Satellite analysis reveals pre-collapse changes in glacier surface and meltwater days before the August 26 disaster that killed over 1,000 and left nearly 4,000 missing near the Nepal-Tibet border.

A rapid scientific assessment has found clear warning signs in the days leading up to a catastrophic glacier collapse in the Himalayas that triggered deadly flash floods along the Nepal-China border. The disaster on August 26 killed more than 1,000 people and left over 3,900 missing, many of them construction workers at hydropower sites along the Trishuli River valley. Experts say better monitoring and early warning systems could have saved lives further downstream even if the initial avalanche moved too quickly for full evacuation at the border.

The HiRISK report, prepared by international scientists and released on August 28, analysed satellite imagery and identified “pre-event indications” of the coming avalanche. These included visible changes in the glacier’s surface, accelerated movement of water within the ice, and a crack spreading into the surrounding bedrock.

Early Signs in Satellite Record

Jakob Steiner, a geologist and co-author of the report who advises the Nepali government on Himalayan disaster risks, said the signs were detectable days before the collapse. Satellite observations from August 24 showed meltwater turning visibly brown, a signal that something was shifting inside the glacier and destabilising the surrounding rock. Water within the glacier also appeared to accelerate, while a propagating crack entered the bedrock slope.

“This is where I think we need to invest more,” Steiner told ABC News. “There are thousands and thousands of glaciers in High Mountain Asia, so we have to find scalable solutions.” He pointed to AI-assisted satellite monitoring as a practical way to cover the vast, remote terrain.

The collapse itself involved a large section of glacier and underlying rock high in the Lhende Khola valley, a tributary feeding into the Bhote Koshi-Trishuli system. Estimates place the starting elevation around 5,200 metres, with material plunging roughly 1,200 metres to the valley floor. The resulting mass of ice, rock and debris generated seismic signals initially mistaken for a magnitude 4.4 earthquake; later analysis linked them to the glacial event itself, registering closer to magnitude 5.2.

Scientists estimate the failed mass released tens of millions of cubic metres of material. Once it reached the river, it created a temporary blockage before releasing a dense, fast-moving slurry often described as “liquid concrete.” Water levels on the Trishuli rose by as much as nine metres in 30 minutes in some places.

Weak Early Warning Systems

The disaster struck several hydropower projects under construction about 35 kilometres downstream from the Gyirong (Rasuwagadhi) border checkpoint. On Monday, Nepali authorities reported at least 933 workers believed trapped across roughly half a dozen sites. About 300 have been evacuated from a collapsed tunnel at the Upper Trishuli project. Twelve hydropower facilities and a solar plant – around 10 per cent of Nepal’s operational capacity – were shut down, with seven damaged or destroyed.

Steiner said early warning systems for construction workers remained “very weak.” Workers often lived in barracks close to the infrastructure, structures he compared to “Lord of the Rings kind of dwarf caves.” Hundreds could be housed in each site.

“Given the rapid speed of the initial avalanche, warning time from conventional … systems would have been minimal and likely insufficient for successful evacuation at the Nepal-China border check point at Rasuwagadhi,” the HiRISK report stated. “Further downstream, with warning times of 10 minutes or more, comprehensive and well-capacitated systems could have saved a significant number of lives.”

Arnold Dix, an Australian geologist and engineer assisting with rescue efforts at the Trishuli-3A site, estimated flood waters moved at more than 160 kilometres per hour. “There were people at the construction camp, 10 minutes wouldn’t be long enough to run from camp to the tunnels (which were more resilient to the flood),” he said. He described the flood’s force as “really the equivalent of like a bomb.”

Tunnel expert Chris Cooper, also involved in the rescue, stressed the need to assess risks beyond individual project boundaries. Standard designs account for annual flood levels, he noted, but this event was “catastrophic” and unlikely to have been fully factored in. He called for improved monitoring and cross-border data sharing.

A Pattern of Repeated Disasters in the Same Valley

The August 26 event was not the first in the area. A glacial lake outburst flood hit the Gyirong port in July 2025, killing at least nine people and destroying roads and a bridge. A rock-ice avalanche affected the same region in July 2024, and an earthquake-triggered ice avalanche occurred in April 2015.

“The magnitude of all of [these events] coming together and then happening after each other in the same location is a concerning thing, and this is what we call multi-hazard events,” Steiner said. He argued the earlier disasters should have prompted stronger action. “We can’t say, oh my God, nobody knew.” After the 2025 event he advocated for staff training and monitoring of early warning signs, yet “there was a lot of hot air, but very little was (done).”

In May, Nepali and Chinese experts and officials met in Kathmandu to discuss border-area disaster risks, including glacial hazards, monitoring and joint response measures. Participants called for monitoring glacial lakes and mapping hazards. No early warning system specifically for glacier-related hazards had been established in the affected area by the time of the collapse.

Chinese Ambassador to Nepal Zhang Maoming said both countries “can learn a lot from responding to this disaster.” “It is imperative for China and Nepal to further strengthen cooperation in disaster prevention, mitigation and emergency management, and continuously enhance our capacity to respond to natural disasters,” he stated.

Climate Pressures and the Challenge of Prediction

High-altitude regions of the Himalayas are warming faster than the global average, accelerating glacier retreat and permafrost thaw. These processes weaken mountain slopes and increase the likelihood of rock-ice avalanches and debris flows. The International Centre for Integrated Mountain Development (ICIMOD) and other bodies have long warned of dozens of potentially dangerous glacial lakes across Nepal, Tibet and neighbouring areas. Around 200 lakes across the wider Himalaya are considered at risk of outburst.

Predicting the exact timing of a glacier collapse remains difficult. Existing warning infrastructure in the region has focused more on glacial lakes than on steep hanging glaciers capable of sudden failure. Scientists note that precursors such as accelerated ice velocity, increased crevassing, surface deformation, changes in meltwater colour and pathways, and unusual seismic signals can sometimes be detected. High-resolution satellite imagery, radar interferometry, optical tracking, time-lapse cameras and seismic networks all offer potential, yet covering thousands of glaciers across rugged terrain requires scalable approaches.

China’s Ministry of Water Resources has since warned that glaciers around the Cuojian river remain at “major risk” of further collapse, raising the possibility of additional surges. Unstable canyon walls, remaining sections of hanging glacier and temporary barrier lakes continue to pose secondary threats.

Calls for Better Protection and Cross-Border Systems

Steiner said every major construction project undergoes environmental and risk assessments, but it remains unclear whether recommendations were fully implemented. Both the Nepali and Chinese governments, he argued, need to do more to protect the large numbers of workers engaged in high-altitude infrastructure.

Cooper and others emphasised the importance of planning for hazards that originate outside individual project boundaries and of establishing reliable cross-border data-sharing mechanisms. Downstream communities and workforces need warning times measured in minutes rather than seconds if lives are to be saved, he said.

The disaster has already prompted international assistance, including satellite damage mapping and support for rescue operations. As recovery continues and the full human cost becomes clearer, the scientific findings underscore a broader message: the tools to detect many of these threats already exist. The challenge is deploying them at scale, investing in the systems that turn detection into timely action, and treating the Himalayas as the interconnected, multi-hazard landscape they have become.