Where a massive 1835 rockfall into Lake Geneva displaced enough water to create a tsunami that destroyed villages 70km away. Freshwater tsunamis are rare, but Alpine lakes surrounded by steep slopes have produced several.
SimonWaldherr, CC BY-SA 4.0
A different kind of tsunami
Tsunamis are not exclusive to the ocean. Any large body of water can produce a devastating wave if a massive volume of water is displaced rapidly. In deep Alpine lakes flanked by steep, unstable mountainsides, the trigger is not seismic uplift but catastrophic landslides. When millions of cubic meters of rock and sediment suddenly plunge into a lake, the energy transfer creates a displacement wave that can travel across the entire body of water with immense destructive power.
Lake Geneva, or Lac Léman, has the ideal conditions for such an event. The lake is deep, and the surrounding mountains, part of the Chablais Alps, are a complex mix of limestone and sedimentary rocks. At the lake's eastern end, the Rhône River has spent millennia depositing vast, unconsolidated layers of sediment, creating a massive and unstable underwater delta. A significant rockfall in this area can destabilize these sediments, causing a submarine landslide far larger than the initial rockfall. This secondary collapse is what can generate a truly massive lake-wide tsunami.
The Tauredunum event
The best-documented and most significant such event on Lake Geneva occurred in 563 AD. Historical accounts from two chroniclers of the era, Gregory of Tours and Marius of Avenches, describe a catastrophe that began with the collapse of a mountainside called Tauredunum. For centuries, this was a legend. Modern science, however, has confirmed the event's reality. A 2012 study by the University of Geneva used seismic surveys to map the lakebed and discovered a gigantic deposit of sediment, known as a megaturbidite. This deposit is over 10 kilometers long, 5 kilometers wide, and has an average thickness of 5 meters.
Radiocarbon dating of organic material within the deposit places its formation squarely in the 6th century. Computer simulations based on this data reconstruct the event with startling clarity. The initial rockfall destabilized the Rhône delta, triggering a submarine landslide of approximately 250 million cubic meters of material. This displacement generated a tsunami with an initial height of up to 16 meters (52 feet). The wave propagated down the 70-kilometer length of the lake at about 70 km/h. It struck the shoreline of what is now Lausanne within 15 minutes at a height of 13 meters and reached Geneva in roughly 70 minutes, where it was 8 meters (26 feet) high. The wave overtopped the city's walls, destroying a bridge and mills.
💡Fun Facts
Scientists have found evidence in the lakebed sediments of at least five major tsunami events on Lake Geneva over the last 4,000 years.
The 563 AD tsunami was not caused directly by the rockfall into the lake, but by the rockfall triggering a much larger collapse of underwater sediments at the Rhône River delta.
Other Swiss alpine lakes have histories of landslide-tsunamis, including a deadly event on Lake Lauerz in 1806 and another on Lake Lucerne in 1601.
The total volume of displaced sediment in the 563 AD event was estimated at 250 million cubic meters.
The lakeside and viewpoints are public and accessible at all times.
Admission
Free.
Accessibility
The shores of Thonon-les-Bains are accessible with paved promenades. The surrounding mountainsides offer varied hiking trails, from easy to challenging.
