Nepal Flash Floods: What Triggered the Deadly Deluge? Avalanche, Earthquake or GLOF?

Kathmandu: The devastating flash floods that swept through Nepal's Rasuwa district on Wednesday did not appear to be a conventional monsoon flood. Preliminary evidence points towards a sudden geological event near the Nepal-Tibet border that may have released a huge volume of water and debris into the Bhote Koshi River.

Nepal's Foreign Minister Shishir Khanal said an earthquake occurred at around 8:37 am, followed by a large landslide that blocked a river and appears to have caused the flood. The US Geological Survey recorded a 4.4-magnitude earthquake in the region around the same time.

What Triggered the Devastating Floods?

The precise cause is still being investigated, but preliminary assessments point towards a geological event on the Tibetan side of the border.

Nepal's Foreign Minister Shisir Khanal said an initial assessment suggested that an earthquake may have triggered an avalanche, which then contributed to the sudden release of water. Another possibility being examined is that an ice-and-rock avalanche or landslide temporarily blocked the Lhende Khola, creating a natural dam. If that blockage suddenly failed, the trapped water could have rushed downstream into the Bhote Koshi with enormous force.

Authorities are also examining whether a glacial lake outburst flood (GLOF) may have played a role. Satellite imagery detected significant rainfall on the Tibetan side, while Nepal's flood authorities indicated that rainfall within Rasuwa itself was unlikely to explain the scale and suddenness of the flooding.

That distinction is important. The available evidence suggests this was not simply a case of heavy monsoon rain overflowing a river. Instead, the disaster appears to have involved a sudden upstream release of water, potentially linked to an avalanche, landslide, glacial event or earthquake-triggered geological disturbance.

Was Heavy Rain Responsible?

Rainfall is part of the investigation, but authorities have indicated that rainfall in Rasuwa itself does not fully explain the suddenness or scale of the flooding.

The focus has therefore shifted towards what happened upstream on the Tibetan side of the border and whether a landslide, avalanche, glacial event or combination of these factors suddenly released the water.

The possibility of an unstable blockage upstream has raised concerns that another sudden release could occur.

Reuters reported that authorities were monitoring the possibility of a second flood and had issued warnings downstream, including in parts of northern India.

This is particularly concerning because the Bhote Koshi eventually feeds into the wider Trishuli river system, meaning an upstream event can have consequences far beyond the immediate disaster zone.

Disaster With Multiple Possible Triggers

The evidence currently points towards a chain of events rather than a single straightforward cause: an earthquake may have destabilised an already fragile mountain slope, triggering an ice-and-rock avalanche or landslide; that event may then have blocked the Lhende Khola before the blockage failed and released a destructive surge into the Bhote Koshi.

However, the precise mechanism is still being established.

Experts are continuing to analyse satellite imagery, seismic data and conditions on the Tibetan side to determine exactly what triggered the flood.

What is already clear is that the disaster demonstrates the extreme vulnerability of Himalayan river valleys to sudden geological and glacial events - where a disturbance high in the mountains can become a deadly wall of water within minutes.

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