The Himalayan flash floods that killed at least 489 people and left over 900 missing across Nepal and Tibet were not triggered by an earthquake, contrary to initial reports, but by a glacier collapse so violent that it produced its own seismic signal, the United States Geological Survey said.
The agency initially recorded a magnitude 4.4 earthquake along the Nepal-China border, north of Kathmandu, at about 8.37am local time on Wednesday, before revising its assessment in a statement issued that evening.
It said the seismic signal had come from “a glacial collapse and debris flow” rather than a tectonic quake, and that the collapse alone had caused the catastrophic flooding downstream. Once reclassified, the event registered at magnitude 5.2, a measure of the force of the ice and rock striking the valley floor rather than of any movement underground.
An initial study of satellite imagery from Planet Labs showed a large section of the lower end of a glacier breaking away at an altitude of about 5,200m and falling roughly 1,200m onto the valley floor, gathering rock and debris as it went.
The resulting avalanche struck the Lhende river in Tibet, about 20km northeast of the Rasuwagadhi border crossing with Nepal, temporarily damming the water before the barrier gave way and released a powerful flood downstream.
The surge fed into the Bhote Koshi river and crossed into Nepal at around 9am local time, then continued into the Trishuli river, where water levels rose by as much as nine metres in just half an hour.
Officials were caught off guard because there had been no rain, according to local news reports, and some residents ignored early warnings from neighbours precisely because the danger did not resemble the monsoon flooding they were used to.
Nepal’s Rasuwa district, immediately south of the border, bore the worst of the damage, with the flood also reaching Nuwakot and Dhading further downstream. At least 19 bridges and around 40km of roads were destroyed, and damaged hydropower infrastructure left some areas without electricity.
Adrian McCallum, a glaciologist and polar engineer at the University of the Sunshine Coast, said the disaster had the hallmarks of a glacial lake outburst flood. “As glaciers melt and recede, ice turns to water, the water is stored behind natural moraine dams, but when those dams breach, tremendous amounts of water can be released down a valley, taking out everything in its path,” he explained.
Ruth Gamble, environmental historian and director of La Trobe Asia at La Trobe University, said that the flood behaved “like an inland tsunami”, travelling 170km down the Bhote Koshi valley into central Nepal, along a route also used by tourists travelling between Kathmandu and Lhasa.
Why are experts worried a second flood is coming?
Both Nepal and China have issued warnings about fresh flooding in the Himalayas as two lakes that have developed on either side of their border threaten to burst.
Experts were assessing whether debris from the flood had temporarily blocked Nepal's Trishuli river at a narrow section, forming a landslide dam, the Nepal-based International Centre for Integrated Mountain Development said.
Such a blockage can create a secondary hazard if impounded water is suddenly released.
“With a blockage still lodged upstream on the Nepal-China border, authorities warn a second flood is imminent,” Qianggong Zhang, head of climate and environmental risks at ICIMOD, said.
He added that evacuation orders were in place for Jilong Port along the border based on the risk of a second wave of flooding.
China warned that 3 million cubic meters of water — the equivalent of about 1,200 Olympic swimming pools — was expected to flow into an already dammed lake on its side of the border over the next three days, creating a high risk of a breach, with peak flow expected around September 1.
Rainfall forecast over the coming week could exacerbate the threat, Chinese state media said, citing the water resources ministry.
"The volume of water is continuing to increase, and so is the risk," Zhu Jinfeng, a researcher at the water resources ministry's hydrology department, told CCTV.
Scientists cautioned it was too early to say how much climate change contributed to the collapse itself. Glaciers across the wider Hindu Kush Himalaya region, spanning Afghanistan, Pakistan, India, China, Nepal, Bhutan, Myanmar and Bangladesh, lost ice 65 per cent faster between 2011 and 2020 than in the previous decade. Nepal’s own glaciers have lost about a third of their ice volume over roughly the past three decades.
Satellite observations showed unusually warm conditions and reduced snow cover around the glacier before it collapsed, exposing more bare ice, though researchers were yet to determine a direct link to this week’s disaster.
Nader Naderpajouh, head of the school of project management at the University of Sydney, said the disaster could prompt the introduction of early warning systems similar to those developed after the 2004 Indian Ocean tsunami, since a water build-up behind a glacial dam could sometimes be detected before it burst.
“Such communications and preparations can save lives and reduce the impact at scale,” he said.
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