As the world marks the International Day for Disaster Risk Reduction, Nepal responded to a second major Himalayan disaster in less than seven weeks.
On Sunday, a flash flood in the Budhi Gandaki River swept away houses, bridges and also damaged hydropower projects in the Gorkha district.
The disaster struck roughly a month and a half after the devastating Bhotekoshi and Trishuli river floods on August 26.
The devastating flash floods left at least 1,451 people dead and 5,745 remain missing, according to the National Disaster Risk Reduction and Management Authority (NDRRMA). On the Chinese side, the authorities reported 43 deaths and 519 missing.
A new study by the International Centre for Integrated Mountain Development (ICIMOD) released on Tuesday concludes that the mountain slope causing the Gyirong-Rasuwa disaster was deforming by up to 40 centimetres between July 7 and August 24.
It said Sentinel-1 InSAR satellite observations recorded up to 40 centimetres of line-of-sight deformation between July 7 and August 24, indicating measurable movement of the slope before failure.
Pema Gyamtsho, Director General of ICIMOD, released the report, "Understanding the Gyirong-Rasuwa disaster of August 26" at Nepal's National Conference on Disaster Risk Reduction, where policymakers, practitioners and researchers gathered to discuss lessons from the recent disasters and emerging risks across the Himalayas.
"The finding comes as scientists investigate a recent flood in the Budhi Gandaki basin, where satellite imagery has confirmed another ice-rock avalanche and glacier collapse," reads an ICIMOD press release.
Assessments of the event sequence, impacts and implications for future risk reduction are continuing, adds the press release.
"Researchers conclude the two disasters, occurring within seven weeks of each other, highlight the growing challenge of cascading mountain hazards across the Himalayas."
The analysis found significant ground deformation at the collapse site before an estimated 132 million cubic metres of rock and glacier ice detached from the northern face of Langtang Lirung and plunged into the Lhende River valley.
Sentinel-1 InSAR satellite observations recorded up to 40 centimetres of line-of-sight deformation between July 7 and August 24, a 48-day period before the collapse, suggesting progressive destabilisation of the rock and ice mass prior to failure.
Researchers caution that the observed movement could not have predicted exactly when the collapse would occur.
However, the findings suggest that some large mountain failures may exhibit detectable signs of instability before failure and that satellite monitoring could help identify high-risk slopes elsewhere in the Himalayas for closer observation.
The report concludes that "the disaster was not a conventional glacial lake outburst flood, as initially suspected.
"Instead, a massive bedrock failure sheared away part of an overlying glacier, triggering a cascading chain of hazards involving rock, ice, water and sediment."
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"This tragedy shows that some dangerous mountain slopes may exhibit measurable signs of instability before catastrophic failure," said Gyamtsho.
"The challenge now is turning scientific insight into practical risk reduction through stronger monitoring and regional cooperation."
The findings highlight important lessons for disaster risk reduction. The collapse occurred at 08:37 Nepal Standard Time and the debris surge reached the border corridor within approximately seven minutes.
SMS alerts were issued at approximately 09:15 to 09:16, around 25 to 30 minutes after the corridor had already been impacted.
"Researchers say warning systems designed around rainfall and river levels are not equipped to detect hazards triggered by sudden mountain collapse."
"Comparing hazard mechanisms and community responses associated with the Gyirong-Rasuwa and Budhi Gandaki events could provide important lessons for improving future preparedness," it said.
While investigations into the latest event are continuing, scientists say understanding community response, warning dissemination and local preparedness will be critical to reducing future losses from fast-onset mountain hazards.
Researchers identified several possible factors that may have contributed to the collapse, including geological weaknesses, damage associated with the 2015 Nepal earthquake, thawing permafrost, glacier change and the gradual accumulation of snow and ice, according to ICIMOD.
"The Gyirong-Rasuwa corridor has experienced two catastrophic transboundary disasters in just 13 months," according to researchers.
In July 2025, a rapidly draining supraglacial lake triggered a destructive flood through the same river system.
In August 2026, a massive rock and ice collapse devastated the same corridor through an entirely different hazard mechanism.
Combined with the recent Budhi Gandaki event, researchers noted these disasters underscore the need to better understand how climate change, cryospheric change and geological instability are reshaping risks across the Hindu Kush Himalaya.
ICIMOD and its partners are already moving into the next phase of investigation. "A joint field mission involving ICIMOD and China ICIMOD begins this week to collect additional evidence from the disaster area and support the identification of potential hazard hotspots elsewhere in the region," according to the press release.