What Caused Nepal's Deadly Flash Flood? Scientists Probe Earthquake, Ice Avalanche And Glacial Lake Theories

Scientists and authorities in Nepal and China are racing to determine what triggered the devastating flash flood in the Bhote Koshi River on Wednesday, as the death toll has climbed to 166 and more than 800 people, including foreign travellers, remain missing.

The exact cause of the disaster remains unclear. Authorities are examining several possibilities, including an earthquake, an ice avalanche and a possible glacial lake outburst, as they investigate what has become Nepal's worst disaster since the 2015 earthquakes that ravaged villages, killed scores of people and destroyed critical infrastructure.

The floods have severely disrupted communication, road networks and internet services in areas along the Nepal-China border, further complicating the rescue and investigation efforts. Laxmi Prasad Niraula, Nepal's consul general in Lhasa, said the root cause was still unknown, adding that while some preliminary information had emerged, the Chinese side was also "clueless" about what caused the sudden outburst, The Kathmandu Post reported.

One possibility being investigated is that an earthquake may have triggered the flash flood. Nepal Foreign Minister Shishir Khanal told Parliament on Wednesday that an earthquake struck at 8:37 am, followed by reports of a flash flood just three minutes later at 8:40 am.

Khanal, however, cautioned that the connection between the two events still needs to be verified. The US Geological Survey reported a magnitude 4.4 earthquake in Tibetan territory at 8:37 am on Wednesday.

The close timing has prompted authorities and experts to examine whether the earthquake may have destabilised ice, snow or other material upstream, potentially setting off the sudden release of water into the river system.

An ice avalanche is another possible explanation being examined by scientists. Rijan Bhakta Kayastha, a professor at Kathmandu University and climate researcher, said an initial assessment based on preliminary findings indicated that an avalanche may have triggered the flash flood.

According to Kayastha, an ice avalanche may have fallen into and blocked the Lhende River, allowing water to accumulate before the blockage eventually gave way and sent a surge downstream.

"There is information that an ice avalanche from the Nepal side blocked the Lhende river," Kayastha told Nepali daily Kantipur. He also pointed to the earthquake recorded on the Tibetan side around the same time, saying the two events would need to be examined together.

A glacial lake outburst is also being considered, particularly because similar incidents have occurred in the region before. However, Kayastha said it could take several days to establish whether a glacial lake was responsible for the latest flooding.

The Bhote Koshi River experienced a similar sudden flood on July 8, 2025. The Department of Hydrology and Meteorology subsequently concluded that a glacial lake outburst had caused flooding in the Lhende River near the Nepal-China border in Rasuwa district, the area that has again been among the worst hit.

Kayastha said the region contains several large glacial lakes and noted that the Lhende River had also been blocked during the previous incident. "There are large glacial lakes in that area," he said. "The Lhende river was also blocked last year." He added that the initial assessment this time was that an ice avalanche and possibly a glacial lake outburst had caused the flooding.

Kayastha ruled out heavy rainfall as the main cause of the latest disaster, pointing instead to the sudden nature of the flooding.

According to the climate researcher, the speed and intensity of the water surge suggested that a large volume of water may have been released after an upstream blockage gave way.

The exact cause remains unclear, but authorities are investigating possibilities such as an earthquake, an ice avalanche, or a glacial lake outburst. The disaster has severely disrupted communication and road networks.