Flood risk threatens Swiss valley after ‘millennium event’ glacier collapse

A view of the destruction caused by a massive landslide. Keystone / Jean-Christophe Bott

Two days after a gigantic avalanche of ice, mud and debris buried much of the village of Blatten in southwestern Switzerland, the danger is not over. There is a risk that the water dammed up by the masses of debris in the Lötschental valley could overflow, according to the authorities.

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Three million cubic metres of rock fell on Blatten on Wednesday afternoon at the same time as part of the Birch Glacier. The canton put the total volume of ice and rock deposits on the valley floor at ten million cubic metres.

The site on the Kleine Nesthorn in canton Valais, where the landslide began, is also not stable. Further rockfalls and debris flows are therefore feared. Finance Minister Karin Keller-Sutter, who holds the rotating Swiss presidency this year, is visiting the disaster area on Friday afternoon.

Due to the danger of a possible flood wave from the Lötschental valley, on Thursday night the regional command centre in Gampel-Steg called on residents to prepare for a rapid evacuation. Those affected are asked to organise accommodation outside the valley floor and outside the Gampel-Steg perimeter. Relatives who may not be contactable and animals should also be considered, they said.

So far 365 people have been evacuated from the Lötschental valley.

Leaving valley ‘not an option’

As for Blatten, houses lie buried 50 to 200 metres under rubble. Not even the church is visible. Only a few roofs peek out at the very edge.

Keystone / Jean-Christophe Bott

A village has disappeared from the map, said Valais politician Christophe Darbellay. The people of Blatten have lost everything. “Their homes, their souvenirs, their church, their cemetery,” he said the day after the landslide. The farmers no longer even had any land, their life’s work was gone, he said. Not a single green plant grows in the grey scree. It will be a long time before grass grows over it.

+ Why do Swiss mountains collapse? It’s complicated

The Valais authorities are convinced that Blatten will be rebuilt, but nobody can say when, where and how at the moment.

For Darbellay, however, it is clear that the Lötschental will remain inhabited. “Leaving the valley is not an option,” he said.

Keystone / Jean-Christophe Bott

‘Millennium event’

“Only the residents themselves can answer the question of whether Blatten will be rebuilt,”  Valais politician Beat Rieder told the Neue Zürcher ZeitungExternal link (NZZ) on Friday.

Rieder called for more federal funding for protection against natural hazards in the Alps. If the federal government can’t find any money for the mountain population in an CHF85 billion ($103 billion) budget, “what else we have money for?” he wondered.

“We’ve experienced a millennium event here, and that’s why nobody knows yet how to deal with it,” Rieder said. “We have nothing to compare it to.”

SRF

The population needs to be reassured and informed soberly about the situation, he said. The crucial question now is how the valley can be protected from further damage. The next few days will be crucial for the crisis team, he said.

Rieder emphasised that Switzerland needed more protective structures and, where this was not enough, comprehensive monitoring systems. With regard to Blatten, he said there are numerous expenditure items in the federal budget that could be cut. “We could already restore a lot in Blatten with this money,” he said. But the first job for the authorities, he added, was to enable people to return.

Keystone / Jean-Christophe Bott

Staying prepared

Nevertheless, the situation remains tense in the Lötschental valley. The water in the lake which has formed above the debris continues to rise, but for the time being there has been no overflow.

Interviewed on Swiss public radio, RTS, cantonal geologist Raphaël Mayoraz reckoned “the current situation is fairly favourable. To put it plainly, the water is beginning to make its way through the 2.5km long deposit. As the hours go by, we can reduce the risk of a catastrophic scenario. We do know, however, that we need to keep such a risk in mind.”

The authorities remain on alert and the municipalities located downstream of the landslide, including those in the Rhône valley, are preparing themselves.

“This scenario of a possible evacuation will last for weeks,” Mayoraz said on Friday. “It will continue until the River Lonza manages to create a relatively stable channel through the entire deposit. There’s also a risk that pockets of water will form. Above all, we mustn’t let our guard down. That would be the worst thing we could do when we are not certain that the danger has been averted.”

Blatten before and after the landslide: on November 3, 2024, and May 29, 2025:

‘Never been seen before’

The acceleration in the movement of the Birch glacier is linked to global warming, according to Christophe Lambiel, a specialist in permafrost and the evolution of Alpine landscapes at the University of Lausanne.

“The 500-metre wall above the glacier is in permafrost” which is deteriorating as a result of global warming, he said in an interview broadcast on Friday by Le Nouvelliste and ArcInfo.

As the permafrost deteriorates, “the rock becomes unstable, blocks fall and weigh down the glacier, which accelerates down a steep slope. The heavier the glacier and the steeper the slope, the faster it can advance”.

The northern slopes of mountains above 3,000 metres are frozen ground, Lambiel said. “The permafrost has warmed considerably over the past ten years or so, particularly since 2022.”

He said he knew of no similar landslide in the Alps. Three million cubic metres of rock fell on the village of Blatten at the same time as part of the glacier.

“A mountain landslide evolves in successive rockfalls, which overload a moving glacier,” he pointed out. “This glacier, which is already moving fast, accelerates even more and eventually collapses. It’s a sequence that’s never been seen before,” he added, referring to a glacial collapse.

In his view, the vulnerability of this area is due to a combination of glacier and mountain instability. “There is clearly a geological component,” he said. As the movement of this mountain is old, “it’s a deep slide, which has accelerated sharply in recent days”.

Source: ATS

Translated from German and French by DeepL/ts

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