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.