The devastating flood that swept through Rasuwa on August 26 has raised questions about whether such an event could have been anticipated.
While experts say it is generally impossible to predict the exact date, location or scale of many mountain disasters, decades of research have identified areas and conditions where the risks are higher.
Studies have repeatedly warned that Nepal’s glaciers, glacial lakes, landslides and downstream settlements require continuous monitoring and effective early-warning systems.
The Rasuwa disaster, however, has also highlighted an important limitation: monitoring glacial lakes alone may not be enough to understand all the hazards originating in the high mountains.
What experts had warned about
Environmental journalist Ramesh Bhusal says recent disasters across Nepal and the wider Himalayan region show that extreme mountain events can no longer simply be treated as unforeseeable.
Floods and debris-flow disasters have occurred in places including the Seti River in Pokhara, Melamchi, Roshi and Thame, as well as in other parts of the Himalayan region, including Uttarakhand, Pakistan and northeastern India.
“When similar events occur three times within five years, calling them unimaginable simply means we failed to anticipate them,” Bhusal says.
Experts estimate that around 83 percent of Nepal’s mountainous and hilly areas are exposed to risks associated with landslides and glacial lake outburst floods.
Disaster forecasting has evolved significantly over time. Researchers now use rainfall and river-gauge data, satellite imagery, remote sensing, geological studies and other technologies to identify areas exposed to different hazards. Artificial intelligence is also increasingly being explored as a tool for forecasting and risk assessment.
But forecasting a hazard does not necessarily mean predicting the exact moment when a disaster will occur.
Why glacial lakes are monitored
A glacial lake forms when water accumulates in or around a glacier, often behind a natural dam made of ice, rock or debris.
If the natural dam fails, a large volume of water can suddenly move downstream. This is known as a glacial lake outburst flood (GLOF).
Such floods can travel long distances through mountain valleys, carrying rocks, sediment and debris and damaging settlements, roads, bridges, hydropower projects and other infrastructure downstream.
Geography professor Dr Narendra Raj Khanal, who has studied glacial lakes and floods, says researchers have repeatedly highlighted the need for regular glacier monitoring and functioning early-warning systems.
“We may not be able to say that a particular glacial lake will burst on a particular date or that a landslide will occur at a particular location,” Khanal says. “But based on our studies, we have consistently said that regular monitoring of glaciers and the establishment and operation of early warning systems are essential.”
Which glacial lakes pose a risk?
A study by professors Narendra Raj Khanal, Pradeep Kumar Mool, Arun Bhakta Shrestha, Golam Rasul, Pawan Kumar Ghimire, Rajendra Bahadur Shrestha and Sharad Prasad Joshi assessed the potential human impact of glacial lake outburst floods.
According to an article published in the International Journal of Water Resources Development, a GLOF from Lumuchimi in the Bhote Koshi basin could potentially affect 16,313 people.
The study estimated that an outburst from Imja in the Dudh Koshi basin could affect 7,762 people, while an outburst from Tsho Rolpa in the Tamakoshi basin could affect 5,183 people.
A separate study by the International Centre for Integrated Mountain Development (ICIMOD) and the United Nations Development Programme (UNDP) also highlighted the potential danger posed by glacial lakes to downstream communities and infrastructure.
“Glacial lake outburst floods are an important problem for people living in the Himalayan region,” the 2020 report said, noting that lakes at high elevations can generate floods capable of travelling long distances downstream.
The report identified potentially dangerous glacial lakes as an important target for risk-reduction planning.
47 potentially dangerous lakes identified
The ICIMOD-UNDP study assessed 3,624 glacial lakes across Nepal’s Koshi, Gandaki and Karnali river basins and identified 47 as potentially dangerous. Of these, 25 are in China, 21 in Nepal and one in India. The Koshi basin accounts for 42 of the potentially dangerous lakes, while three are in the Gandaki basin and two in the Karnali basin.
In Nepal, the potentially dangerous lakes are spread across several river systems, including four in the Tamor basin, five in the Arun, nine in the Dudh Koshi, one in the Trishuli, one in Mugu and one in the Tamakoshi basin. In China, 13 are located in the Arun basin, seven in the Tamakoshi, four in the Sunkoshi and one in the Trishuli basin. The remaining lake is located in India’s Kali (Mahakali) basin.
The study identified six lakes as being at particularly high risk: Tsho Rolpa, Lower Barun, Imja Tsho, Lumding, West Chamjang and Dona (Thulagi).
Why the Rasuwa flood is different
Experts now say the Rasuwa flood was not a conventional glacial lake outburst flood.
According to Education Minister Sasmit Pokharel, who is also the government spokesperson, glaciers appear to have collapsed, causing water to surge downstream while carrying large amounts of sediment.
Experts believe the flooding in the Bhote Koshi was triggered by the collapse of an ice-rock formation near the Lhende River, which flows from China.
This distinction matters because a GLOF and a glacier or ice-rock collapse can produce different warning signals and require different monitoring approaches.
Professor Khanal says the Rasuwa disaster has demonstrated that focusing only on glacial lakes leaves an important gap in understanding high-mountain hazards.
“The Rasuwa flood has sent a message that studying only glacial lakes is not sufficient,” Khanal says. “It appears that forecasting such risks requires detailed study of glaciers, snow and ice alongside glacial lakes.”
Why glaciers, snow and ice matter
Glacial lakes are only one component of a much larger mountain system.
Changes in glaciers, snow and ice can affect the stability of slopes and river systems. A collapse involving ice and rock can generate a sudden surge of water, sediment and debris even when there is no glacial lake outburst.
This means that risk assessments need to consider the entire chain of possible hazards—from changes in glaciers and snow to ice or rock collapse, debris flows and downstream flooding.
The Rasuwa disaster has therefore raised the question of whether Nepal’s existing monitoring systems are broad enough to capture these interconnected risks.
Can scientists predict exactly when a disaster will happen?
Not usually. Scientists can identify hazards, assess the likelihood of certain events, monitor changes and estimate which communities and infrastructure could be exposed.
But that is different from predicting that a particular glacier will collapse on a particular day or that a particular landslide will occur at a precise location.
This distinction is particularly important in the Himalayas, where terrain is difficult to access and mountain conditions can change rapidly.
The objective of monitoring and early-warning systems is therefore not necessarily to predict every disaster with certainty. It is to identify dangerous changes early enough to reduce exposure and give people time to act.
Monsoon can amplify mountain hazards
Glacial and ice-related hazards can become particularly dangerous during Nepal’s monsoon season.
Nepal’s monsoon generally begins around June 13 and continues until October 2, bringing around 80 percent of the country’s annual rainfall.
Heavy rainfall can increase river levels and destabilise slopes. When intense rainfall coincides with water released from a glacial lake, glacier collapse or other high-mountain event, the resulting flood or debris flow can become significantly more destructive.
Historical records show that glacial lake outburst floods have occurred between March and September, with nearly two dozen events causing significant damage in the wider Himalayan region.
Have warnings been turned into action?
Climate analyst Dr Ngamindra Dahal says studies have repeatedly identified the risks associated with glacial lakes and high-mountain hazards, but implementation has remained limited.
“We know there is a risk. Plans have also been prepared. But those plans have not been turned into projects,” he says.
A National Disaster Risk Reduction and Management Authority official, however, says implementing every recommendation immediately is difficult.
“Not all the glacial lakes that can affect us are within Nepal,” the official says. “Implementation requires a large amount of money, which is not easy to secure.”
The cross-border nature of many Himalayan hazards adds another layer of complexity. Several potentially dangerous glacial lakes lie outside Nepal, meaning effective risk reduction can require cooperation between countries as well as monitoring within Nepal.
What the Rasuwa disaster has taught us
The Rasuwa flood does not necessarily show that scientists should have been able to predict the exact event.
Instead, it highlights the difference between predicting a disaster and understanding disaster risk.
Researchers have already identified potentially dangerous glacial lakes, assessed the number of people who could be affected and repeatedly called for monitoring and early-warning systems.
But the Rasuwa event shows that risk assessment must extend beyond glacial lakes.
Monitoring glaciers, snow, ice, glacial lakes, slopes and rivers together may provide a more complete picture of the hazards developing in Nepal’s high mountains.
The challenge now is turning that knowledge into continuous monitoring, effective warnings and practical preparedness—particularly for communities and infrastructure located downstream.
The precise timing of the next mountain disaster may remain impossible to predict. But understanding where the risks are, how hazards can cascade and who is exposed can make the consequences far less unpredictable.
