Glacier Collapse, Avalanche Likely Fueled Deadly Nepal Flood

Scientists are rushing to understand the exact causes of the deadly flash flood that sent a wall of mud and water tearing through Himalayan river valleys on the China-Nepal border.

What’s clear so far is that a huge chunk of a glacier collapsed and that a 5.2 magnitude seismic event occurred at around 8:37 a.m. local time Wednesday, which in turn led to a catastrophic flow of debris downstream, the US Geological Survey said.

“What we’ve certainly confirmed – there was a large ice avalanche, probably a mixture of ice and rock, which detached and then became fluid and then transformed into this devastating flood,” said Simon Allen, a research associate at the University of Zurich who studies glacial hazards in the Himalayas.

The mountains have been shrouded in clouds, meaning the precise chain of events is likely to better understood once satellites allow a clearer examination. Initially, a magnitude 4.4 earthquake had been reported, though a subsequent analysis of imagery and nearby seismic stations confirmed the incident was actually a glacial collapse and debris flow, the USGS said.

Analysis so far indicates the snout of a glacier at an elevation of roughly 5,200 meters broke away, and began to make a steep descent to the valley floor, melting ice and gathering sediment as it did so, said Dan Shugar, a geomorphologist and an associate professor at the University of Calgary. “When you have a lot of rock falling a great distance it generates a lot of heat and that can melt a lot of ice, so that could explain part of the water that we see in the absolutely horrific videos,” he said.

So far, there’s no evidence to suggest that a glacial lake outburst or an earthquake were major contributors to the incident, said Shugar, who is among several dozens global scientists who’ve been collaborating over the past 24 hours to identify the cause.

“At the moment we’ve got the hydrology data, have some seismology data, we’ve got some remote sensing data, we have the videos,” he said. “We don’t yet have eye witness accounts from right in the valley, or a helicopter flyover of the source area.”

Scientists at the Nepal-based International Centre for Integrated Mountain Development are working to confirm a preliminary assessment that a large volume of ice and rock debris entered the Lende Khola, a tributary of the Bhote Koshi River, causing an event that triggered an abrupt surge of water, sediment and boulders downstream. The same area was impacted last July by a major flood, according to the center.

“It operated like an inland tsunami, traveling nearly 170 kilometers (106 miles),” and impacting a major highway, said Ruth Gamble, director of La Trobe Asia and lead author of Rivers of the Asian Highlands: from Deep-Time to the Climate Crisis. “It is a popular route for international tourists travelling from Kathmandu to Lhasa.”

Researchers have long sounded alarm bells on the vulnerability of the so-called Third Pole to warming temperatures, and about the rising potential for disasters.

The high-altitude region, with a large population spanning parts of China, India, Nepal and several other countries across the Tibetan plateau, is warming at nearly double the global average pace. That’s leading to rapid glacial retreat, with ice masses in the area shrinking twice as fast since 2000 compared to the previous decades, according to a 2019 study.

Records in the region don’t extend back long enough to say with statistical certainty whether ice avalanches like the one that occurred Wednesday are becoming more frequent, though researchers say that’s the pattern they are observing.

Glacial lake outburst floods — when water bursts through the ice or rock holding it in place — “are increasingly common in alpine regions globally as our climate changes,” said Adrian McCallum, a discipline lead in engineering at the University of the Sunshine Coast.

As the Nepal disaster struck, many experts in the field were convening in Chengdu, China, for an annual summit to discuss how to mitigate these very risks.

“When you put together all the physical understanding that the glaciers are retreating, permafrost is thawing, it certainly adds up,” said the University of Zurich’s Allen. Experts are concluding “that we are seeing and we are expecting these events to become more frequent,” he said.

Wednesday’s incident follows a pattern of similar events which have hit the Himalayan region in recent years, including a landslide that triggered a flood in Chamoli, India, in 2021, which left about 200 people dead or missing.

“The increasing frequency of cryosphere hazards is now visible more than ever,” said Mohd. Farooq Azam, a senior cryosphere specialist at the ICIMOD. “The pace is so fast that our ongoing efforts are not able to cope with this rapid change.” Regional governments should “take some joint, consolidated steps to react to such cross-border hazards,” he said.

Communities have been evacuated ahead of the collapse of other glaciers, including the major slide that buried the Swiss village of Blatten, in recent years.

“The Himalayan Region has had a history of deadly flash floods. The event appears to have occurred without warning, catching those downstream unaware,” said Andrew Gissing, chief executive officer of Natural Hazards Research Australia. “Early warning saves lives, and there are efforts globally to improve early warning systems.”

Such evacuations could have spared lives Wednesday had the right equipment and communications been in place, according to Allen.

“This is looking like it will be the biggest Himalayan disaster that we’ve seen. So I really hope this will be a turning point — that there will be more focus, more emphasis now on reducing the risks for these communities,” Allen said.

Photograph: Flash floods inundated villages and swept away buildings and people in Nepal; photo credit: Prabin Ranabhat/AFP/Getty Images

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