When the mountain falls

The mechanism

A whole dam, not a single crack

On 9 October 1963 about 270 million m³ of rock slid into the reservoir — which held only 115 million m³ of water. The rock displaced the water and it washed 250 metres over the crest. The thin arch dam held completely. What wiped out the village was the wave — in under 15 minutes.

That is why „how fast can the reservoir be drawn down?“ is the wrong question. Drawing down takes hours. Displacement takes seconds. An intact dam is no protection against it.

What is changing

The mountains are falling far more often

Over roughly the last 15 years, large rock and mountain falls in the Alps have clearly increased. Specialists attribute this squarely to climate change: thawing permafrost and retreating glaciers that used to buttress the rock.

Per statements from SLF Davos (Samuel Weber), geologist Michael Krautblatter (TU Munich) and the German Alpine Club. The ~450 events in the Mont Blanc massif refer to the extreme summer of 2025.

The silent assumption

Why the builders could not have known

High rock faces above 2500 m are held together by permafrost — frozen water as the „cement of the Alps“, the foundation of the mountain. The dams of the 1950s and 60s were designed for exactly that world: frozen, stable slopes.

That was not negligence in 1960. Man-made warming was not an engineering input — the first IPCC report came in 1990, and permafrost as a driver of rockfall only became a research field after the hot summer of 2003.

The difference from Vajont matters: there the slip surface was known and ignored. Here it is the other way round — sound construction, with the world changing underneath it. The cement is thawing. The glaciers that buttressed the foot of the wall are retreating. The safety calculation rests on an assumption that is quietly expiring.

Blatten, May 2025: the instability at the Kleines Nesthorn was detected in time — the village had been evacuated nine days earlier when the mountain came down onto the Birch glacier and buried 90% of Blatten. Monitoring saved the people (one fatality). It did not save the village — and a reservoir cannot be evacuated.

• 2017 · Bondo GR
  About 3 million m³ off the Piz Cengalo, 8 hikers killed — no storm as the trigger, but slow thawing. The village escaped destruction only narrowly.
• 2023 · Brienz/Brinzauls GR
  Village evacuated ahead of a threatened collapse of about 1.2 million m³.
• 2025 · Blatten VS
  A rock and ice avalanche buried 90% of the village — nine days after the precautionary evacuation. One fatality.

The bill comes later

A burden across generations

Well over 200 large dams are under federal supervision, plus hundreds of smaller structures. Almost all date from the building boom of 1950 to 1970. They need permanently: instrument monitoring, earthquake strengthening, sediment management — and now on top of that the monitoring of every slope above every reservoir, an item the builders never budgeted for. Georges Darbre of the federal office has already ordered operators to strengthen several dams against earthquake risk, and demands emergency plans and alarm chains.

The glaciologist Wilfried Haeberli has been giving a double warning for years: rockfalls can trigger tsunami-like surge waves that would be felt as far as the Rhine — and future generations may invest too little in keeping the dams up once the builders' generation is gone.

This is the classic infrastructure trap: deferred maintenance is invisible — until it is not. Saving today does not postpone a cost, it postpones a risk onto the next generation. And unlike a road, a 285-metre wall cannot simply be left to itself.

For investors this is an open-ended liability that nobody prices publicly — and a climate risk on ageing assets that is being repriced right now.

Why we need them anyway

The country's battery

The dams cannot simply be given up. Hydropower supplied about 60% of Swiss electricity in 2024. Roughly half of that comes from storage plants — the big walls in the mountains.

So the dams alone supply close to a third of all Swiss electricity. And not just any electricity: they supply winter and peak power, dispatchable at the push of a button.

That is the real bind: the same structures are at once the biggest natural-hazard risk and the backbone of supply. Giving them up is not an option — only maintaining them properly.

Figures rounded, production mix 2024 (Swiss Federal Office of Energy). Split of hydropower by mean production expectation.

Not fear — prevention