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France’s Guerlédan Lake Drained in 2015 Revealed a Forgotten Valley

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France's Lake Guerlédan was drained in 2015, revealing 17 submerged canal locks, slate quarries and historic remains beneath the reservoir.

CENTRAL BRITTANY, FRANCE | November 2, 2026

A remarkable sight in central Brittany, France, once attracted enormous crowds: a lake that had covered an entire valley was drained, exposing a landscape hidden beneath water for around 85 years.

The event took place in 2015 at Lake Guerlédan, where the temporary disappearance of the reservoir revealed parts of the historic Nantes–Brest Canal, abandoned slate quarries, old buildings and 17 submerged canal locks.

The exposed valley offered visitors a rare opportunity to walk through places that are normally beneath the lake.

How the valley disappeared beneath the water

The story dates back to the construction of the Guerlédan hydroelectric dam on the Blavet River.

Construction began in 1924, and the dam was inaugurated in 1930. Its completion transformed the valley into a reservoir approximately 12 kilometres long, with a surface area of around 304 hectares.

The rising water submerged sections of the Nantes–Brest Canal, including 17 locks, lockkeepers' houses, quarry workers' shelters and slate quarries. It also permanently interrupted navigation along this section of the canal.

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Why was the lake drained in 2015?

The reservoir was emptied in 2015 to allow a major technical inspection and maintenance programme at the hydroelectric dam.

The operation was organised by EDF, the French electricity company, to inspect sections of the dam normally underwater and carry out repairs. The work included improvements to the upstream waterproofing and the renovation of two bottom outlet valves and their operating systems.

The last complete emptying had taken place in 1985. The 2015 operation therefore came 30 years later. The lake remained largely empty for approximately six months, from May to October, before the reservoir began filling again.

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As the water disappeared, the former valley gradually re-emerged.

Visitors could see:

  • The remains of 17 locks from the old canal.

  • Former slate quarries and quarry workers' shelters.

  • Old lockkeepers' houses and other structures.

  • Exposed sections of the historic canal route.

  • Trees and other remains preserved beneath the reservoir.

The unusual landscape attracted visitors, photographers, historians and tourists from across France and beyond.

Did 400,000 people visit?

The claim that approximately 400,000 people walked across the dry lake bed is not confirmed by the strongest historical sources located for this report.

In fact, the official Lake Guerlédan tourism website says that approximately two million visitors explored the exposed valley during the 2015 draining operation. Other contemporary reports published before the event gave different visitor forecasts, so estimates should not be confused with final attendance figures.

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Can the lake be drained again?

Complete emptying was historically carried out at intervals of several decades, but that does not mean the lake will automatically be drained every 30 years.

Modern underwater inspection methods allow engineers to examine parts of the dam without necessarily emptying the entire reservoir. The official tourism website says the timing of another complete draining remains uncertain.

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Today, Lake Guerlédan is once again a major recreational destination in central Brittany. Its approximately 304-hectare reservoir attracts visitors for boating, swimming, walking, cycling and other outdoor activities.

The 2015 event remains a striking example of how engineering maintenance can temporarily reveal an entire chapter of a region's forgotten history.

By News National Editorial Team

Source transparency

This article is based on information from the official Lake Guerlédan tourism website, local tourism authorities, EDF's 2015 technical press dossier and contemporary French media coverage.

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