Lake Annecy water

Mar 2022
2 min.

Lake Annecy: H2O like no other

Visitors just can’t say enough about Lake Annecy’s breathtaking landscapes and pristine, crystal clear waters.

The lake is a source of pride for locals, who are proud to maintain such a close relationship with this beautiful natural wonder. Everyone keeps a watchful eye on the watershed’s health: for drinking (since 1908), for fishing, and for swimming. Fed by seven small rivers and mountain streams, as well as an underground spring beneath the lakebed, the lake empties into the Thiou River, which then flows through Old Town’s canals after having spent an average time of 4 years in the lake.

Taking care of the lake

If today, the waters of Lake Annecy are reputed to be some of the most pure in Europe, it is the result of the area’s citizens and governments making it so.

Through the 1940s wastewater was still poured into the lake. As water-quality visibly deteriorated, scientists and local officials took action by creating, in 1957, SILA (Lake Annecy Water Treatment District), with the backing of then Mayor Charles Bosson. Its primary mission at the time: to build a runoff and sewage collection belt around the lake as well as a wastewater treatment plant. The first of its kind in France, the townships around neighboring Lake Bourget and Lake Geneva quickly followed Annecy’s lead. Today, SILA oversees all scientific analysis of water quality, reed bed restoration (natures tool for wastewater treatment), and improving rainwater management. Siloé, the area’s main wastewater treatment plant and located in Cran-Gevrier, organizes tours for school groups and the general public.



Vue du lac d'Annecy depuis Duingt

© Gilles Piel

The Alpine Lakes Regional Observatory also provides scientific data to increase our understanding of the ever-changing lake environment. Located within the walls of Annecy’s Castle-Museum, it maintains a permanent exhibit on alpine lakes, where each room focuses on a specific topic: archeology, ethnology, hydrology, biology, ecology, etc.

Continuous monitoring to keep a close eye on the effects of climate change and measure pollution indicators confirm that Lake Annecy’s waters are still incredibly clean


Alpine Lakes Regional Observatory

Observatoire Régional des Lacs Alpins

© Teo Jaffre

Clean drinking water almost straight from the lake

Lake water is pumped from 40 meters deep to the La Puya pump and pre-filtering station, built in 1908 and recently renovated.

After passing underground through Mt. Semnoz, the water reaches the Espagnoux water treatment plant (equipped with an ultra-filtration system), where a very small amount of chlorine is added. Lake Annecy supplies 70% of the territory of Grand Annecy with drinking water, i.e. more than 200,000 inhabitants.

Paddle sur le lac d'Annecy

© Gilles Piel


  • © Gilles Piel

Journalist: Aude Pollet-Thiollier

Translation: Darin Reisman