Lac de Longemer, Glacial lake in Xonrupt-Longemer, France.

Lac de Longemer is a glacial lake in the Vosges that stretches about 2 kilometers in length with a maximum width of 550 meters and depths reaching 34 meters. The water is surrounded by forested slopes that descend steeply to the shoreline, creating a narrow valley setting.

The lake formed during the last ice age when glaciers carved out a basin in the Vosges bedrock, leaving behind a deep depression. After the glaciers retreated, this depression filled with water and has remained a stable water body throughout the millennia since.

The lake's name appears in local patois as Longemô. On the west bank stands the chapel of Saint-Florent-des-Graviers, already recorded in the mid-15th century, rebuilt in 1449 and again in 1727; it occupies the site of an oratory to Saint Bartholomew linked by legend to the hermit Bilon, a former officer of Duke Gérard I of Lorraine, around 1056. In July 2002 the lake hosted rounds of the world fly-fishing championship.

A walking path circles the entire lake with multiple access points for swimmers and those wanting to use boats along the shore. Weather in the Vosges can change rapidly, so checking conditions beforehand helps ensure safe footing on the sometimes muddy trail.

The lake's water feeds into the Vologne River system, which ultimately contributes to the Rhine basin, creating a surprising hydrological link across hundreds of kilometers. This connection reveals how mountain water sources fuel major European river networks despite their remote location.