Disappearing river in Bellegarde-sur-Valserine

In Bellegarde-sur-Valserine, located in the Ain department, the Rhône River played a natural game of hide and seek with visitors during periods of very low water levels. The river would almost disappear entirely, vanishing under a chaotic arrangement of rocks before reappearing further downstream.

Submerged by a dam in 1948

Between January 19 and 24, 1948, the filling of the Génissiat dam caused the Pertes du Rhône to disappear beneath the reservoir. This site, known for the river's dramatic disappearance, was submerged under water, marking the end of its natural spectacle.

Geological significance of the Pertes

The Pertes du Rhône were a geological marvel, measuring approximately sixty meters in depth. The river carved through a narrow gorge in limestone, forming what geologists classify as a fluvio-karstic canyon. This type of landscape results from the combined effects of river erosion and the dissolution of rock.

Misleading term for a natural phenomenon

The term "Pertes" originally referred to the complete disappearance of water, as ancient authors described the river vanishing entirely. However, the river did not flow underground but instead passed under fallen rocks and a rocky arch, depending on water levels. At high water levels, the river remained visible, but at low levels, it nearly disappeared.

Historical interest and changes over time

The site has long attracted travelers, with Alexandre Dumas describing it in his Impressions de voyage en Suisse. By the late 18th century, the narrowest point of the gorge had banks less than two feet apart. Over time, dynamite was used to modify the rocks, allowing floating timber to pass, but the river still disappeared under the remaining arch at low water levels.

Industrial impact and hydroelectric development

In 1873, a water-powered mill was built at the confluence of the Valserine and Rhône rivers. Later, hydroelectric plants on both sides of the confluence were also submerged when the Génissiat dam was filled in 1948. This dam, the first project of the Compagnie nationale du Rhône, was constructed about ten kilometers downstream of Bellegarde and reached a height of 104 meters.

Legacy of the submerged site

The Pertes du Rhône are now listed in the regional geological inventory as a "Degraded, Submerged" site, reflecting the impact of the dam on the landscape. The former gorge is now under water, and visitors can no longer observe the river’s dramatic disappearance from the edge of the gorge.

A similar phenomenon nearby

Nearby, the Pertes de la Valserine continues to exist. Here, water disappears into the ground, and visitors can still cross the river using a short bridge, preserving a similar but distinct natural wonder.