Hyderabad: A catastrophic flash flood tore through northern Nepal on August 26, killing at least 165 people and leaving hundreds missing. It was a torrent of water, mud, ice and rocks that swept through communities along the Nepal-China border.
The flood struck the Rasuwa district before moving downstream through the Bhote Koshi and Trishuli river systems. Bridges, roads, homes and hydropower infrastructure were swept away, while the force of the debris-laden water travelled deep into the valleys.
Cause of catastrophe
The disaster initially raised questions about whether a glacial lake had suddenly burst, triggering what is known as a Glacial Lake Outburst Flood, or GLOF.
But preliminary satellite analysis points to a more complicated chain of events.
An ice-rock avalanche or glacier collapse appears to have sent a huge volume of debris into the Lhende River. The debris blocked the river, creating a temporary natural dam. Water accumulated behind the blockage before the barrier gave way, sending a sudden surge of water, rocks, mud and ice downstream.
The National Disaster Risk Reduction and Management Authority (NDRRMA), citing an initial analysis of Planet Labs satellite imagery, said, “An initial study based on the analysis of satellite images by Planet Labs indicates that a glacier-inclusive flood occurred in the Lhende River due to a snow-rock landslide that took place on the Nepal-China border approximately 20 km northeast of the Nepal-China Rasuwagadhi border crossing.”
So, what exactly is a GLOF, and how can a disturbance high in the Himalayas turn into a destructive flood hundreds of metres lower in the valley?
What is a GLOF?
A Glacial Lake Outburst Flood occurs when water stored in a glacial lake is suddenly released.
As glaciers retreat and melt, meltwater can accumulate in depressions around the glacier. The lake may be held back by a moraine, a natural ridge made of loose rocks, gravel, soil and, in some cases, buried ice.
Unlike an engineered dam, a moraine is not a reinforced structure. It can be weakened by erosion, melting ice, rising water levels or a sudden impact.
If the natural barrier fails, the stored water can escape within minutes.
Because Himalayan valleys are steep, the released water can accelerate rapidly as it moves downhill. Along the way, it can pick up rocks, boulders, mud, trees and other material, turning a flood into a destructive debris flow.
Was the Nepal flood a GLOF?
That has not been conclusively established.
This distinction is important.
The August 26 disaster appears to have involved a glacier collapse or ice-rock avalanche followed by river blockage and sudden release of water and debris.
Nepal’s Department of Hydrology and Meteorology said preliminary satellite imagery indicated that debris had blocked the river, creating a temporary lake that subsequently burst. Officials said the large amounts of debris made the resulting flood much more destructive.
The NDRRMA also said its preliminary Planet Labs analysis showed that an ice-rock landslide had triggered a debris-laden flood in the Lhende River, about 20 km northeast of the Rasuwagadhi border crossing.
That means the event may be better understood as a glacier-collapse and debris-dam outburst, rather than automatically being labelled a conventional GLOF.
Scientists are still examining whether a pre-existing glacial lake also burst as part of the chain of events.
So how can a glacier collapse cause a flood?
The key is to understand the cascade.
1. Glacier and ice become unstable
High in the Himalayas, glaciers and surrounding slopes contain enormous quantities of ice and rock.
If a section of a glacier collapses, the falling mass can gather rocks, soil and sediment as it moves down the mountain.
Satellite imagery from the disaster indicates that a substantial section of glacier broke away at an elevation of around 5,200 metres and fell roughly 1,200 metres towards the valley floor.
2. Ice and rocks enter the river
The collapsing material eventually reaches the river system below.
In this case, the ice-rock avalanche appears to have entered the Lhende River, a tributary of the Bhote Koshi.
This is where the disaster changed from a mountain collapse into a potential flood-generating event.
3. The river gets blocked
Imagine a narrow mountain river suddenly receiving an enormous pile of ice, rocks and mud.
The debris can form a temporary natural dam across the river.
Water continues flowing in from upstream, but it cannot pass through the blockage at its normal rate.
It begins to accumulate behind the debris.
The Kathmandu Post reported that preliminary satellite imagery showed debris blocking the river and forming a temporary lake before the barrier subsequently burst.
4. The temporary dam breaks
The natural blockage cannot necessarily withstand the increasing force of the accumulated water.
Once it fails, the stored water is suddenly released.
But this is no longer clear water.
The water has mixed with the material that formed the blockage, including rocks, mud, ice and sediment.
That is why the resulting flood can become much more destructive.
As Binod Parajuli, chief of the Flood Forecasting Division at Nepal’s Department of Hydrology and Meteorology, explained, the debris significantly increased the destructive power of the flood.
5. The debris accelerates downstream
Once the blockage gives way, gravity takes over.
The water rushes down steep Himalayan valleys, carrying boulders, trees, ice and sediment with it.
The river itself becomes the pathway for the disaster.
In the August 26 event, the flood moved from the Lhende River into the Bhote Koshi and then into the Trishuli system, carrying the debris and floodwater into populated areas downstream.
Hydrological observations showed just how quickly the flood wave moved. Water levels in parts of the Trishuli rose by as much as 9 metres in about 30 minutes.
6. The flood reaches settlements and infrastructure
By the time the flood reaches the lower valleys, it is no longer simply a surge of water.
It is a rapidly moving mixture of water, mud, rocks, trees and debris.
That mixture can smash into bridges, roads, homes, power infrastructure and other structures.
At least 19 bridges and around 40 km of roads were reported damaged or washed away in the disaster.
The devastation therefore does not necessarily occur at the point where the glacier collapses.
It unfolds along the entire river pathway.
Why is distinction between a GLOF and Nepal disaster important?
A conventional GLOF begins with water stored in a glacial lake and the sudden failure of the natural barrier holding that water.
The Nepal disaster appears to have involved another possible pathway:
Glacier collapse → ice-rock avalanche → river blockage → temporary debris dam → sudden breach → debris-laden flood
There could still have been a glacial lake involved somewhere in this chain. Authorities have said that possibility is being investigated.
But scientists have cautioned that it is too early to conclusively classify the event as a GLOF.
The distinction matters because not every glacier-related flood is a GLOF.
A glacier collapse, landslide, avalanche or rockfall can itself create a temporary dam and trigger a sudden flood.
Did an earthquake trigger the glacier collapse?
The answer is still evolving.
An earthquake with a magnitude initially reported at 4.4 was detected around the same time as the disaster. Early reports suggested that the earthquake may have triggered the avalanche.
However, the US Geological Survey later concluded that the seismic signal was generated by the collapse and subsequent debris flow rather than by a tectonic earthquake. The event was subsequently assessed as a magnitude 5.2 seismic signal generated by the collapse.
That means the apparent earthquake may have been a result of the massive ice-rock collapse rather than its cause.
The exact sequence of events is still being investigated.
What does climate change have to do with it?
The immediate trigger of a specific disaster cannot automatically be attributed to climate change.
But the broader Himalayan landscape is changing.
Rising temperatures are contributing to glacier retreat and altering the stability of high-altitude ice, snow, permafrost and glacial lakes.
As glaciers retreat, new lakes can form. As natural ice and sediment structures weaken, slopes and moraine dams can become more vulnerable to collapse.
This creates the possibility of cascading hazards, where one event triggers another:
Warming → glacier or slope instability → collapse or avalanche → river blockage → sudden release → downstream flood
The August 26 disaster is therefore important not only because of the immediate destruction, but because it demonstrates how a disturbance high in the Himalayas can rapidly become a major downstream hazard.
Why should India be concerned?
The flood did not stop at the point where the glacier collapsed.
The Lhende flows into the Bhote Koshi, which joins the Trishuli system. The Trishuli then forms part of the larger Gandaki or Narayani river system, which eventually enters India as the Gandak.
This creates a transboundary risk.
A disaster that begins in a remote high-altitude valley can therefore affect communities, infrastructure and river systems much farther downstream.
The August 26 flood prompted warnings for communities along downstream rivers, including the Trishuli and Narayani systems.
For India, particularly downstream areas of the Ganga basin, the incident is a reminder that Himalayan flood risk cannot be assessed only by looking at rainfall in the plains.
A flood can begin with an avalanche or glacier collapse far upstream.
Why are such events difficult to predict?
One of the biggest challenges is that the trigger may occur in an extremely remote, high-altitude location.
There may be no people present to report what is happening.
Weather conditions can make direct observation difficult.
And unlike conventional floods driven by heavy rainfall, glacier collapses and landslide-dam failures can occur suddenly.
That makes satellite monitoring, seismic monitoring, river-level sensors and early-warning systems increasingly important.
The Nepal disaster also shows why monitoring must extend across borders. The source of a flood can be upstream of the communities that eventually face its consequences.