Glacial Lake Outburst Floods
A glacial lake outburst flood — known to specialists as a GLOF and to Icelanders as a jokulhlaup — is what happens when a body of water held back by ice or by a moraine suddenly drains. The biggest historical events have moved more water in a few hours than entire rivers move in a year, with devastating downstream consequences.
How they form
There are three basic ways:
- Subglacial. Water trapped beneath a glacier — often heated by a volcano or by geothermal flow — overpressures and bursts out. The Icelandic jokulhlaup is the classic example.
- Ice-dammed. A glacier blocks a side valley, water builds up behind the dam, then either the dam floats free or the ice fails. Russell Fjord behind Hubbard Glacier is one example.
- Moraine-dammed. A loose terminal moraine holds back a proglacial lake. As the moraine erodes, slowly or suddenly, the lake drains.
Where they matter most
- Iceland. Vatnajokull events after volcanic eruptions; well monitored, low casualty risk now.
- Himalaya. Imja Tsho and dozens of other lakes pose serious risk to downstream villages.
- Andes. Cordillera Blanca in Peru lost thousands of lives to GLOFs in the 20th century.
- Karakoram. Ice-dammed lakes form and drain each year on Shimshal and Hopar systems.
Famous events
- 1996 Vatnajokull. Skeidararsandur flood after Gjalp eruption destroyed infrastructure but no fatalities.
- 1941 Huaraz, Peru. Glacial lake outburst killed 1 800 people.
- 2023 South Lhonak Tsho, India. GLOF destroyed a major hydropower dam and killed dozens.
Monitoring and mitigation
ICIMOD monitors over 25 000 glacial lakes in the Himalaya. Many at- risk lakes are partially drained by engineering — siphons, channels, controlled spillways — to reduce burst risk. New satellite mapping is revealing previously unknown lakes.
Visitor implications
Avoid camping immediately downstream of moraine-dammed lakes. Heed warning signs and ranger advice in known GLOF corridors. In Iceland, roads sometimes close after volcanic activity in anticipation of floods.
Explore on the map
The interactive map flags every known GLOF-risk lake worldwide. Filter by region for trekking planning.