More than 1,500 people are missing across Nepal and China’s Tibet, including more than 800 foreign tourists, after rare flash glacial-lake outburst floods submerged towns and communities under a sludge of mud and water. More than 270 people have been killed, but officials say the toll is expected to rise as rescue teams attempt to reach farther areas that have been cut off by the floods, with several key bridges and highways also devastated. With skies still clear above the Nepal-China border, and no rain pouring, officials earlier believed that a magnitude 4.4 earthquake could have triggered the downstream floods.
However, several experts and the United States Geological Survey have confirmed that “the seismic event” was a consequence of “a glacial collapse and debris flow that caused subsequent catastrophic impacts downstream” – rather than the other way around. So, what is a glacial collapse, and is this a catastrophic outcome of worsening climate change? In simple terms, a glacial collapse is the detachment of a large mass of glacier ice from its bed – sometimes involving millions of cubic metres of ice, rock, and water – that can transform into a highly mobile ice-rich avalanche or debris flow.
This collapse is not a formally defined scientific term in glaciology, but is widely used to describe the event. Earth System Science Data, an international open-access scientific journal, describes this as gravity-driven failures of glacier ice on mountain slopes, including ice avalanches, glacier detachments, and rock-ice avalanches. An initial study of Planet Labs satellite imagery showed a landslide of ice and rock unleashed a flood carrying debris in the Lhende River in Tibet, about 20km (12 miles) northeast of the Nepal-China border crossing in Rasuwagadhi, Nepal’s disaster management officials said on X.
Television images showed collapsed homes, floating cars and a metal bridge being washed away, while witnesses said the waters engulfed entire villages. The disaster struck the mountainous Nepal-Tibet border region, with some of the worst damage in Nepal’s Rasuwa district, immediately south of the Chinese border. The initial surge entered the Lhende Khola, which feeds into the Bhote Koshi and then the Trishuli river, carrying floodwaters south through Nepal.
Water levels in the Trishuli reportedly rose by as much as 9 metres (27ft) in just 30 minutes. Nepal’s National Disaster Risk Reduction and Management Authority said the “glacier-inclusive flood” occurred in the Lhende River due to “an ice-rock avalanche on the Nepali side”, according to Prakash Pokharel, a geologist at the authority. Vikram Gupta, who specialises in engineering geology and teaches at Sikkim University in India’s northeast state bordering Nepal, said it was “confirmed that a substantial portion of that glacier snout collapsed to initiate the event – probably including or entraining a fair amount of rock and sediment”.
Earth scientist Dan Shugar, an associate professor at the University of Calgary, said the imagery seemed to show that a large amount of snow had melted in the 24 hours before the disaster. The disaster comes against a backdrop of rapid warming and glacial loss across the Himalayas. Glaciers across the Hindu Kush Himalaya mountain range – which spans Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan – lost ice 65 percent faster between 2011 and 2020 than during the previous decade.
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