Changing Risks in the Himalayas: From Glacial Lakes to Glacier Collapse

Kathmandu, Nepal: The Himalayas have long been regarded as one of Nepal’s greatest natural assets, providing water for millions of people, supporting agriculture and hydropower, sustaining tourism and shaping the country’s cultural and economic identity. But the mountains are no longer as stable as they once appeared. Rapid changes in glaciers, the expansion and formation of glacial lakes, unstable mountain slopes, avalanches, landslides and increasingly unpredictable weather are creating a new and more complex pattern of disaster risk. The devastating disaster in Rasuwa on August 26 has brought that changing reality into sharp focus, showing that Nepal’s Himalayan hazards extend far beyond the risk of glacial lake outburst floods.

The Rasuwa disaster was particularly significant because the available scientific assessments do not indicate that it was a conventional glacial lake outburst flood. Preliminary evidence points instead to the collapse of a large mass of glacier ice and rock in the Himalayan region near the Nepal-China border, which generated a powerful debris flow and sent water, mud, ice, rocks and other material downstream. The sudden release of this material caused catastrophic flooding along the river system, destroying homes, roads, bridges and infrastructure and leaving a large number of people dead or missing. By August 29, Nepalese authorities reported at least 669 deaths in Nepal and seven in Tibet, while nearly 3,000 people remained missing. More than 3,700 people had been rescued, although difficult terrain and continuing risks were complicating the search.

The scale of the disaster has also demonstrated how quickly a high-altitude geological event can become a humanitarian crisis far downstream. Communities located nowhere near the glacier itself were suddenly exposed to a destructive torrent carrying enormous quantities of rock, mud and debris. Roads and bridges that normally connect remote settlements were washed away, while hydropower facilities and other infrastructure were damaged. The disaster has also raised concerns about the vulnerability of workers at hydropower projects, people travelling through the Himalayan border corridor and communities whose livelihoods depend on roads, tourism and river systems. The government has estimated that reconstruction could cost between US$4 billion and US$5 billion, underlining the economic consequences of mountain disasters in addition to their human cost.

The human consequences are more profound than the statistics alone suggest. Behind every missing-person figure is a family waiting for information about a father, mother, son, daughter, relative or friend. Survivors have been forced to leave their homes with little more than the clothes they were wearing, while relatives have crowded hospitals and help centres searching for information about loved ones. In some of the worst-affected areas, communications and transportation infrastructure were destroyed at the same time, making it difficult for families to establish contact. The geography of the Himalayas further complicates rescue operations because narrow valleys, steep slopes, damaged roads and unstable terrain can prevent rescuers from reaching affected communities quickly.

The Rasuwa disaster therefore requires Nepal to reconsider the way it defines Himalayan risk. For many years, discussions about climate-related hazards in the mountains have focused heavily on glacial lake outburst floods, or GLOFs. Those risks remain serious, but the latest disaster demonstrates that a glacier itself can become the source of a catastrophic event. A glacier can partially collapse, an ice-rock mass can fall into a river, a landslide can temporarily block a river, or a sudden release of water and debris can trigger further flooding downstream. These processes can interact with one another, producing what scientists increasingly describe as cascading or compound hazards.

Nepal already has strong evidence that such complex events are possible. The August 2024 disaster in Thame, in the Everest region, was initially understood primarily as a glacial lake outburst flood. A detailed ICIMOD study published in 2025, however, showed that the event involved a much more complicated chain reaction. A large rock avalanche struck a glacial lake at high altitude, generating a displacement wave and causing the lake to breach. The resulting flood then affected another lake, producing a second release of water and greatly increasing the destructive power of the downstream flow. The study estimated that the combined event released about 459,000 cubic metres of water and debris, destroying much of the settlement of Thame and displacing 135 people.

The lesson from Thame is particularly relevant to Rasuwa. Himalayan hazards should not always be examined as isolated events. A glacier, glacial lake, rock slope and river are physically connected. A change in one part of the system can trigger a chain of consequences elsewhere. This means that simply preparing a list of dangerous glacial lakes is not enough. Nepal also needs to monitor glacier movement, unstable slopes, rock formations, river channels, snow and ice conditions and potential natural dams. The focus of disaster management must gradually shift from individual hazards toward the entire mountain system.

Nepal’s concerns about glacial lakes are nevertheless well founded. A joint inventory by the International Centre for Integrated Mountain Development (ICIMOD) and the United Nations Development Programme (UNDP) identified 3,624 glacial lakes across the Koshi, Gandaki and Karnali river basins in Nepal, Tibet in China and India. Of these, 47 were classified as potentially dangerous, including 21 in Nepal, 25 in China and one in India. Forty-two of the 47 potentially dangerous lakes were located in the Koshi basin.

The figure of 21, however, needs to be presented carefully. It does not mean that 21 glacial lakes in Nepal are about to burst or that all 21 carry the same degree of immediate danger. They were identified as potentially dangerous based on scientific assessment and therefore require monitoring and risk-reduction measures. Using the phrase “21 potentially dangerous glacial lakes” is more accurate than describing them as “21 extremely high-risk lakes.” Such precision is particularly important because inaccurate or sensational reporting can create public fear without improving public understanding.

A glacial lake by itself is not a disaster. Many glacial lakes form naturally as glaciers retreat, and their existence does not automatically mean that an outburst will occur. The danger becomes greater when a lake expands rapidly, when the natural moraine dam holding it becomes unstable, or when an avalanche, rockfall or other mass enters the lake and displaces a large volume of water. If the natural dam fails, water can move rapidly downstream, carrying rocks, mud, ice and other debris. Communities many kilometres away can then face devastating consequences within a very short period.

Nepal has experienced glacial lake outburst floods before, and the risks are not theoretical. Scientific records document numerous GLOF events in the country, while glacial lakes such as Tsho Rolpa have been the subject of long-term monitoring and risk-reduction efforts. The experience of Tsho Rolpa has demonstrated that human intervention can reduce risk, but it has also shown that mountain environments cannot be treated as static. Conditions change continuously, requiring long-term observation rather than one-time intervention.

The changing condition of the Himalayan cryosphere provides an important background to these risks. ICIMOD has reported that glaciers across the Hindu Kush Himalaya lost ice at a rate 65 percent faster during 2011–2020 than during the previous decade. The organization has repeatedly warned that accelerating glacier loss is affecting water systems and increasing the vulnerability of mountain communities.

Climate change, however, must be discussed with scientific caution. It would be inaccurate to claim that climate change directly caused the Rasuwa disaster unless scientific investigations establish such a causal relationship. The more appropriate conclusion is that rising temperatures, glacier retreat, changing snow and ice conditions and other effects of a warming climate are altering the physical environment in which Himalayan hazards occur. Climate change can increase or modify certain risks, but determining the precise cause of an individual glacier collapse or flood requires detailed geological, hydrological and climatic analysis.

This distinction is particularly important for journalism. There is a substantial difference between saying that “climate change caused the disaster” and saying that “climate change may be increasing the conditions under which such hazards occur.” The first is a definitive causal claim, while the second reflects scientific uncertainty. Responsible reporting should not turn a scientific possibility into an established fact merely because it makes a stronger headline.

At the same time, there is little doubt that the changing Himalayan environment requires stronger preparedness. In July 2025, the Green Climate Fund approved a US$36.1 million grant to help Nepal reduce the threat posed by glacial lake outburst floods. The programme is intended to strengthen early-warning systems, reduce water levels in selected dangerous glacial lakes and protect vulnerable mountain communities. The project focuses on four lakes—Thulagi, Lower Barun, Lumding Tsho and Hongu-2—and is expected to benefit more than 2.2 million people.

Such investment represents an important shift from responding to disasters after they occur toward preventing or reducing their impact before they happen. But the Rasuwa disaster suggests that Nepal needs to broaden this approach. A programme focused primarily on glacial lakes cannot fully address hazards arising from glacier collapse, rock avalanches, unstable slopes and cascading events. Future investment should therefore integrate glacial-lake monitoring with wider cryosphere and mountain-slope monitoring.

Early-warning systems will be particularly important, but their effectiveness depends on what they are designed to detect. A conventional river-level warning system may provide valuable time during a rainfall-induced flood, but it may offer little or no warning when an ice-rock avalanche suddenly enters a river upstream. Nepal therefore needs a more comprehensive mountain monitoring system that combines satellite imagery, remote sensing, glacier observation, weather stations, river gauges, ground sensors and field assessments.

Technology, however, is only one part of an effective warning system. The warning must reach people, and people must understand what to do when they receive it. A siren is of little value if residents do not know where to evacuate. An emergency message is ineffective if roads to the designated safe area have already been destroyed. A warning system also needs to consider children, elderly people, persons with disabilities and people who may be alone when an emergency occurs.

For this reason, local communities should be treated as a central part of Nepal’s disaster-management system rather than simply as recipients of government warnings. Vulnerable settlements should identify safe areas, maintain evacuation routes and conduct regular preparedness exercises. Local residents can also be trained in first aid, emergency communication, search and rescue and basic disaster assessment. In remote mountain areas, local people are often the first responders because outside rescue teams may need many hours to arrive.

The Rasuwa disaster also demonstrates that Himalayan disaster management cannot be confined within national borders. Many of Nepal’s major river systems originate in high-altitude areas connected to Tibet. A glacier, lake or river event on the Chinese side can therefore have consequences downstream in Nepal. Scientific information about changing glaciers and lakes needs to be shared across the border, while emergency communication mechanisms should allow potentially dangerous developments to be communicated quickly.

Cross-border cooperation should not be viewed primarily through the lens of political relations. It is a matter of human safety. Rivers do not recognize national boundaries, and neither do floods, avalanches or debris flows. Nepal and China have a shared interest in reducing the loss of life and damage to infrastructure caused by mountain hazards. Regular scientific information exchange, joint assessments and emergency communication could significantly improve preparedness.

The disaster also raises broader questions about Nepal’s development model. Mountain roads, bridges and hydropower projects are essential for economic development, but they are frequently constructed in valleys and along rivers where the consequences of extreme events can be severe. Development planning must therefore account for future hazards rather than relying only on historical experience. A bridge that has survived decades of normal floods may still be vulnerable to an extreme debris flow. A hydropower project may be exposed to a hazard originating many kilometres upstream.

Risk mapping should consequently become an essential component of major infrastructure planning. Before approving a road, bridge, hydropower project or settlement in a vulnerable mountain corridor, authorities should consider the possibility of extreme flooding, landslides, avalanches, glacier collapse and glacial lake outburst floods. The question should not simply be whether infrastructure can be built, but whether it can remain safe under changing environmental conditions.

The economic consequences of failing to make that assessment can be enormous. The latest Rasuwa disaster has already damaged critical infrastructure and disrupted transport and energy systems. Nepal’s Finance Minister has estimated that reconstruction could require US$4 billion to US$5 billion—an amount equivalent to a substantial share of the national economy. Reuters reported that hydropower projects representing more than 12 percent of Nepal’s energy capacity were affected.

The impact also extends beyond infrastructure. Mountain tourism, trekking, pilgrimage and border trade depend heavily on roads, bridges and safe access. When a major disaster destroys a transport corridor, the economic effects spread from construction workers and farmers to hotel owners, guides, porters, transport operators and traders. Disaster preparedness is therefore not merely about saving lives during an emergency; it is also about protecting livelihoods and the wider economy.

Health risks can become equally serious after the floodwater disappears. When homes and water systems are destroyed, access to safe drinking water and sanitation can be disrupted. Damaged health posts and difficult transportation can prevent people from receiving essential treatment. Displaced families may also face increased exposure to infectious diseases, malnutrition and psychological trauma. Disaster preparedness must therefore include emergency health services, safe water, sanitation, medicines and long-term support for displaced communities.

The Rasuwa disaster should also encourage Nepal to reconsider how it defines recovery. Rebuilding the same road, bridge or settlement in exactly the same vulnerable location may restore connectivity, but it does not necessarily reduce future risk. Reconstruction should be an opportunity to build more resilient infrastructure, relocate particularly exposed facilities where necessary and incorporate updated hazard information into planning.

Nepal has already demonstrated that risk reduction is possible. Work on Tsho Rolpa, Imja and other glacial lakes, the development of early-warning systems and the new international financing for GLOF risk reduction all show that the country has both experience and institutional capacity. The challenge is to move from individual projects toward a coordinated national system that considers the Himalayas as a connected and changing environment.

The most important lesson from Rasuwa is therefore not that Nepal has 21 potentially dangerous glacial lakes. Nor is it simply that climate change is melting the Himalayas. The deeper lesson is that the nature of Himalayan risk is becoming increasingly complex. A glacier can collapse. A rock avalanche can trigger a flood. A flood can damage a bridge. A damaged bridge can isolate communities. A destroyed road can delay rescue. A disaster that begins high in the mountains can therefore develop into a national humanitarian and economic crisis within hours.

Nepal cannot prevent every natural process in the Himalayas. It cannot stop every glacier from retreating, prevent every avalanche or control every unstable rock slope. But it can invest in science, monitoring, early warning, safer infrastructure and stronger communities. It can improve coordination among government agencies and scientific institutions. It can strengthen cooperation with neighbouring countries. And it can ensure that development decisions are based on the risks that Nepal is likely to face in the future, rather than only the disasters it has already experienced.

The tragedy in Rasuwa should therefore become a turning point in Nepal’s approach to Himalayan disaster management. The country needs to move beyond the question of whether a particular glacial lake will burst and begin asking a much broader question: How is the entire Himalayan system changing, and what does that change mean for the people living below it?

The answer will require more than emergency rescue operations after each disaster. It will require continuous scientific observation, responsible journalism, community preparedness, cross-border cooperation and political commitment.

Nepal may never be able to predict exactly when the next Himalayan disaster will occur. But it can increasingly identify where the danger is growing, understand how different hazards interact and take measures to reduce the consequences.

The mountains may be changing faster than Nepal is accustomed to. The country’s disaster-management system must now change with them.