In simple terms
A glacial lake outburst flood is the sudden release of water held in a lake fed by a melting glacier. The UN Office for Disaster Risk Reduction defines it as a sudden release of a significant amount of water retained in a glacial lake, whatever the cause.
The short form is GLOF. The water can arrive in a valley within minutes, far downstream from any ice a person can see.
How it works
As a glacier retreats, meltwater collects in the hollow it leaves behind. ICIMOD describes lakes forming between the shrinking ice and the ridge of rock debris, called a moraine, that the glacier pushed ahead of it.
The dam holding that water can be loose moraine, glacier ice or solid bedrock. Moraine and ice dams are the weak ones.
A dam fails when the source glacier breaks up, when a landslide or ice avalanche drops into the lake and sends a wave over the rim, when water seeps through the dam, or when a rising lake simply overtops it. Earthquakes can set off the same collapse. In Alaska the US Geological Survey describes a different route: water pressure opens a drain under the glacier itself.
Peak flows often run several times higher than an ordinary flood. The water tears out rock and soil as it goes, so a GLOF frequently becomes a debris flow.
Why it matters
Mountain valleys downstream hold villages, roads, bridges and hydropower plants. One pass of a GLOF can remove them, hours before anyone knows a lake has burst.
The exposure is large. An ICIMOD assessment in 2018 mapped more than 25,000 glacial lakes across five river basins of the Hindu Kush Himalaya, and a 2020 inventory listed 47 as potentially dangerous. Satellites, drones and river sensors are the main way these lakes are now watched.
Where you’ll see it
- Flood warnings in Nepal, Bhutan, Pakistan and northern India
- Alaska, where the Mendenhall River flooded in August 2023 with no heavy rain
- Satellite monitoring programs that track glacial lakes as they grow
- Road, bridge and hydropower design in high mountain countries
Example
A block of ice falls into a moraine-dammed lake, the wave washes over the rim, and the loose dam cuts through within the hour, sending water and boulders down the valley.
Often confused with
A GLOF is not the same as ordinary meltwater flooding. Seasonal melt swells a river over weeks. A GLOF empties a stored lake at once.
Key facts
- UNDRR defines a GLOF as a sudden release of a significant amount of water retained in a glacial lake, whatever the cause.1
- Glacial lakes can be held back by moraine, glacier ice or bedrock, and those dams fail through overtopping, seepage, landslides or breakup of the source glacier.2
- ICIMOD mapped more than 25,000 glacial lakes across five river basins of the Hindu Kush Himalaya in 2018, and a 2020 inventory listed 47 as potentially dangerous.2
- GLOF peak discharges often run several times above ordinary flood maximums and carry exceptional erosive power, frequently triggering debris flows.1
- A USGS study of 120 glacier-dammed Alaskan lakes counted 1,150 drainage events since 1985, with no change in how often they occur.3
Related concepts
In the news
Quick checkWhat holds back the water in a glacial lake before an outburst flood?Show answer
A dam of loose moraine debris, glacier ice or bedrock. The moraine and ice dams are the ones that give way.
Sources
- United Nations Office for Disaster Risk Reduction. Glacial Lake Outburst Flooding (MH0607). 7 June 2023 (accessed 17 September 2026)
- International Centre for Integrated Mountain Development. Glacial Lake Outburst Floods. 2 July 2021 (accessed 17 September 2026)
- U.S. Geological Survey. An Increased Understanding of Alaska and its Glacial Lake Outburst Floods. Undated (accessed 17 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 17, 2026Researched and drafted with AI assistanceReport an issue