In a warming world, rising temperatures in high mountain regions are driving glacier retreat and melting, contributing to an increase in the number and area of glacial lakes. As glaciers retreat, they can create new lakes or expand old ones, sometimes contained by natural dams made of ice or moraines — ridges of rock, sediment and debris left behind by glaciers.
If these dams fail, glacial lake outburst floods (GLOFs) can release huge volumes of water and debris, severely flooding downstream communities and infrastructure. In some cases, GLOFs involve rockfalls or the collapse of entire mountain slopes with cascading impacts. Or there can be avalanches — massive, rapid downhill flows of rocks, ice and snow.
Scientists have improved glacier and lake monitoring in the Himalayan region, but large gaps remain in data collection, sharing and processing. This affects real-time early warning for river systems that run across international borders. In the aftermath of the recent Nepal flood disaster, this explainer looks at the relevance of these issues.
Initial reports suggested that a massive mountain slope failure involving glacier ice in Nepal’s Langtang National Park, close to the Chinese border. The mass of debris and flood waters travelled nearly 100 kilometres, gathering more water and rock as it gushed downhill, swelling and overflowing streams and rivers.
In Nepal, scientists at the International Centre for Integrated Mountain Development (ICIMOD) and their partners investigated whether the event was a high-altitude ice-rock avalanche. Rock-snow-ice avalanches — rapid, massive, downhill movements of detached rock with snow and ice — are expected to become more frequent as glaciers retreat and the permafrost thaws due to global warming.
ICIMOD reported that huge masses of ice and rock entered the Lende Khola, a tributary of the Bhote Koshi River. The burst of water downstream had a devastating force and exceptional speed. The waters of the Trishuli River at the riverside town of Galchchi rose by nine metres within 30 minutes, and at Malekhu downstream by seven metres, ICIMOD scientists noted.
“Continued inflow from the Mala River and other tributaries may have masked the upstream blockage. The steep gradient and narrow valley further accelerated and concentrated the flood, while the heavy debris load increased its destructive force,” reported Reliefweb, a specialised digital service of the UN Office for the Coordination of Humanitarian Affairs (UNOCHA). It caused a massive catastrophe along the Bhotkoshi-Trishuli corridor.
Internationally, scientists initially considered an earthquake and a glacial-lake outburst. Seismic analysis, however, showed that the mass movement itself generated energy equivalent to a magnitude-5.2 earthquake, and no earthquake has occurred.
Satellite imagery showed that the glacier–rock mass was gaining momentum in the days and weeks leading up to failure, as reported. Citing Manoochehr Shirzaei, a geophysicist at Virginia Tech University in Blacksburg, USA, Nature added: “(S)atellite images from just days before the disaster suggest that a section of the glacier and the rocks it sat on top of were accelerating in the weeks before the collapse, a concerning sign of an unstable system.” The precise sequence of how this disaster unravelled remained uncertain: it could have been a massive rockslide that pulled a part of the glacier downhill, or the glacier might have failed first, triggering a landslide.
A glacial lake outburst flood, or GLOF, is the sudden release of a large volume of water stored in a glacial lake. Glacial lakes can form beside, in front of, within, beneath or on the surface of a glacier. The barriers holding back the water may consist of glacial ice, moraine debris or bedrock.
GLOFs can produce exceptionally high peak flows, sometimes far greater than floods caused by heavy rainfall or snowmelt. Their force can erode and transport large amounts of sediment and debris, and in some cases the flood can develop into a fast-moving debris flow.
Himalayan flood risk is changing not only because the climate and cryosphere — frozen water part of the Earth system — are changing, but because the source of a disaster and the people affected by it can increasingly be separated by hundreds of kilometres and one or more international borders, pointed out Vijaykumar P., Assistant Professor in the Department of Environmental Sciences, University of Kerala.
Downstream communities — not only in the mountains, but also the plains — demand science-based inputs for disaster risk reduction. Still, hydrological data-sharing mechanisms between China and India are inadequate at present, despite earlier agreements, as government reports suggest.
Glaciers in the Hindu Kush Himalaya (HKH) are shrinking due to climate change, having steadily lost mass since 2000. More than 50 years of observations show that about 89% of recorded years experienced a negative glacier mass balance, marking a continuing and widespread decline, as ICIMOD studies show. That means less water and potentially more GLOFs – all affecting the livelihoods of nearly two billion people who depend on these water systems.
The number, area and volume of glacial lakes have increased — globally by 53%, 51% and 48%, respectively — in most parts of the world where they exist. Still, there is only “limited evidence” to show that GLOFs have changed, the Intergovernmental Panel on Climate Change (IPCC) points out. Expanding lakes could potentially lead to GLOF events. Glaciers that end in lakes retreat faster than those that terminate on land – thereby increasing glacial lake expansion and glacial recession rates. Consequently, GLOFs become a major hazard in the Himalayas, as studies show.
As rising temperatures melt glaciers and thaw permafrost, destabilising mountain slopes and expanding glacial lakes, downstream exposure to GLOFs has also increased — with more people, denser infrastructure, hydropower development, and intensified agriculture. It’s a cascading risk, as ReliefWeb calls it.
The flood event in Nepal illustrates a wider Himalayan problem. Even under a scenario that limits global warming to 1.5°C, the higher central and eastern parts of High Mountain Asia are projected to lose around 60% of their existing glacial ice. That means urgent measures are needed to address highland hydrometeorological disasters, Vijaykumar noted.
Hazards originating in remote places where intense precipitation, avalanches or GLOFs occur — even across international borders — can cause destructive floods and landslides far downstream. Monitoring and forecasting can be complicated by sparse stations, lack of real-time dissemination, damaged equipment and data gaps across nations and regions, Vijaykumar pointed out.
As Vijaykumar explained: “While accelerating glacial retreat expands high-altitude lakes and increases the frequency of GLOFs worldwide, the human toll is overwhelmingly concentrated across the Hindu Kush Himalaya region — spanning Nepal, northern India, and downstream Bangladesh.”
“Unlike polar regions like the Arctic and Antarctica, or rugged sectors of the Andes and Alps where meltwater often drains into remote, sparsely populated terrain or benefits from extensive early-warning infrastructure, the Himalayan valleys and downstream floodplains feature extreme population density, critical transport arteries, and hydropower infrastructure built directly within narrow outburst corridors,” Vijaykumar added.
“Glaciological events become severe humanitarian disasters.”
“The mountainous Himalayan states in India have been facing geological disasters and climate change-induced hydro-meteorological disasters and extreme weather events,” said N. Vinod Chandra Menon, Founder Member, National Disaster Management Authority (NDMA), Government of India
Menon said: “Hyper-local weather patterns can be monitored by Automatic Weather Stations (AWS) in the Himalayan States, supported by sensors for water level monitoring in the dams, rivers, lakes and other water bodies. India has more than 10,000 glaciers, and with the heatwaves and global warming, the Himalayan States must deploy early warning systems to monitor glacier melting, glacier retreats and the formation of glacial lakes.”
The Wadia Institute of Himalayan Geology (WIHG) tracks glaciers and glacial hazards to support early warning and disaster preparedness, and has inventoried 1,266 glacial lakes in Uttarakhand and 958 in Himachal Pradesh. The Central Water Commission (CWC) monitors 902 glacial lakes and water bodies for changes in their extent. The Centre has also approved a ₹150-crore National GLOF Risk Mitigation Project covering Arunachal Pradesh, Himachal Pradesh, Sikkim and Uttarakhand.
On a broader scale, despite improvements in glacier monitoring, only a small fraction of glaciers are closely studied, and data coverage remains uneven across the region.
Early warning systems for early action to save lives and protect livelihoods, property, assets and infrastructure must become a priority agenda, experts pointed out.