Nepal faces new flash flood risk from lake formed after glacial avalanche

Two days after a glacial avalanche caused severe flash floods in parts of Nepal and Tibet (China) on Wednesday, a newly formed river burst its banks, raising the risk of further flooding. The breach on Friday temporarily halted rescue operations in the area.

The flash floods and mudslides triggered by the Himalayan glacial avalanche have killed at least 469 people. More than 1,000 others remain missing in Nepal and China's Tibet region.

Narendra Pariyar, the top official of Nepal's Rasuwa district, said local authorities had been informed that the lake had begun to overflow. "We can see the river level rising. We don't know how big the lake is or how high the water level is," Pariyar said.

Two new lakes formed

Scientists say the disaster has created not just one but two new lakes. One formed at a high altitude near the glacier, where meltwater is being trapped. The other, much larger and more dangerous, lies on a river in Nepal, where a massive landslide blocked the valley and stopped the river's flow.

The lake is near the confluence of the Chhochen Khola and Purepu Tsangpo rivers in Nepal's Lhende river system, which flows into the upstream of the Bhotekoshi and Trishuli rivers, carrying water south toward Kathmandu.

Nitesh Khadka, a Nepali researcher and physical geographer specializing in glacial flood outbursts, explained that the upper lake formed when glacier meltwater accumulated and could not drain naturally. "The lake on the Lhende River formed when a landslide/mudslide blocked the flow and water was trapped. This is a natural process and cannot be prevented," Khadka said.

Risk of lake outburst

Nepali and Chinese authorities warned on Thursday that the lower lake, formed by accumulated debris in the flood zone, was a threat. Nepal's National Disaster Risk Reduction and Management Authority (NDRRMA) warned that the lake's water level was rising and could cause a breach.

China's Ministry of Water Resources estimated Thursday that the lake holds about 1.5 to 2 million cubic meters of water and warned that a further 3 million cubic meters could pour in within three days, equivalent to about 1,200 Olympic-size swimming pools.

"This is a threat, because if a newly formed lake suddenly bursts, it could trigger another flood; it could mobilize sediments, hindering search and rescue operations and endangering survivors in hydropower tunnels," Khadka warned. The floodwater would then carry mud and rocks, making the flow more destructive and complicating efforts to find the missing.

Difficult access to the site

The terrain where the landslide-dammed lake formed is remote and rugged, making it difficult to bring teams or heavy equipment. "After the glacial avalanche created the lake, many isolated and steep cliffs appeared along it," Chen Hanxiao, an engineer with a Chinese technical team, told CCTV.

Chinese authorities said they had deployed an eight-member technical team to conduct aerial surveys and 3D modeling of the lake in Nepal.

Rescue efforts and damage

Rescue operations in the hard-hit Rasuwa district were suspended Friday due to the lake breach but later resumed. China's state broadcaster CCTV said the risk from the overflowing lake was assessed as "controllable."

According to a World Health Organization (WHO) official on Friday, six health facilities were damaged by the flooding and eight health workers are missing.

Nepal said it does not need foreign assistance. "Offers of help are natural. Our security agencies are carrying out search and rescue operations. At this moment, we do not need such support," Lok Bahadur Chhetri, a spokesman for Nepal's Ministry of Foreign Affairs, said Friday.

Background of similar disasters

Glacial avalanches and glacier lake outburst floods (GLOFs) have occurred before in the Himalayas. A glacial avalanche occurs when a large mass of ice detaches from its base, sometimes involving millions of cubic meters of ice, rock, and water, and can turn into an avalanche or a fast-moving debris flow.

In July 2016, the lower part of a valley glacier in the Aru range, Tibet, broke off, sending about 68 million cubic meters of ice down the mountain, killing nine herders and hundreds of livestock. Two months later, a nearby glacier collapsed, releasing about 83 million cubic meters of ice.

In November 2022, about 40 million cubic meters of ice broke off the Bukadaban Feng glacier in the Kunlun Mountains, on the northern edge of the Tibetan Plateau in Qinghai Province, China. The avalanche crashed into a lake below, generating a 13-meter-high wave, according to an analysis by the European Space Agency.

GLOFs have also occurred in northern Pakistan. In May 2022, floods hit the Hassanabad area of Hunza Valley after a glacial lake burst suddenly due to the surging Shishper glacier.