in autumn 1874. The dam was raised 2.5 m in 1910, when the Oelberg power station opened, and the lake has since silted up, forming small islands.

The lake is named after the Pérolles plateau above it, where engineer Guillaume Ritter sent power from the Magere Au turbines via a teledynamic cable about 1 km long, crossing a height difference of some 70 m. An 8 km circular trail with information boards explains the lake's fauna, flora, geology, hydrology and history. Fishing is allowed from the bank outside the reed belts.

The area can be explored on foot, with visitors expected to stay on marked trails to protect the sensitive environment. Dogs must remain on a leash, and while photography is permitted, visitors should respect the quiet of nature and avoid disturbing wildlife.

The dam that holds the lake back, the Maigrauge gravity wall, was completed in 1872 and is considered Europe's oldest concrete dam. Engineer Guillaume Ritter designed it to supply Fribourg with drinking water and to drive a teledynamic cable, about 1 km long with a 70 m height difference, that carried power up to the Pérolles plateau for sawmills.