Ice-rock avalanche likely triggered deadly Nepal flash floods, says ICIMOD

Kathmandu,UPDATED: Aug 26, 2026 21:36 IST

An ice-rock avalanche from a high-altitude area likely triggered the flash floods that came down from Tibet into central Nepal on Wednesday, experts at the Centre for Integrated Mountain Development (ICIMOD) said. The disaster killed at least 95 people and left nearly 500 missing, including 133 Indians, as floodwaters swept through multiple districts, destroying towns and villages, washing away bridges and damaging at least a dozen hydropower projects.

ICIMOD described the disaster as an example of cryosphere hazards becoming more frequent and more visible, and said it showed the need for early warning systems and stronger regional coordination to deal with cross-border hazards. Scientists from the Kathmandu-based organisation and partner institutions in Nepal and China are investigating the cause and assessing the continuing risk to downstream communities.

The flood surged through the Bhote Koshi river at around 9 am local time before spreading to several other districts. It sent a powerful flow of water, sediment and boulders through the Bhote Koshi and Trishuli river systems. According to ICIMOD, preliminary assessment based on video footage and reports from affected areas suggests a large volume of ice and rock debris may have entered the Lhende Khola, a tributary of the Bhote Koshi. Researchers said this may have caused a sudden surge that carried water, sediment and large boulders downstream.

"Hydrological observations indicate the exceptional speed and magnitude of the flood wave. Water levels on the Trishuli River at Galchchi reportedly rose by as much as nine metres within 30 minutes, while levels at Malekhu increased by seven metres over a similar period," the statement said. It added that several water monitoring stations along the Trishuli were damaged or washed away, though the Galchchi station remains operational.

Experts are also examining whether debris temporarily blocked the river at a narrow section and formed a landslide dam, which could create a secondary hazard if trapped water is suddenly released. Authorities and scientists are continuing to assess the size, location and stability of any such blockage. "The same site devastated by last year's Rasuwa flood is once again under threat," said Qianggong Zhang, Head of Climate and Environmental Risks at ICIMOD. "An ice avalanche in the Lhende Khola is the suspected trigger, but the cause remains unconfirmed. With a blockage still lodged upstream on the Nepal-China border, authorities warn a second flood is imminent and evacuation orders are now in place for Jilong port."

"The increasing frequency of cryosphere hazards is becoming more visible than ever before. The pace of change is so rapid that current efforts are struggling to keep up," said Mohd Farooq Azam, Senior Cryosphere Specialist at ICIMOD. "Today's disaster in the Lhende Khola underscores the need for stronger regional collaboration, with governments taking coordinated action to respond to cross-border hazards," he said. Saswata Sanyal, Disaster Risk Reduction Specialist at ICIMOD, said, "The Lhende Khola has flooded twice in 14 months, this time most probably triggered by an ice avalanche that blocked the river and released a sudden surge downstream. This still needs to be confirmed, as we receive further information. These are cascading hazards: a cryosphere event becomes a flood in a settlement within hours."

Preliminary analysis of seismic records also found unusual signals during the disaster period. Data from a seismometer in Jilong, about 12 kilometres from the affected area, showed anomalous signals, while a second station in Zhangmu also recorded unusual activity. Scientists are examining whether these signals are linked to the suspected avalanche, but ICIMOD said no causal link between seismic activity and the flood has been established so far. The organisation said its experts are continuing to analyse hydrological, seismic, remote-sensing and field data to understand what triggered the event and to assess ongoing risks. Scientists also cautioned that while climate change is affecting glaciers, snow conditions, permafrost and mountain slopes across the Hindu Kush Himalaya, it is too early to say what role it played in this specific event. "Multi-hazard risks are increasing in the Himalayas and the Lhenda Khola case is one such example. The exposure and risk profile of downstream communities is changing dramatically. With these communities at the forefront, real time risk information needs to reach local decision-makers and translate into life-saving decisions," said Basanta Raj Adhikari, Director, Centre for Disaster Studies, Tribhuvan University, Nepal.

Overall, ICIMOD said the flood appears to have been caused by a high-altitude ice-rock event, though investigations are still under way. With the risk of further flooding still being assessed, the organisation has called for faster warning systems, better monitoring and closer coordination between governments to respond to such hazards.

With PTI Inputs

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