Bhote Koshi flood may have been triggered by ice-rock avalanche, say scientists

Scientists say an ice-and-rock avalanche may have blocked the Lhende River and triggered the Bhote Koshi flood. The finding has sharpened warnings over fresh flooding as unstable upstream lakes and debris are monitored.Stock photo used for illustrationScientists are examining whether a possible glacial collapse and an ice-rock avalanche triggered the devastating flash floods along Nepal's Bhote Koshi River. Preliminary assessments suggest that debris may have temporarily blocked the Lhende River, formed a landslide dam and then given way, sending a powerful flood downstream.The flood swept through the Bhote Koshi in Rasuwa district on Wednesday, killing hundreds and destroying bridges and other infrastructure along the river corridor as it moved through multiple districts. While early speculation pointed to a glacial lake outburst flood or an avalanche, scientists studying satellite images now believe a large mass of ice and rock may have fallen into the Lhende River, a tributary of the Bhote Koshi.According to The Kathmandu Post, the National Disaster Risk Reduction and Management Authority has identified an ice-and-rock avalanche as the leading explanation. Satellite images studied by scientists showed what appeared to be a large avalanche about 20 km northeast of the Rasuwagadhi border crossing near Nepal's border with Tibet. The debris appears to have entered the Lhende River and triggered a fast-moving, debris-laden flood.Rijan Bhakta Kayastha, a climate researcher and professor at Kathmandu University, was quoted as saying that an avalanche appeared to have blocked the Lhende River before the flood broke through. "We suspect an avalanche on the Nepal side blocked the Lhende," Kayastha said. "An earthquake occurred at around the same time, but we do not yet know whether it helped trigger the avalanche." The US Geological Survey had initially recorded what it classified as a magnitude 4.4 earthquake in Tibet at 8:37 am on Wednesday, about 47 km north of Gosainkunda. It later revised its assessment, saying analysis of long-period seismic waves showed that the seismic energy had been generated by a glacial collapse and debris flow, not an earthquake. The event was then classified as a magnitude 5.2 seismic event. Reports said the debris blocked the Lhende River, created a temporary lake and, after the blockage gave way, sent a torrent of mud and boulders towards the Bhote Koshi.Sarthak Shrestha, a remote sensing and geoinformation associate at the International Centre for Integrated Mountain Development, said the evidence so far pointed to an avalanche on the Nepal side as the main trigger. "So far, the evidence indicates that an avalanche above Syaphrubesi blocked the river and the flood followed," Shrestha said, adding that it could take another week or two to establish the full picture. ICIMOD said its preliminary assessment, based on video footage and reports from the affected area, indicated that a huge volume of ice and rock debris may have fallen into the Lhende River. It is working with Chinese institutions to determine precisely what happened. ICIMOD said the resulting flood was unusually fast and powerful. At Galchhi, the Trishuli River rose by as much as nine metres in just 30 minutes, while the water level at Malekhu climbed about seven metres in the same period. Another possible explanation considered at first was a glacial lake outburst. The same region saw a sudden surge in the Bhote Koshi on July 8 last year, which Nepal's Department of Hydrology and Meteorology had attributed to a glacial lake outburst in the Lhende River near the Nepal-Tibet border. A study by ICIMOD and the UN Development Programme identified 3,624 glacial lakes across Nepal, India and Tibet, of which 47 were classified as potentially dangerous, including 21 in Nepal.Scientists have also said the risk of further flooding has not been ruled out. Cloud cover has hampered satellite observations, while experts are monitoring the possibility of another temporary water body forming upstream. "The risk is still there," Kayastha said. "Rising temperatures increase the likelihood of glaciers melting, ice and rock breaking apart, and avalanches occurring in the Himalayas. But we cannot say exactly where the next event will happen."Meanwhile, Chinese authorities reported another potentially dangerous development downstream, saying a large barrier lake had formed near the confluence of the Chhochen Khola and Purepu Tsangpo rivers in Nepal. China Central Television said about two million cubic metres of water had accumulated by Thursday morning and water had begun flowing out, while another three million cubic metres could enter the lake over the following three days, raising concerns that the natural dam could fail. The authorities are monitoring the lake and carrying out flood simulations to assess the possible impact of a breach. The Chhochen Khola and Purepu Tsangpo originate in Tibet and merge before crossing into Nepal, where the river is known as the Bhote Koshi, the principal upper tributary of the Trishuli River.Overall, scientists and disaster agencies are increasingly pointing to an ice-and-rock avalanche and a temporary river blockage in the Lhende as the most likely trigger for the Bhote Koshi flash floods, even as monitoring continues over the possibility of more flooding upstream and downstream.With PTI Inputs- EndsPublished By: India Today Web Desk Published On: Aug 27, 2026 18:15 IST