Was there really a tsunami-like wave on Lake Geneva in the 6th century?

When looking at the pristine banks of Lake Geneva (lac Léman) today, we tend not to think that such a tranquil location could produce a natural disaster. However, in the sixth century, a large tsunami-like wave from the lake engulfed a number of towns and cities in Switzerland. Here’s what happened:

Can a lake have a tsunami?

Now, the first question someone might ask is how a lake can produce a tsunami. Typically, tsunamis are the result of large earthquakes caused by tectonic plates in the ocean, creating a huge wave that travels for thousands of kilometres before hitting the coastline, causing devastation and damage wherever it goes.

Despite being associated with the ocean, there have been several cases of huge tsunami-like waves on lakes, caused by either tectonic plate movement or large amounts of material falling into the water. Lake Kivu between Rwanda and the Democratic Republic of the Congo, Lake Tahoe in the US and Lake Wakatipu in New Zealand have all caused a tsunami wave in the past or have the potential to cause one. 

Tauredunum tsunami on Lake Geneva in 563

This phenomenon supposedly happened in Lake Geneva in 563. According to sources from the time, a whole section of the Tauredunum mountainside collapsed into the water, creating a huge wave that swept the length of the lake.

In the words of Roman historian Gregory of Tours, “A curious bellowing sound was heard for more than sixty days: then the whole hillside was split open and separated from the mountain nearest to it, and it fell into the river, carrying with it men, churches, property and houses…The water then flooded the higher reaches and submerged and carried everything which was on its banks.”

Chroniclers of the time described villages being devastated as the wave made its way to the cities of Ouchy - now a part of Lausanne - and Geneva. Once at Geneva, locals described an impact of such force that water washed over the city walls, destroying a bridge and killing several inhabitants. While the exact number of casualties is unknown, it remains one of the deadliest lake tsunamis recorded in history.

Did the tsunami on Lake Geneva actually happen?

For many years, scientists have struggled to prove that the so-called Tauredunum event even took place. The nature of medieval sources makes the claims of giant tsunamis difficult to confirm, as they may be referring to something more spiritual, melodramatic or symbolic in nature, and may be relying on oral accounts from the time.

This doubt remained until a study in 2012 confirmed how the tsunami took place. This was done by analysing a huge deposit of silt collected from the bottom of the lake. According to researchers from Canton Vaud, in 563, a rockslide from a nearby mountain - with a volume of 250 million metres squared - fell into the water near where Lake Geneva meets the River Rhône.

This would have created a wave of up to 13 metres in height, which would sweep across the lakeside over the course of 65 minutes. Archaeologists managed to find Gallo-Roman remains around the lake which proves that the tsunami did in fact take place.

How likely is a tsunami on Lake Geneva?

Of course, such a devastating tsunami is unlikely to happen again. In fact, many Swiss lakes like Brienz in Canton Bern have had their own mini-tsunamis across the ages, with minimal damage caused. If a similar amount of mountainside were to fall into the lake again, it is likely that authorities would be given warning months in advance, and would prepare evacuation plans accordingly.