New barrier lake raises flood fears after Nepal’s Bhote Koshi disaster

Scientists are investigating whether a glacial collapse and ice-rock avalanche created a temporary landslide dam that later burst, triggering the devastating flash floods along Nepal’s Bhote Koshi river.

The disaster struck Rasuwa district on Wednesday, sending a powerful torrent downstream that killed hundreds and destroyed bridges and other infrastructure across several districts.

Initial theories about the sudden surge near the Rasuwagadhi border crossing included a glacial lake outburst flood and an avalanche. However, preliminary analysis of satellite imagery now suggests that a large volume of ice and rock may have fallen into the Lhende River, a tributary of the Bhote Koshi.

The debris may have temporarily blocked the river and created a natural dam. When the blockage gave way, the accumulated water is believed to have surged downstream carrying mud, rocks and boulders.

Nepal’s National Disaster Risk Reduction and Management Authority has also identified an ice-and-rock avalanche as the most likely explanation, according to The Kathmandu Post.

Satellite images examined by scientists showed signs of a major avalanche about 20 km northeast of the Rasuwagadhi border crossing. The debris appeared to have entered the Lhende River and produced the unusually powerful flood.

Rijan Bhakta Kayastha, a climate researcher and professor at Kathmandu University, said the available evidence suggested that an avalanche on the Nepal side had blocked the river before the flood broke through.

“We suspect an avalanche on the Nepal side blocked the Lhende,” Kayastha told The Kathmandu Post.

He said a seismic event was detected around the same time, although it remained unclear whether the movement had played any role in triggering the avalanche.

The US Geological Survey initially classified the event as a magnitude 4.4 earthquake in Tibet at 8.37 am on Wednesday. It was located about 47 km north of Gosainkunda.

The agency later revised its assessment after examining long-period seismic waves. It concluded that the energy had been produced by a glacial collapse and debris flow rather than an earthquake, subsequently classifying it as a magnitude 5.2 seismic event.

Sarthak Shrestha, a remote sensing and geoinformation associate at the International Centre for Integrated Mountain Development, said the evidence currently pointed towards an avalanche above Syaphrubesi as the primary trigger.

“So far, the evidence indicates that an avalanche above Syaphrubesi blocked the river and the flood followed,” Shrestha said. He added that establishing the complete sequence of events could take another week or two.

ICIMOD is working with Chinese institutions to determine the precise cause. Its preliminary assessment, based on videos and reports from the affected areas, also indicates that a substantial volume of ice and rock debris may have entered the Lhende River.

The flood travelled downstream with exceptional speed and force. The Trishuli River rose by as much as nine metres within 30 minutes at Galchhi, while the water level at Malekhu increased by about seven metres over the same period, according to ICIMOD.

Scientists had also considered the possibility of a glacial lake outburst. The region experienced a similar sudden rise in the Bhote Koshi on July 8 last year, which Nepal’s Department of Hydrology and Meteorology attributed to the bursting of a glacial lake on the Lhende River near the Nepal-Tibet border.

A study conducted by ICIMOD and the United Nations Development Programme identified 3,624 glacial lakes across Nepal, India and Tibet. Of these, 47 were classified as potentially dangerous, including 21 in Nepal.

Experts have warned that the danger of further flooding has not been eliminated. Persistent cloud cover has restricted satellite observations, while scientists are monitoring whether another temporary body of water is forming upstream.

“The risk is still there,” Kayastha said, warning that rising temperatures increased the likelihood of glacier melt, fragmentation of ice and rock, and avalanches across the Himalayas.

Chinese authorities have meanwhile reported the formation of another large barrier lake near the confluence of the Chhochen Khola and Purepu Tsangpo rivers in Nepal.

Around two million cubic metres of water had accumulated in the lake by Thursday morning, although water had started flowing out, China Central Television reported. Authorities estimated that another three million cubic metres could enter it over the following three days, increasing the risk of the natural dam failing.

Chinese officials are monitoring the lake and carrying out flood simulations to assess the potential consequences of a breach.

The Chhochen Khola and Purepu Tsangpo originate in Tibet and merge before crossing into Nepal, where the river becomes known as the Bhote Koshi, the principal upper tributary of the Trishuli River.

With PTI inputs