The disaster in the border region between China and Nepal was as sudden as it was unexpected: Within just 10 minutes, enormous volumes of water and debris swept 20 kilometers (12.5 miles) downstream on Wednesday, with devastating consequences for thousands of people in the region.

Flash floods are a frequent occurrence in the Himalayas. "They are more the rule than the exception," said Niels Hovius, head of the Geomorphology Section at the GFZ Helmholtz Center for Geosciences in Potsdam. He has worked in the Himalayas and other mountainous regions for many years and is an expert on early warning systems.

What triggered the flash flood?

Scientific data indicate that the disaster began with a glacier collapse in the northern part of the Langtang massif, when a large amount of ice broke away and turned into a massive ice avalanche. The Langtang massif is located in central Nepal near the border with China.

Devastating flash floods hit northern Nepal

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"This glacier collapse sent huge amounts of ice rapidly downhill," said Hovius. The heat generated by friction melted the ice into water, which then swept up debris and mud. The resulting mudflow moved downhill at speeds "reached by passenger cars on German highways."

Seismological instruments recorded an earthquake measuring magnitude 4.4 at the time of the glacier collapse. However, Hovius said, it was not a "classic" or tectonic earthquake. Instead, the instruments measured the force of the falling glacier.

Why can flash floods in the Himalayas be so devastating?

Flash floods in high mountain regions can be triggered in different ways — by glacier collapses or by so-called glacial lake outburst floods (GLOFs, or Glacier Lake Outburst Floods).

In such cases, the topography around a glacial lake suddenly changes, for example when a slope collapses into the lake. The large volume of water that suddenly spills over the lake's banks can then turn into torrents rushing downstream. In this particular case, however, it's clear that this was not a GLOF, said Hovius.

Landslides can also cause deadly mudflows without any glaciers or glacial lakes being involved. In the Himalayas, all three types of events are possible, and historical data show that all three occur regularly.

Compared with the Alps, the valleys of the Himalayas are relatively narrow. The water and mud cannot spread out but are instead forced into a confined space, increasing their depth and speed.

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In addition to the speed of the water and debris, the extent of the devastation caused by a flash flood depends on how far local populations are from the source.

How can people be protected from flash floods?

Nepal faces two major challenges. Many people have settled along rivers in the valleys of the Himalayas — in precisely the areas that are vulnerable to flash floods. Secondly, there are very few early warning systems in the country, which is one of the poorest in Asia.

Even with a timely warning, the particularly devastating flash flood would have given people only a few minutes to reach safety. Hovius described a flood that occurred in the same region in 2025 — smaller, and therefore less severe. At the time, it took the water an hour to reach a settlement, giving people enough time to escape if they had received a timely warning.

"There are initiatives in Nepal to establish local acoustic early warning systems," said Hovius. However, these systems are available only on a very limited scale. At most, they cover only 1% of areas at risk. Hovius added that mobile phones should be used to send people warnings via push notifications, and he is researching how the data needed for such a system could be generated.

Checks should also be made on how settlements are planned, Hovius said. At present, the bulk of a community's infrastructure is concentrated along rivers. "People have moved down from the mountains and settled in the valleys over the past few decades. Perhaps it would make sense to move back to higher ground," he said.

Are flash floods in mountainous areas more frequent amid climate change?

"Attributing individual events to climate change is always difficult," Hovius said. But it's clear that glaciers in high mountain regions become less stable as temperatures rise. Larger volumes of water are also accumulating in glacial lakes, making GLOFs more likely.

As for the current flash flood, it's too early to say whether climbing temperatures caused the glacier to collapse. Flash floods have long been a regular occurrence in the Himalayas.

"But I think it is likely that we will see more and more of these events," Hovius said, warning that future disasters could be even more serious.

This article was originally written in German.