A massive flash flood originating in Tibet swept into central Nepal on Wednesday, killing more than 160 people, while search and rescue teams continued efforts to locate hundreds of people reported missing.
The flood along the Bhote Koshi-Trishuli river corridor washed away bridges, damaged roads and disrupted hydropower projects as huge torrents of water, mud, rocks and debris surged downstream.
The disaster was initially believed to have been triggered by an earthquake. However, the US Geological Survey (USGS) later clarified that no earthquake had occurred. Instead, seismic signals detected in the region were caused by a massive glacial collapse and debris flow in Tibet, which triggered the deadly flooding in Nepal.
What Happened in Tibet?
The disaster began when a huge mass of ice and rock collapsed from a mountain in Tibet and plunged into a valley below.
The collapse generated seismic waves strong enough to initially be detected as an earthquake. The event was estimated to have produced seismic activity equivalent to about a magnitude 4.4 event, but the shaking was not caused by tectonic movement.
An initial analysis of Planet Labs satellite imagery indicated that an ice-and-rock landslide triggered a debris-filled flood in the Lhende River, a high-altitude mountain stream flowing from Tibet. The location was around 20 km northeast of the Rasuwagadhi Nepal-China border crossing.
The Lhende River, also known as Lhende Khola, is a tributary of the Bhote Koshi River, which flows from Tibet into Nepal.
How Did the Glacial Collapse Trigger the Flood?
The enormous mass of ice, rocks and debris crashed into the river and blocked its natural flow, effectively creating a temporary dam.
Water then built up behind the obstruction. When the blockage eventually failed, the stored water was suddenly released along with the accumulated debris.
The powerful surge entered the Bhote Koshi River and raced downstream into Nepal, carrying huge quantities of water, mud, rocks and debris.
Why Was It Initially Reported as an Earthquake?
In the immediate aftermath, the exact cause of the disaster was unclear.
At around 8:37 am Nepal time on Wednesday, seismic activity was initially classified as a magnitude 4.4 earthquake near the Nepal-China border, north of Kathmandu.
Nepal’s Foreign Minister Shishir Khanal also initially suggested that an earthquake may have triggered a landslide, which then blocked the river and sent a large volume of water downstream.
Other possible explanations included a glacial lake outburst flood (GLOF) or an ice avalanche, according to AFP.
The USGS later reviewed seismic data, satellite imagery and information from nearby monitoring stations. It concluded that the event was actually caused by a glacial collapse and debris flow.
The agency clarified that no earthquake had occurred.
Was It a Glacial Lake Outburst Flood?
The event involved a type of glacial hazard that can produce flooding similar to a glacial lake outburst flood, although the USGS specifically identified this incident as a glacial collapse and debris flow.
A GLOF occurs when a natural dam holding back a glacial lake suddenly fails. Such dams can be made of ice, rocks or loose sediment. Once the barrier breaks, a large volume of water can rush downstream, carrying mud, rocks and other debris.
These failures can be triggered by erosion, rising water pressure, avalanches, rockfalls, earthquakes or the sudden collapse of ice and rock into a glacial lake.
In the Nepal disaster, the immediate trigger identified by the USGS was the glacial collapse and debris flow in Tibet. The resulting blockage was followed by a powerful surge that travelled downstream into Nepal.
Flood Devastates Central Nepal
After entering Nepal, the flood surged through the Bhote Koshi River and affected areas including Rasuwa, Dhading and Nuwakot before continuing downstream.
The destruction was extensive. Videos from affected areas showed muddy torrents filled with rocks and debris sweeping through settlements. Bridges collapsed, vehicles were washed away and buildings were submerged or destroyed.
The disaster also damaged critical infrastructure, including roads and hydropower facilities.
Why Can a Glacial Collapse Be So Dangerous?
The disaster highlights how an event in a high-altitude region of Tibet can quickly develop into a major catastrophe downstream.
A massive collapse of ice and rock can block a river within minutes. If the temporary blockage later breaks, the resulting surge can travel rapidly downstream, carrying enormous amounts of water and debris.
A 2023 study estimated that about 15 million people worldwide are exposed to glacial lake outburst flood hazards, with significant populations at risk in countries including China, India, Nepal, Pakistan and Peru.
The Nepal disaster underscores the growing risks posed by unstable glaciers, landslides and other high-altitude hazards in mountainous regions.