What Triggered Flash Floods In Nepal? Is Another Flood Expected In Next 72 Hours? What We Know

Nepal Flash Floods: A devastating flash flood swept down the Bhotekoshi River in Nepal's Rasuwa district on Wednesday, killing at least 389 people, leaving hundreds missing, and severely damaging infrastructure. Rescue teams have since recovered bodies across several downstream districts, including Nuwakot, Dhading, Chitwan, Tanahun, Gorkha, and Nawalparasi East. As emergency operations continue along the Nepal-China border and affected river basins, scientists and climate experts have launched an investigation to determine the exact cause behind the sudden torrent.

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Here is what is known so far:

The Bhotekoshi River rose sharply at the Rasuwagadhi border crossing on Wednesday morning, sending residents fleeing for safety while sweeping others into the surging waters. While initial speculation focused on a glacial lake outburst flood or a sudden avalanche, no definitive cause for the surge was identified throughout the first day.

The flood began near Rasuwagadhi and engulfed settlements downstream in Rasuwa, Nuwakot, and Dhading, destroying bridges and other infrastructure along its path. The government has since confirmed 389 deaths, while hundreds more remain missing.

Describing the scale of the devastation, Nuwakot Assistant Chief District Officer Krishna Prasad Humagain told Kantipur that these floods are the worst in living memory, according to a report by The Kathmandu Post.

What Triggered The Flash Flood?

Scientists analysing satellite imagery report that a massive fall of ice and rock appears to have blocked a river upstream, creating a temporary dam that later burst and unleashed the catastrophic flash flood.

Adding to the mystery, the US Geological Survey recorded a magnitude 4.4 earthquake in Tibet at 8:37 AM (Nepal Time) on Wednesday, approximately 47 kilometres north of Gosainkunda. However, scientists have not yet established whether the earthquake triggered or contributed to the avalanche.

Was There Any Warning Of The Risk?

The Bhotekoshi river corridor was already known to be prone to flash floods. Last year on July 8, a sudden surge struck the region. Nepal's Department of Hydrology and Meteorology concluded that the prior event was caused by a glacial lake outburst in the Lhende River near the Nepal-China border, a finding supported by Sentinel satellite imagery.

While scientists frequently warn that the wider Himalayan region contains numerous potentially dangerous glacier lakes, there was no specific forecast predicting the ice-and-rock avalanche at the location struck by Wednesday's disaster.

A study by the International Centre for Integrated Mountain Development (ICIMOD) and the UN Development Programme identified 3,624 glacial lakes across Nepal, India, and Tibet. Of those, 47 were classified as potentially dangerous, including 21 in Nepal, the report added.

What Do Scientists Say About The Flash Flood?

According to Nepal's National Disaster Risk Reduction and Management Authority, a massive avalanche of ice and rock was the primary cause of the catastrophic flood. Satellite imagery from Planet Labs shows what appears to be a large avalanche approximately 20 kilometres northeast of Rasuwagadhi, near the Nepal-China border. Debris plunged into the Lendi River, a tributary of the Bhotekoshi, triggering a strong surge filled with rock and sediment.

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Scientists at the International Centre for Integrated Mountain Development (ICIMOD) are investigating whether the debris blocked a narrow section of the river channel, creating what is known as a landslide dam that later burst, unleashing the catastrophic flash flood.

Sarthak Shrestha, a remote sensing and geoinformation associate at ICIMOD, noted that evidence so far points to an avalanche on the Nepal side as the main trigger. Chinese monitoring systems have not detected any similar events on the Tibetan side. ICIMOD has been working with Chinese institutions, including the Chengdu Institute of Biology under the Chinese Academy of Sciences, to determine what precisely happened.

Could Another Flood Occur?

Scientists have not ruled out the possibility of further danger. Home Minister Sudan Gurung noted that the Lhende River has not yet completely cleared. According to Kayastha, there were initial concerns that a large body of water might have accumulated upstream, but there was not yet enough evidence to confirm the existence of a lake.

"The risk is still there," Kayastha said, adding, "Rising temperatures increase the likelihood of glaciers melting, ice and rock breaking apart, and avalanches occurring in the Himalayas. But we cannot say exactly where the next event will happen."

Barrier Lake On Nepal-Tibet Border Could Breach Within 72 Hours

A large barrier lake formed near the confluence of the Chhochen Khola and Purepu Tsangpo rivers in Nepal, according to Chinese authorities cited by the Global Times on Thursday.

According to China's Ministry of Water Resources, the lake accumulated approximately 2 million cubic metres of water by Thursday morning and is already overflowing. Another 3 million cubic metres of water is expected to enter the lake over the next three days, significantly increasing the risk of a breach, Chinese state broadcaster CCTV reported.

In response, the ministry is closely monitoring the situation and conducting flood simulations to assess the possible consequences of a breach and support emergency rescue operations. Authorities have urged residents to exercise necessary caution and remain highly vigilant.

(With Inputs From Agencies)

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