Published : 27 Aug 2026, 07:08 PM
Updated : 08 Sep 2026, 09:36 AM
There had been no torrential downpour to signal what was coming.
Yet a huge surge of mud, rock, ice and water suddenly tore through communities along the Nepal-Tibet border, sweeping away homes, roads and bridges and catching many residents by surprise.
At least 165 people have been confirmed dead in Nepal and China’s Tibet region, while nearly 1,500 remain missing, according to Reuters. Rescuers continue searching the devastated Himalayan valleys for survivors.
Scientists now believe the disaster was triggered not by unusually heavy rainfall, but by the collapse of part of a glacier high in the mountains, setting off a chain of events that sent a sudden flood rushing downstream.
So how can a catastrophic flood happen when it is not raining, and why was it so difficult to warn people in time?
What Happened?
The disaster struck the mountainous border region between Nepal and Tibet on Wednesday.
A substantial section of a glacier broke away and plunged hundreds of metres onto the valley floor, sending a huge mass of ice, rock and debris downhill, experts told Reuters after examining satellite imagery.
That material then surged towards the Lhende Khola, a tributary of the Bhote Koshi River, triggering catastrophic flooding downstream.
The torrent devastated settlements and infrastructure on both sides of the border, with roads, bridges and electricity facilities among those destroyed.
Hundreds of foreign nationals were also reported missing, many of them tourists and pilgrims travelling through the region towards Mount Kailash and Lake Mansarovar.
How Did the Flood Occur without Heavy Rain?
That is what initially made the disaster so puzzling.
Flash floods in Nepal are often associated with intense monsoon rainfall. But people in the affected area had not experienced the kind of downpour that would normally signal an imminent flood.
Kosh Raj Koirala, an editor at Nepal’s Republica Daily, told Al Jazeera that many people were caught off guard because there had been no heavy rainfall.
Climate specialists at the International Centre for Integrated Mountain Development (ICIMOD) believe an ice-rock avalanche from a glacier triggered the disaster.
“The Lhende Khola has flooded twice in 14 months, this time triggered by an ice-rock avalanche from a glacier on the Nepal side that blocked the river and released a sudden surge downstream,” ICIMOD disaster-risk specialist Saswata Sanyal told Al Jazeera.
In other words, the flood appears to have been generated high in the mountains rather than by rain falling directly on the communities below.
How does a collapsing glacier create a flood?
Large sections of glaciers can fracture and collapse, particularly on steep mountain slopes.
When that happens, enormous volumes of ice, rock and sediment can race downhill at high speed.
According to the account given by ICIMOD, the avalanche in this case temporarily blocked the Lhende Khola.
Such a blockage can restrict the river’s normal flow and cause water to accumulate behind ice, rock and debris.
If the obstruction then fails or is overwhelmed, the accumulated water can be released rapidly, carrying mud, boulders, ice and other debris downstream.
That helps explain why the flood arrived so suddenly.
Communities below did not need to be experiencing heavy rain themselves: the trigger was occurring kilometres away and far higher in the mountains.
Was There an Earthquake?
There was initially confusion over that too.
The US Geological Survey recorded seismic activity around the time of the disaster, initially appearing to indicate a magnitude 4.4 earthquake.
But Al Jazeera reported that the USGS later said the seismic energy had instead been generated by the collapse of glacial rock and ice and the subsequent movement of debris.
Large landslides and avalanches can generate vibrations strong enough to register on seismic instruments.
Reuters reported separately that Nepali officials had initially considered an earthquake as one possible trigger, but experts examining satellite imagery increasingly pointed towards the glacier collapse itself.
Was This a Glacial Lake Flood or Something Different?
Something different, according to the evidence so far. Unlike a typical glacial lake outburst flood, the latest disaster appears to have begun when part of a glacier collapsed, sending an ice-rock avalanche into the Lhende Khola and temporarily blocking the river before a sudden surge was released downstream.
The same broader river system experienced serious flooding last year after the draining of a supraglacial lake in Tibet, according to regional climate researchers cited by Al Jazeera.
But ICIMOD says the latest disaster appears to have followed another sequence.
This time, an ice-rock avalanche apparently blocked the river before releasing a sudden surge downstream.
Why Was There So Little Warning?
The unusual trigger is part of the answer.
Flood-warning systems are generally better equipped to detect hazards that develop in ways that can be monitored.
Meteorologists can track intense rainfall. River gauges can record steadily rising water. Known glacial lakes can also be watched for dangerous changes.
A sudden collapse high on an isolated glacier can be much harder to anticipate.
The avalanche itself may unfold in minutes. If it blocks a river and that obstruction subsequently releases water quickly, downstream communities may have only a narrow window in which to react.
That appears to have contributed to the surprise in Nepal, where residents had no obvious weather warning that a major flood was imminent.
Climate campaigners and disaster specialists speaking to Al Jazeera have consequently stressed the need for stronger early-warning systems in vulnerable Himalayan communities.
The challenge is that those systems may increasingly need to account for unstable glaciers and slopes as well as rainfall and established glacial lakes.
Does Climate Change Have a Role?
Scientists have not established that climate change directly caused this individual glacier collapse.
But the wider changes taking place across the Himalayas are well documented.
Reuters has reported that Nepal has lost nearly a third of its glacier ice over roughly three decades, with melting accelerating sharply in recent years.
As glaciers retreat, the surrounding landscape can also become more unstable.
Changes in ice can alter the support provided to rock faces, reshape drainage systems and contribute to the formation or expansion of glacial lakes.
That means warming can create several different kinds of mountain hazard rather than simply causing glaciers to shrink gradually.
The wider Himalayan region has suffered a series of destructive floods, landslides and glacier-related disasters in recent years, including the 2023 glacial lake outburst flood in Sikkim and the 2021 disaster in India’s Uttarakhand.
Has This Area Flooded Before?
Yes.
The Bhote Koshi-Lhende Khola corridor has already suffered serious flood damage.
Al Jazeera reported that flooding in the Bhote Koshi last year killed nine people and left 24 missing, while washing away the Friendship Bridge linking Nepal and China and disrupting transport and trade for months.
That event was attributed to the draining of a supraglacial lake in Tibet.
The latest flood is therefore particularly striking because the same region has now suffered two destructive glacier-related disasters within little more than a year, apparently through different mechanisms.
