A huge collapse of glacier ice and rock, rather than an earthquake, triggered the catastrophic flood that tore through communities along the Nepal-Tibet border, the strongest available seismic and satellite evidence indicates.

Scientists believe the avalanche struck the Lhende Khola, temporarily obstructed the river and released a violent surge into Nepal’s Bhote Koshi and Trishuli river systems.

The disaster struck on the morning of August 26, sending water, sediment and large boulders through steep Himalayan valleys with little warning.

At least 160 deaths had been confirmed across Nepal and Tibet by Thursday, while hundreds of residents, workers, pilgrims and foreign tourists remained unaccounted for.

The toll was expected to change as rescuers reached areas isolated by destroyed roads, bridges and communications.

The flood demolished homes, swept away vehicles and damaged hydropower facilities, monitoring stations and the strategically important border crossing between Nepal and China.

Nepalese authorities reported hundreds missing, including foreign nationals travelling through Rasuwa district toward Mount Kailash in Tibet.

Chinese authorities reported deaths and hundreds missing around Gyirong, the Tibetan county adjoining the border.

The disaster was initially confused with an earthquake.

The United States Geological Survey first registered shaking near the border as a magnitude 4.4 earthquake.

After examining long-period seismic waves and other data, it concluded that no earthquake had preceded the flood.

The signal was generated by the immense movement of ice, rock and debris itself and was subsequently assessed as equivalent to a magnitude 5.2 seismic event.

That distinction reverses the suspected chain of events.

An earthquake did not cause the mountainside to collapse; the collapse produced shaking powerful enough to resemble an earthquake on monitoring instruments.

The exact mechanism that converted the avalanche into such an enormous flood remains under investigation.

The leading explanation is that ice and rock plunged into the narrow Lhende Khola valley and blocked the river, allowing water and debris to accumulate behind a temporary natural dam.

When that obstruction failed, the impounded material rushed downstream as a dense, exceptionally destructive flood wave.

Scientists have not yet established how long the river was blocked, how much water accumulated or whether meltwater stored within or beneath the glacier contributed to the surge.

Field observations, higher-resolution satellite imagery and analysis of hydrological and seismic records will be needed to reconstruct the sequence conclusively.

Measurements farther downstream show the scale and speed of the event.

Preliminary hydrological observations indicated that the Trishuli River rose by as much as nine metres in approximately 30 minutes at Galchhi and by about seven metres at Malekhu.

Several monitoring stations were damaged or swept away, complicating efforts to track the flood in real time.

The lack of preceding heavy rainfall made the event especially difficult to anticipate using conventional flood-warning systems.

Unlike a monsoon flood that develops as rain accumulates across a river basin, an avalanche-generated surge can form high in the mountains and reach settlements within hours—or considerably less time in confined valleys.

Authorities were also monitoring debris remaining upstream because another blockage could retain water and then fail without warning.

Evacuations were ordered around the Gyirong border area, while downstream communities were warned to remain away from riverbanks.

Rescue work continued amid damaged transport links and concern over additional landslides.

Climate change cannot yet be identified as the direct cause of this particular collapse.

Establishing such a connection requires detailed evidence about the glacier, slope conditions, temperatures, precipitation and the physical failure that initiated the avalanche.

The wider risk, however, is well documented.

The Himalayas are warming rapidly, glaciers are losing mass, high-altitude lakes are expanding and thawing permafrost can weaken rock faces that were previously held together by ice.

More intense rainfall can further destabilize slopes.

These changes interact with the region’s steep terrain to produce cascading hazards in which a glacier or mountainside collapse becomes a river blockage, debris flow and destructive flood.

The Lhende Khola basin has now experienced two major floods within approximately 14 months.

The recurrence has intensified calls for Nepal and China to share satellite observations, river measurements and automated warnings across the border.

Investigators are continuing to analyse the remaining upstream blockage while authorities search for survivors and assess whether another sudden release threatens communities along the connected river system.