Why a newly formed lake threatens Nepal with another flood
Glacier collapse in Nepal triggered flash floods and mudslides, while a landslide-dammed river has created a dangerous barrier lake
A newly formed lake on the Nepal-China border is raising fears of another devastating flood, two days after a glacier collapse triggered catastrophic flash floods and mudslides across parts of Nepal and China's Tibet.
The disaster has so far left 547 people confirmed dead, while more than 1,000 remain missing across Nepal and Tibet.
On Friday (28 August), officials in Nepal's Rasuwa district said the newly formed barrier lake had begun overflowing, forcing a temporary halt to rescue operations and raising concerns that a sudden breach could send another destructive surge downstream.
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What happened?
The original disaster occurred after a glacier collapse in the Himalayan region triggered a massive flash flood and mudslides, reports Al Jazeera.
The collapse sent huge amounts of ice, rock, mud and other debris into the river system. The debris blocked the river in parts of the valley, creating a natural dam and causing water to accumulate behind it.
Scientists say the disaster has created two new lakes.
One formed at a higher elevation near the glacier in China, where meltwater has become trapped. The second, and potentially more dangerous, lake formed on the Lhende river in Nepal after a massive landslide blocked the valley.
On Friday, officials in Rasuwa said the lower lake had begun overflowing.
"We can see that the level of water in the river is rising," said Narendra Pariyar, the district's most senior bureaucrat.
He said officials were still unable to determine the size of the lake or its exact water level.
Why is the new lake dangerous?
A barrier lake is formed when rocks, mud, ice and other debris block a river, creating a natural dam.
The problem is that such a dam is often unstable. If the water rises high enough, it can erode the debris barrier and cause it to collapse suddenly.
That could release a huge volume of water and debris downstream with little warning.
"The danger is not just the lake itself, but the possibility of sudden dam failure," said Pawan Bhattarai, an assistant professor in the civil engineering department of Nepal's Tribhuvan University.
"In a steep valley like this, even a short-lived breach can generate a fast-moving debris-laden flood with very little warning for downstream communities," he said.
China's Ministry of Water Resources estimated that the lake was holding around 1.5 to 2 million cubic metres of water on Thursday and warned that another 3 million cubic metres could flow into it over three days.
That is roughly equivalent to 1,200 Olympic-sized swimming pools.
Where is the lake?
The lake is located near the confluence of the Chhochen Khola and Purepu Tsangpo rivers in the Lhende river system in Nepal.
The river system lies upstream of the Bhotekoshi and Trishuli rivers, which carry water south towards Kathmandu.
A sudden breach could therefore send a large volume of water, mud and debris downstream, threatening communities and infrastructure along the river system.
How was the lake formed?
The initial glacier collapse caused an avalanche of ice and rock that entered the river valley.
The enormous amount of material blocked or severely restricted the river's natural drainage, creating a natural dam.
"The new lake appears to be an avalanche-induced barrier lake," Bhattarai said.
"The ice-rock avalanche deposited a huge volume of rock, ice and debris in the river valley, blocking or severely constricting the drainage. Water then accumulated behind this natural barrier, forming the lake."
Nepali researcher and physical geographer Nitesh Khadka said satellite images and drone footage showed two lakes had formed following the disaster.
The upper lake formed when meltwater accumulated near the glacier, while the lake on the Lhende river was created after a landslide blocked the river.
Can another flood be prevented?
Experts say completely preventing such a natural process is difficult, particularly because the affected area is remote and mountainous.
However, the risk can potentially be reduced through continuous monitoring and early intervention.
Possible measures include monitoring the lake's water level, safely lowering the water where feasible, strengthening early-warning systems and evacuating downstream communities when necessary.
Bhattarai said better satellite monitoring, rapid detection of river blockages and lake formation, real-time river monitoring and stronger Nepal-China transboundary early-warning systems could help reduce the impact of such disasters.
The difficult terrain, however, makes it extremely challenging to bring heavy equipment and large rescue teams to the site.
What are authorities doing?
Nepali and Chinese authorities have been monitoring the lake and assessing the risk of another flood.
China has deployed an eight-member engineering team to conduct aerial surveys and develop 3D models of the barrier lake, according to state broadcaster CCTV.
Engineers have faced difficulties accessing the site because the glacier collapse created isolated and steep cliffs around the lake.
Rescue operations in the hard-hit Rasuwa district were temporarily halted on Friday after the barrier lake overflowed but were later resumed. CCTV reported that the risks posed by the overflowing lake were considered manageable.
The floods have also damaged six health facilities, while eight health workers are missing, according to a World Health Organization official.
Nepal has said it does not currently require foreign assistance.
"The offer for help is natural. Our security agencies are doing the search and rescue operation. For now we don't require such help," Nepal's Foreign Ministry spokesperson Lok Bahadur Chhetri said on Friday.
Has this happened before?
Similar glacier-related disasters have occurred in the Himalayas before.
Bhattarai cited Nepal's 2012 Seti flood as an example in which rockfall, temporary river blockage and an ice-rock avalanche combined to produce a destructive flood.
A glacier collapse occurs when a large mass of glacier ice detaches from its bed. Such an event can involve millions of cubic metres of ice, rock and water, transforming into a highly mobile avalanche or debris flow.
A glacial lake outburst flood (GLOF) occurs when a glacier-fed lake suddenly releases a large volume of water.
In July 2016, a section of a valley glacier in Tibet's Aru Range collapsed, sending around 68 million cubic metres of ice down the mountain and killing nine herders and hundreds of animals. A nearby glacier collapsed two months later, releasing around 83 million cubic metres of ice.
In November 2022, around 40 million cubic metres of glacier ice detached from the Bukadaban Feng glacier in China's Qinghai province. The resulting ice avalanche crashed into a lake and generated a wave around 13 metres high, according to analysis by the European Space Agency.
Similar GLOF events have also occurred in northern Pakistan. In May 2022, a sudden breach of an ice-dammed lake formed by the surging Shishper Glacier caused flooding in the Hassanabad area of Hunza Valley.
