Glacial Lake Outburst Flood (GLOF)

A Glacial Lake Outburst Flood (GLOF) is a sudden, very powerful flood that happens when water held in a lake near a glacier suddenly breaks out. These lakes are often held back by weak walls of loose rock and ice. When the wall breaks, millions of cubic metres of water rush down the valley within minutes or hours, carrying rocks, mud and trees. A GLOF can travel tens of kilometres and destroy villages, roads, bridges and dams far below, often with very little warning.

Why do glacial lakes form?

A glacier is a huge, slow-moving river of ice. As the climate warms, glaciers melt and shrink back up the mountain (this is called glacier retreat). The meltwater collects in hollows in front of, on top of, or beside the glacier. Over time these pools grow into lakes. So, as glaciers melt faster, more glacial lakes form and existing lakes grow bigger. This is why GLOFs are closely linked to climate change.

What holds the water back?

Glacial lakes are classified by what blocks them in:

• Moraine-dammed lakes: A moraine is a heap of loose rocks, gravel and soil that a glacier pushes and leaves behind. Many Himalayan lakes are held back by such moraines. They are the most dangerous type, because loose material can fail easily.
• Ice-dammed lakes: Held back by the glacier's own ice. If the ice cracks or floats up, the water escapes.
• Bedrock-dammed lakes: Held in hollows of solid rock. These are usually more stable.
• Supraglacial lakes: Pools on the surface of the glacier itself. They can join together and grow over time.

What triggers a GLOF?

A lake can sit quietly for years. A trigger is needed to make it burst. Common triggers are:

1. Avalanche or landslide into the lake: Ice or rock falls into the water and pushes a huge wave over the dam, like a stone thrown into a full bucket. This is what happened at South Lhonak in 2023.
2. Heavy rain or fast snowmelt: The lake fills too quickly and water flows over the top (overtopping), cutting through the dam.
3. Piping or seepage: Water slowly leaks through the loose moraine, washing away material from inside until the dam collapses.
4. Melting of buried ice: Many moraine walls have ice inside them. When it melts, the wall weakens.
5. Earthquakes: Shaking can crack the dam or cause landslides into the lake.

How does a GLOF cause so much damage?

Once the dam breaks, water rushes out at great speed. It picks up boulders, mud and trees, turning into a thick, destructive debris flow. Narrow Himalayan valleys squeeze the water, so it rises very high. Settlements, roads and hydropower projects are often built right next to rivers in these valleys, so they lie directly in the path.

Where is the risk in India?

India's GLOF risk is in the Himalaya: Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh. As of 2024, about 7,500 glacial lakes had been surveyed in the Indian Himalaya. A 2023 global study found that about 15 million people worldwide live in areas that could be hit by GLOFs. High Mountain Asia (the Himalaya and nearby ranges) has the highest danger, with about 9.3 million people exposed. India and Pakistan together account for about 5 million exposed people, around one-third of the world's total.

Major Indian GLOF events

• Kedarnath, Uttarakhand (June 2013): Very heavy rain (about 325 mm in 24 hours) and an avalanche caused the moraine-dammed Chorabari lake (also called Gandhi Sarovar), about 2 km above Kedarnath, to breach on 17 June 2013. Together with extreme floods, it devastated Kedarnath and the Mandakini valley.
• South Lhonak, Sikkim (October 2023): South Lhonak is a moraine-dammed lake at about 5,200 metres in North Sikkim, in the Teesta basin. On the night of 3–4 October 2023, a large landslide into the lake caused it to burst. The flood washed away the 60-metre-tall concrete dam of the 1,200 MW Teesta III project at Chungthang and damaged towns downstream.

South Lhonak was known to be dangerous for years. In 2016, the Sikkim State Disaster Management Authority and the state's science and technology department siphoned water out of it using three long HDPE pipes (strong plastic pipes) to lower the water level. This shows that risk reduction is possible, but needs to be done at a much larger scale.

How is India managing GLOF risk?

• NDMA Guidelines on Management of GLOFs (October 2020): Prepared by the NDMA with the help of the Swiss Agency for Development and Cooperation (SDC). They lay out how to identify and map dangerous lakes, take structural steps to prevent a sudden breach, and set up warning and evacuation systems.
• Monitoring by CWC and NRSC: The CWC (under the Ministry of Jal Shakti) monitors glacial lakes and water bodies using satellite images, especially during the June–October season. The NRSC (part of ISRO) prepares inventories of glacial lakes.
• National GLOF Risk Mitigation Programme (NGRMP): A ₹150 crore programme run by the NDMA, approved by the Ministry of Home Affairs in 2024. It focuses on four states (Sikkim, Himachal Pradesh, Uttarakhand, Arunachal Pradesh) and also includes J&K and Ladakh. The NDMA has identified about 189 high-risk lakes for action. The programme's four parts are: hazard and risk assessment of lakes, monitoring and early warning, mitigation (such as lowering lake water levels), and community awareness and preparedness. Expert teams have been sent to high-risk lakes to study them on the spot.
• Checking dams for GLOF risk: After the Teesta III loss, the Central Electricity Authority (the government's main technical body for the power sector) found 47 dams that could be hit by GLOFs: 38 already working and 9 under construction (Ministry of Power, August 2024). The CWC is now checking whether these dams can safely pass a GLOF on top of a big flood. GLOF studies are now required for new dams planned in glacial lake areas.

What tools reduce the risk?

• Lowering the lake level: Siphoning through pipes, or digging a controlled outlet channel, so there is less water to escape.
• Early warning systems (EWS): Sensors, cameras and automatic weather stations at the lake that send alerts to people downstream when water levels change suddenly.
• Strengthening the moraine dam and making dams and bridges downstream able to handle big floods.
• Land-use rules: Keeping new houses and projects away from the riskiest stretches of river.
• Community preparedness: Evacuation drills, safe high-ground shelters and local warning chains.

Commonly confused concepts

• GLOF vs cloudburst: A cloudburst is extreme rainfall in a small area in a short time (the India Meteorological Department uses about 100 mm in an hour over a small area). A GLOF is the sudden release of stored lake water. A cloudburst can trigger a GLOF, but they are different events.
• GLOF vs flash flood: A flash flood is any flood that rises very fast, usually within a few hours. A GLOF is one cause of flash floods; heavy rain is another.
• GLOF vs rock-ice avalanche (Chamoli 2021): The 7 February 2021 Chamoli disaster in Uttarakhand was first thought to be a GLOF. Scientists later found it was caused by a massive rock and ice avalanche (about 27 million cubic metres) from Ronti peak, which turned into a debris flow. No glacial lake burst.
• Landslide lake outburst flood (LLOF) vs GLOF: An LLOF happens when a landslide blocks a river and the temporary lake later bursts. A GLOF involves a glacial lake.
• Moraine vs glacier: A glacier is moving ice. A moraine is the pile of rock and soil the glacier leaves behind.

Issues, criticism and the way forward

• Too many lakes, too few sensors: Tens of thousands of glacial lakes exist, but only a small share have on-site monitoring. Many lakes are at heights above 4,500 metres and are hard to reach.
• Short warning time: A GLOF can reach villages within minutes to hours. Warning systems must work instantly and reach every household, including through mobile alerts and sirens.
• Hydropower and roads in risky valleys: Many dams and highways are built in narrow Himalayan valleys. Experts warn that dam safety and GLOF risk must be studied before approval, as the Teesta III loss showed. As of August 2024, 47 dams in India were known to be at GLOF risk.
• Climate change makes it worse: As warming continues, glaciers will keep retreating and new lakes will form, so risk will keep rising.
• Cross-border lakes: Some risky lakes lie in Nepal or the Tibet region of China, upstream of India. Cooperation and data-sharing with neighbours is needed.
• Way forward: Experts recommend faster use of sanctioned funds, real-time monitoring of all high-risk lakes, early warning that reaches the last mile, hazard-zone building rules, a national GLOF database, use of artificial intelligence and satellites, more research spending, and close teamwork between the Centre, states and district administrations.

Concepts to Know

• Glacier retreat: When a glacier melts faster than snow can add new ice, so its front edge moves back up the valley.
• Hindu Kush Himalaya (HKH): The mountain region stretching across eight countries: Afghanistan, Pakistan, India, Nepal, China, Bhutan, Bangladesh and Myanmar. It holds the largest amount of ice outside the polar regions and feeds major rivers like the Indus, Ganga and Brahmaputra.
• Debris flow: A fast-moving mix of water, mud, rocks and trees that behaves like wet concrete and is far more destructive than clean floodwater.
• Hectare: A unit of area. One hectare is 10,000 square metres, roughly the size of a large football ground.
• Automatic weather station (AWS): A machine that measures rain, temperature, wind and similar data by itself and sends it out regularly without a person on site.