Scientists investigating Nepal’s deadly Rasuwa district floods have found that a sudden, undetected ice-rock avalanche high in the Himalayas triggered the disaster, exposing the limits of conventional early-warning systems.
Scientists investigating last week’s deadly floods that tore through Nepal’s Rasuwa district say the disaster was most likely triggered by a sudden ice-rock avalanche high in the Himalayas, rather than a glacial lake outburst flood (GLOF).
On 26 August, a huge mass of glacial ice and rock broke away at an altitude of about 5 200m and plunged roughly 1 200m into the valley below, triggering a devastating cascade of hazards.
More than 1 100 people have now died in the flooding and landslides across Nepal, with thousands still missing, as rescue teams continue to search devastated communities and hydropower sites. The toll is continuing to rise as bodies are recovered and teams reach previously inaccessible areas.
Scientists have used satellite imagery, seismic data, video footage, numerical modelling and regional climate data to determine that an ice-rock avalanche was the most likely trigger for the disaster, rather than a GLOF.
The precise sequence remains under investigation, including whether longer-term changes such as permafrost degradation and rock-slope instability contributed to the collapse.
The collapse occurred at about 8.37am Nepal standard time and generated a 5.2-magnitude seismic signal. Within minutes, debris and floodwaters had moved downstream towards the Bhotekoshi River.
Initial reports suggested that the flooding may have resulted from the failure of an artificial dam. However, subsequent assessments indicate that the direct impact of the ice-rock avalanche and resulting debris flow were primarily responsible for the catastrophic flooding, according to the International Centre for Integrated Mountain Development (ICIMOD).
The event illustrates a growing challenge for the Hindu Kush Himalaya: hazards can cascade rapidly, turning an ice-rock avalanche into a debris flow and destructive flood within minutes, it said.
Dr Farooq Azam, senior cryosphere specialist and intervention manager at ICIMOD, said the event was unprecedented and unexpected.
“There was simply no detection and no time to issue a warning,” he said in a statement. “The event was a sudden-onset hazard that could not be captured by conventional warning systems.
“However, if there had been an early warning system, it could have saved lives to some extent. But in such rock-ice avalanches, establishing a warning system is very difficult to ascertain the place of the breakpoints, which can sometimes be below the bedrock or glaciers and not easily visible with satellite or eyes.”
The difficulty is that conventional early-warning systems are generally designed around hazards that can be monitored through indicators such as rising river levels or known glacial lakes, he said.
An ice-rock avalanche can instead originate from an unstable mountain slope, glacier or subsurface area, making it extremely difficult to determine where or when a failure might occur.
Azam said the response therefore needed to go beyond early-warning systems to include preparedness, land-use planning and more resilient infrastructure.
“The immediate priorities therefore include stronger land-use planning, resilient infrastructure and greater community awareness of high-altitude hazards.”
There needs to be a reconsideration of how infrastructure is planned in the Himalayas, Azam said.
“Environmental impact assessments are routinely conducted before infrastructure is built; increasingly, there is a need to assess how changing environmental and mountain conditions could affect that infrastructure over its lifetime and, lastly, the community awareness of such hazards.”
ICIMOD’s recommendations following the disaster include stronger enforcement of land-use planning in areas exposed to river, slope and mountain hazards.
It also argues that governments need to look beyond conventional environmental impact assessments carried out before infrastructure is built, and assess how changing environmental conditions could affect infrastructure throughout its lifetime.
A third priority is to incorporate indigenous knowledge into contemporary disaster-risk planning. Historically, many mountain communities settled on higher ground and away from river channels. More recent development has increasingly moved towards riverbanks and, in some cases, onto riverbeds.
Indigenous knowledge, Icimod argues, should therefore inform modern land-use and disaster-risk planning. The need for such preparedness is becoming more urgent as the Himalayan cryosphere changes.
The latest Icimod assessment of glaciers in the Hindu Kush Himalaya found that glacier mass loss has accelerated significantly since 2000.
Over 50 years of observations, about 89% of recorded years had negative glacier mass balance, indicating a sustained decline. The report also warns that only a small fraction of the region’s glaciers have been studied and that data coverage remains uneven.
The Himalayan region has also experienced significant warming in recent decades, although rates vary substantially across locations.
Glacier loss threatens water availability, increases the risk of GLOFs and puts the livelihoods of nearly two billion people dependent on Himalayan water systems at risk.
But the relationship between climate change and individual mountain disasters is not always straightforward.
ICIMOD has cautioned that while climate change is altering glaciers, snow conditions, permafrost and mountain slopes across the Hindu Kush Himalaya, it is too early to determine what role climate change played in the Rasuwa event specifically.
There are, however, growing signs of broader changes across the Himalayan cryosphere. The cryosphere refers to all the frozen parts of the Earth.
Research by Climate Trends indicates that black carbon, produced largely by biomass burning and fossil-fuel use, has increased in parts of the eastern and central Himalayas.
Black carbon absorbs sunlight and can darken snow surfaces, reducing their reflectivity and contributing to surface warming and accelerated snow loss.
The study found that average snow-surface temperatures rose from -11.27°C during 2000-2009 to -7.13°C during 2020-2023.
Declining snow cover is also changing the timing of water flows, research has revealed. Earlier snowmelt can shift peak runoff into earlier months, potentially increasing water shortages later in the year when demand is high.
At higher altitudes, permafrost degradation is another concern. As permanently frozen ground thaws, previously stable slopes can become more vulnerable to erosion, landslides and instability, posing risks to roads, railways, hydropower plants and other infrastructure.
Anjal Prakash, professor of public policy at FLAME University and an Intergovernmental Panel on Climate Change author, said, “Nepal’s mountains have been sending warnings for a decade: thinning glaciers, retreating snowlines, permafrost turning to slush under roads and hydropower plants we kept building anyway.
“We didn’t lack the science, as researchers have tracked this decline for fifty years. We lacked the political will to act on it. Early warning systems are being sold as the fix, but no siren can outrun a mountainside collapsing in seconds.
“The real failure sits upstream, in land-use decisions that pushed settlements and infrastructure onto riverbanks our ancestors deliberately avoided and in a global economy still burning the fossil fuels that are cooking these glaciers from the inside out.
“Every villager who watched that valley disappear paid for emissions decisions made thousands of kilometres away. Until governments treat the Himalayas as a climate frontline rather than a postcard backdrop, Rasuwa won’t be the last name we’re forced to memorise.”
Looking ahead, the risks are expected to intensify. Global climate models project that the Hindu Kush Himalaya could warm by between 2°C and 5°C by the end of the century, depending on future emissions, research has found.
That warming is expected to accelerate glacier mass loss. Under a high-emissions scenario, studies project that Himalayan glaciers could lose more than 60% of their current ice volume by 2100. Even under a more moderate emissions scenario, up to 35% could be lost.
The consequences would extend far beyond the mountains. As glaciers retreat, increased meltwater may temporarily boost river flows, a phenomenon known as “peak water”.
But once glacier volumes decline substantially, the amount of meltwater feeding glacier-dependent rivers is also expected to fall, increasing the risk of water scarcity for communities downstream.
