In the sixth century, Gregory of Tours, a chronicler of the Germanic people known as the Franks, told of an extraordinary event in what is now Switzerland, where the Rhône River spills into Lake Geneva. He wrote of a big rockfall in the year 563 AD in the vicinity of a place called Tauredunum. The debris plunged into the river, and a great mass of water ‘overwhelmed with a sudden and violent flood all that was on the banks as far as the city of Geneva’, more than 64 kilometres across the lake. The Geneva bridge was demolished, and several people inside the city walls were killed. Historians and scientists have long believed that Gregory and another chronicler, Marius of Avenches, who told a similar tale, were describing a tsunami that raced across the lake. But there has not been any direct evidence of it until now.

Researchers at the University of Geneva now say they have found that evidence in the form of a large deposit of sediment in the middle of the lake. In a study published in the journal Nature Geoscience, they also propose the sequence of events that caused the deadly surge. The researchers think that large boulders crashed down onto soft sediments which had accumulated at the river mouth because of the slowing of the river’s flow when it enters the lake. These sediments formed an underwater delta that had several canyon-like channels. When the falling rocks hit the delta, they destabilised the sediments and caused the canyons to collapse. It was this collapse that created the tsunami. The sediments from this collapse would have been propelled towards the lake’s centre.

Guy Simpson, a lecturer in the University of Geneva’s Department of Geology and Palaeontology, says the thick layer of sediment, which has the same curved shape as a lens, lies more than 305 metres down in the deepest part of the lake and was found largely by chance. Katrina Kremer, a University of Geneva doctoral student and the study’s lead author, had been conducting seismic soundings, searching for thin sediment layers that might be evidence of major floods that had taken place in prehistoric times, long before the event described by Gregory of Tours. ‘But we came across this enormous deposit,’ Simpson says. ‘We didn’t know straight away that it was the deposit that caused the sixth-century tsunami. But it was a jumbled mess of sediment. It was quite obvious that it was deposited rapidly.’

The researchers then took samples of the sediments and used carbon-dating techniques on remains of leaves and other organic matter they found to determine when the deposit formed. This narrowed the date to a period between the late fourth century and the early seventh century. Other than the rockfall, there is no record of any special event during that period, Simpson says.

The researchers estimated that the deposit, which is at least 9.6 kilometres long by 4.8 kilometres wide and averages about five metres thick, contains more than 248 million cubic metres of material. They ran multiple computer simulations showing that the collapse of that much sediment at the mouth of the Rhône would have caused a tsunami with an estimated height of 7.9 metres at Geneva, where it would have arrived in about 70 minutes. The rockfall itself may have been set off by a major earthquake, as some scientists have speculated.

Lake tsunamis, although unusual, are not unknown, says Richard Schweickert, an Emeritus Professor of Geology at the University of Nevada in Reno, United States. He cites evidence that the collapse of part of the shoreline of Lake Tahoe in northern California within the past 20,000 years caused a tsunami with wave heights of about 30 metres. There are two faults beneath the lake that could have caused an earthquake, he says, and the collapse of the Rhône delta sediments, as calculated by the Swiss researchers, would certainly have been capable of moving a large amount of material into the lake. He suggests that the findings could be corroborated by carefully mapping the shoreline to look for unusual deposits or erosion left behind by the giant waves.

Simpson says the Rhône delta sediments might collapse again, perhaps because of an earthquake or even their own weight. In the sixth century, Geneva was a small community, mostly behind walls on a hill, whereas today it is home to international organisations and about 200,000 people, many of whom live in low-lying areas near the water. Testing the stability of nearby slopes and creating more detailed models of how a tsunami could affect Geneva today would provide a more accurate assessment of whether this is something the lakeside city should be concerned about.

Most tsunamis occur in oceans and are generated by earthquakes. However, the study is a reminder that even a landlocked nation such as Switzerland is not immune to catastrophic waves.