How a Himalayan Glacier Collapse Triggered Nepal’s Deadly Flood

RASUWA, Nepal: A mass of ice and rock, estimated to be dozens of times larger than the Empire State Building, broke away from the Langtang Lirung glacier on the morning of Aug. 26.

Within minutes, it triggered a devastating mudflow that killed at least 682 people across Nepal and China, with nearly 3,000 people still missing as of the latest reports.

The disaster also completely destroyed the Gyirong Port complex on the China-Nepal border, long one of the main trade routes between the two countries.

The force of the flood was so immense that the bodies of some victims were swept as far as 240 kilometres downstream, reaching Indian territory.

For scientists now investigating its cause, the event has become the latest case study of how global warming is reshaping mountain hazards in the Himalayas, while exposing significant gaps in disaster-warning systems that communities in these valleys had long relied upon for protection.

According to the United States Geological Survey, or USGS, more than 100 million cubic metres of ice, rock and debris were displaced during the event, generating seismic vibrations comparable to a magnitude 5.2 earthquake and detected by monitoring stations as far away as the United States and Germany.

The collapse plunged into the Lhende Khola, a tributary of the Bhote Koshi, before surging into the Trishuli River and striking settlements along a 72-kilometre stretch across the Rasuwa and Nuwakot districts.

The speed of the flood wave also stunned experts.

Jeffrey Kargel, a senior scientist at the Planetary Science Institute, estimated that the surge travelled through its first 22 kilometres at an average speed of 193 kilometres an hour, fast enough to cover the distance in less than seven minutes.

Geomorphologists describe such a phenomenon as a “hazard cascade”: a relatively small failure at an elevation above 5,000 metres that can rapidly develop into a major disaster for communities far downstream, according to Jon Tunnicliffe, an associate professor of environmental science at the University of Auckland.

Some scientists are also investigating another possibility: whether debris from the collapse temporarily blocked the river before the resulting natural “dam” burst, abruptly releasing the accumulated water in what is known as a landslide-dam outburst flood.

It was not the first time the valley had experienced a similar disaster.

A glacial lake at the same location burst in 2025, another ice-and-rock collapse occurred nearby in 2024, while the 2015 earthquake also triggered a major landslide in the Langtang region.

According to research led by Gunjan Silwal and cited by Bethan Davies, chair of glaciology at Newcastle University, the Langtang catchment lost ice 1.5 times faster between 2000 and 2023 than during the previous 36 years. The rising snowline is now intersecting areas of steep ice-covered slopes, causing glaciers to become increasingly fractured and unstable.

The disaster also exposed weaknesses in the region’s early-warning systems.

Although Chinese and Nepalese scientists have maintained a joint flash-flood warning system for years, it failed to detect this disaster in time because water-level monitoring instruments were designed primarily for monsoon or seasonal flooding, rather than sudden glacier failure.

Four major hydrological monitoring stations — at Bhote Koshi Rasuwa, Syabrubesi, Betrawati and Malekhu — were themselves swept away by the flood they were supposed to detect.

Because the flood originated on the Tibetan side of the border, the absence of a real-time cross-border information-sharing channel meant the warning reached Nepalese authorities too late.

It took 16 minutes after the flood crossed the border before the Rasuwa District Administration Office received any notification, according to Santosh Nepal, an international researcher at the International Water Management Institute.

By then, the flood had already travelled 14 kilometres to Syaphrubesi in less than 10 minutes, moving faster than any conventional warning system could respond.

Major trade routes, strategic bridges and hydropower projects have continued to be built along the Bhote Koshi and Trishuli valleys, areas that the International Centre for Integrated Mountain Development, or ICIMOD, said were also affected by the Rasuwa floods last year.

Hundreds of hydropower project workers were reportedly trapped inside tunnels when the disaster struck.

The disaster also involved Malaysian nationals.

The number of Malaysians who had lost contact rose from 17 to 54 within a single day, before climbing again to 91 across Nepal and China, although 72 of them in Darchen, Tibet, were later confirmed safe.

Prime Minister Datuk Seri Anwar Ibrahim and Yang di-Pertuan Agong Sultan Ibrahim Sultan Iskandar issued public statements directing Wisma Putra and the Malaysian Embassy in Kathmandu to accelerate efforts to locate those affected.

For scientists, the Rasuwa disaster also carries a much larger long-term warning.

For now, the accelerating loss of Himalayan glaciers is creating more acute disasters of this kind. But as those glaciers continue to shrink over the coming decades, the threat is expected to shift toward a water-supply crisis affecting millions of people who depend on rivers originating in the Himalayas.