Scientists Say Collapsed Himalayan Glacier, Not an Earthquake, Triggered Deadly Nepal-Tibet Flash Floods

KATHMANDU, Nepal — Scientists say a collapsed glacier near the Himalayan peak of Langtang Lirung, not an earthquake, was the likely trigger behind the devastating flash flood that struck the Nepal-Tibet border this week, killing at least 177 people and highlighting the dangers of rapidly melting ice across the world’s highest mountain range.

Preliminary reports in the immediate aftermath of Wednesday’s disaster suggested a tremor may have caused a landslide that in turn triggered the flooding. But the U.S. Geological Survey has since clarified that the event was instead caused by a “glacial collapse,” a process in which a glacier or a portion of one disintegrates suddenly. The impact of the falling ice hitting the valley floor below was powerful enough to register as a magnitude 5.2 seismic event, according to USGS data and outside scientists who have since analyzed the disaster.

Pinpointing the source

Dr. Simon Cook, a glacier expert at the University of Dundee, told the BBC that his team had identified the lower portion of a glacier near the Langtang Lirung peak — located roughly 15 kilometers, or about 9 miles, west of the flood site — as the likely source of the collapse. Cook said his analysis indicates the glacier broke away and moved initially in a northwesterly direction before traveling westward downstream toward the communities and infrastructure that were ultimately destroyed.

A separate analysis by the International Centre for Integrated Mountain Development, an intergovernmental scientific institution known as ICIMOD, concluded that the disaster likely began with an ice-rock avalanche originating from a high-altitude area. That avalanche sent a large volume of ice and rock debris into the Lende Khola, a tributary of the Bhote Koshi river, generating a sudden surge of water, sediment and large boulders that swept downstream with tremendous force.

According to ICIMOD’s findings, water levels in rivers downstream rose between seven and nine meters within just 30 minutes of the collapse, a surge so powerful that it washed away or damaged several river monitoring stations positioned to track exactly this kind of hazard. The steep, narrow topography of the valley funneled the resulting torrent of water and debris at high speed through populated areas along the riverbanks.

A pattern scientists have long warned about

While it remains unclear precisely what caused the glacier to collapse on Wednesday, researchers have spent years warning that rising temperatures and broader climate change are accelerating the melting of ice and permafrost throughout the Himalayas, raising the risk of exactly this type of catastrophic event.

An ICIMOD analysis released earlier this year found that glaciers across the Hindu Kush Himalaya region are now losing ice at double the rate recorded since 2000, a trend that directly increases the danger of flooding and related hazards across the region.

This week’s disaster is not an isolated case. Cook pointed to similar incidents involving floods and landslides triggered by glacier collapses in recent years in India, Switzerland and Italy. In 2021, a large section of a Himalayan glacier collapsed in the Chamoli valley in northern India, unleashing a cascade of debris and water downstream that killed 200 people; scientists later estimated the force of that collapse was roughly equivalent to 15 atomic bombs. Even within Nepal, the same general region experienced flooding last year after a glacial lake in Tibet burst its banks.

“The pace of change is so rapid”

Mohd Farooq Azam, a cryosphere specialist with ICIMOD, said the growing frequency of hazards tied to the world’s frozen regions has become impossible to ignore. “The pace of change is so rapid that current efforts are struggling to keep up,” Azam said.

Cook was careful to note the scientific difficulty of attributing any single disaster directly to climate change, but said the broader pattern points clearly in that direction. “When you look at the pattern of these events, you do think climate change could have an impact,” he said.

He explained the underlying mechanism driving the trend: as the planet warms, glaciers shrink and generate more meltwater, while permafrost — the centuries-old frozen soil that helps hold many mountainsides together — is thawing and degrading. “So you are going to get more landslides, debris flows, glacial meltwater, lake outbursts, glacier collapses, because climate warming ultimately destabilizes these high mountain environments,” Cook said.

Visible evidence from above

Satellite imagery comparing the Trishuli river before and after the disaster illustrates the scale of the transformation. Images taken Aug. 23, just three days before the flood, show a comparatively clear river channel. By Aug. 26, the same stretch of river appears choked with brown sediment spilling out well beyond its normal banks, visual confirmation of the massive volume of debris and water that surged through the valley.

A disaster still unfolding

The human toll from the flood continues to climb as search operations proceed. More than 800 people remain missing across Nepal, according to authorities, with rescue teams working under difficult conditions in the steep, debris-choked terrain to reach survivors and recover victims. Video and helicopter footage from the disaster zone show the scale of destruction wrought by the flash flood as it tore through the Rasuwa district.

A warning for the future

For scientists tracking the health of the Himalayan cryosphere, this week’s disaster serves as the latest and most severe example of a danger they have been documenting for years. With glacier loss accelerating and permafrost destabilizing across the range, researchers say events like the one that struck the Nepal-Tibet border this week are likely to become more frequent rather than less, adding urgency to calls for better monitoring systems, early warning infrastructure and international cooperation to protect vulnerable communities living downstream from the world’s highest and most rapidly changing mountains.