Réservoir Saint-Michel

Réservoir de Saint-Michel, also known as Lac Saint-Michel or Lac de Brennilis, is a man-made lake tucked into the Monts d'Arrée hills of inland Brittany, in the Finistère commune of Brennilis. Sitting at an elevation of 227 metres, it covers roughly 4.5 square kilometres, making it one of the largest bodies of water in the region.

How did the lake come to exist?

The reservoir was built between 1929 and 1936, when the Société Hydro-électrique des Monts d'Arrée dammed the Ellez river as part of a drive to bring electricity to rural inland Brittany. The project submerged more than 400 hectares of the Yeun Elez basin, a broad depression of marshland and peat bogs ringed by the gentle slopes of the Monts d'Arrée.

What does the dam look like?

The retaining structure is a 510-metre arch dam, combining a straight section with a series of adjoining arches spanning 25 metres each. It regulates the flow of the Ellez to feed the Saint-Herbot hydroelectric plant, which had been running since 1928; the new reservoir was designed to boost its output.

Why is the lake linked to a nuclear power station?

From 1967 to 1985, water drawn from the reservoir cooled the reactor of Brennilis nuclear power plant, one of EDF's early gas-cooled, graphite-moderated stations. The plant shut down in 1985 and has been under decommissioning ever since, a process not expected to finish until around 2040. On the former plant site, EDF now runs a visitor centre, the Espace découverte, explaining the station's history and the dismantling work.

What is there to see around the lake?

The shoreline slopes gently into meadows, heathland and peat bogs; one of them, the Vénec bog, is protected as a national nature reserve. A footpath loops around the Yeun Elez wetlands for about 15 kilometres, a moderate-to-strenuous walk of four to five hours with little elevation change. Along the way, walkers pass standing stones and sites tied to Breton folklore recorded by writers such as Anatole Le Braz.

What legends surround the Yeun Elez?

Local tradition long held the Yeun Elez basin to be one of the gates of hell: the bottomless bog known as the Youdig was said to swallow the souls of the dead and careless travellers alike. Another tale explains a nearby cluster of standing stones as a wedding procession turned to stone for failing to kneel before a priest carrying the Blessed Sacrament.

Frequently asked questions

Is Réservoir de Saint-Michel a natural lake? No. It is an artificial reservoir, created between 1929 and 1936 by damming the Ellez river.

How large is the lake? It covers about 4.5 square kilometres (450 hectares) and sits at an elevation of 227 metres.

Can visitors swim or take a boat out? The reservoir serves an energy and ecological function, and much of its shoreline consists of protected peat bogs, so it is worth checking local signage before planning any activity on the water.

Is the lake connected to a former nuclear plant? Yes. Between 1967 and 1985, its water cooled the reactor at Brennilis nuclear power station, which is now shut down and being decommissioned.

How long is the walk around the lake? The loop trail through the Yeun Elez wetlands covers about 15 kilometres and takes roughly four to five hours to complete.