A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a jökulhlaup. The dam can consist of glacier ice or a terminal moraine. Failure can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, volcanic eruptions under the ice, or massive displacement of water in a glacial lake when a large portion of an adjacent glacier collapses into it.
Increasing glacial melting because of climate change, alongside other environmental effects of climate change (i.e. permafrost melting) mean that regions with glaciers are likely to see increased flooding risks from GLOFs. This is especially true in the Himalayas where geologies are more active.
A 2023 study found 15 million people at risk from this hazard, mostly in China, India, Nepal, Pakistan, and Peru.New study finds 15 million people live under the threat of glacial floods
Definition
A glacial lake outburst flood is a type of outburst flood occurring when water dammed by a glacier or a moraine is released. A water body that is dammed by the front of a glacier is called a ice-marginal lake, and a water body that is capped by the glacier is called a sub-glacial lake. When a marginal lake bursts, it may also be called a marginal lake drainage. When a sub-glacial lake bursts, it may be called a jökulhlaup.
A jökulhlaup is thus a sub-glacial outburst flood. Jökulhlaup is an Icelandic term that has been adopted into the English language, originally referring only to glacial outburst floods from Vatnajökull, which are triggered by volcanic eruptions, but now is accepted to describe any abrupt and large release of sub-glacial water.
Glacial lake volumes vary, but may hold millions to hundreds of millions of cubic metres of water. Catastrophic failure of the containing ice or glacial sediment can release this water over periods of minutes to days. Peak flows as high as 15,000 cubic metres per second have been recorded in such events, suggesting that the v-shaped canyon of a normally small mountain stream could suddenly develop an extremely turbulent and fast-moving torrent some deep. Glacial Lake Outburst Floods are often compounded by a massive river bed erosion in the steep moraine valleys, as a result, the flood peaks increase as they flow downstream until the river reaches, where the sediment deposits. On a downstream floodplain, it suggests a somewhat slower inundation spreading as much as wide. Both scenarios are significant threats to life, property and infrastructure.
Monitoring The United Nations has a series of monitoring efforts to help prevent death and destruction in regions that are likely to experience these events. The importance of this situation has magnified over the past century due to increased populations, and the increasing number of glacial lakes that have developed due to glacier retreat. While all countries with glaciers are susceptible to this problem, central Asia, the Andes regions of South America and those countries in Europe that have glaciers in the Alps, have been identified as the regions at greatest risk.
There are a number of imminent deadly GLOF situations that have been identified worldwide. The Tsho Rolpa glacier lake is located in the Rolwaling Valley, about northeast of Kathmandu, Nepal, at an altitude of . The lake is dammed by a high unconsolidated terminal moraine dam. The lake is growing larger every year due to the melting and retreat of the Trakarding Glacier, and has become the largest and most dangerous glacier lake in Nepal, with approximately of water stored.
Examples Asia India
In June 2013, Kedarnath in Uttarakhand witnessed flash floods along with a GLOF caused by Chorabari Tal, killing thousands of pilgrims, tourists and residents who came to visit the place.
Pakistan Pakistan has more than 7000 glaciers, which is more than anywhere else in the world, except for the polar regions. As of 2018, more than 3,000 glacial lakes had formed in Gilgit-Baltistan, with 30 identified by the UNDP as posing an imminent threat of glacial lake outburst flooding. In 2017, a "Scaling up of Glacial Lake Outburst Flood Risk Reduction in Northern Pakistan Project" was continued. In 1929, a GLOF from the Chong Khumdan Glacier in the Karakoram caused flooding on the Indus River 1,200 km downstream (a maximum flood rise of 8.1 m at Attock).Hewitt, K. (1982) Natural dams and outburst floods of the Karakoram Himalaya
Bhutan
GLOFs occur with regularity in the valleys and low lying river plains of Bhutan. – Conference web site (NOAA), Abstract (pdf) In the recent past, flash floods have occurred in the Thimphu, Paro and Punankha-Wangdue valleys. Of the 2674 glacial lakes in Bhutan, 24 have been identified by a recent study as candidates for GLOFs in the near future. In October 1994, a GLOF upstream from Punakha Dzong caused massive flooding on the Pho Chhu River, damaging the dzong and causing casualties.
In 2001, scientists identified Lake Thorthormi as one that threatened imminent and catastrophic collapse. The situation was eventually relieved by carving a water channel from the lip of the lake to relieve water pressure.
Nepal
Even though GLOF events have been occurring in Nepal for many decades, the 1985 Dig Cho glacial lake outburst has triggered detailed study of this phenomenon. In 1996, the Water and Energy Commission Secretariat (WECS) of Nepal reported that five lakes were potentially dangerous, namely, Dig Tsho, Imja, Lower Barun, Tsho Rolpa, and Thulagi, all lying above 4100 m. A 2001 study done by ICIMOD and UNEP reported 20 potentially dangerous lakes in Nepal. In ten of them GLOF events have occurred in the past few years and some have been regenerating after the event. Additional dangerous glacial lakes may exist in parts of Tibet that are drained by streams crossing into Nepal, raising the possibility of outburst incidents in Tibet causing downstream damage in Nepal. The Gandaki River basin is reported to contain 1025 glaciers and 338 lakes.
The Thulagi glacier located in the Upper Marsyangdi River basin, is one out of the two moraine-dammed lakes (supra-glacial lakes), identified as a potentially dangerous lake. The Kreditanstalt für Wiederaufbau, Frankfurt am Main, the BGR (Federal Institute for Geosciences and Natural Resources, Germany), in cooperation with the Department of Hydrology and Meteorology in Kathmandu, have carried out studies on the Thulagi Glacier and have concluded in 2011 that even assuming the worst case, a disastrous outburst of the lake can be excluded in the near future.BGR/NLfB/GGA: Gletschersee Thulagi
Recent destructive glacial lake outburst floods in Nepal include the 2024 Thame flood, and two of the 2025 Nepal floods. (note article is marked as having AI assistance)
China Longbasaba and Pida lakes are two moraine-dammed lakes at an altitude of about in the Eastern Himalayas. Due to the rise of temperature, the areas of the Longbasaba and Kaer glaciers decreased by 8.7% and 16.6% from 1978 to 2005. Water from glaciers directly flowed into Longbasaba and Pida lakes, and the area of the two lakes increased by 140% and 194%. According to the report of the Hydrological Department of Tibet in 2006, if a GLOF had occurred at the two lakes, 23 towns and villages, where more than 12,500 people live, would have been endangered.
In Tibet, one of the major barley producing areas of the Tibetan Plateau was destroyed by GLOFs in August 2000. More than 10,000 homes, 98 bridges and dykes were destroyed and its estimated cost was about $75 million. The farming communities faced food shortages that year by losing their grain and livestock.WWF Nepal Program. "An Overview of Glaciers, Glacier Retreat, and Subsequent Impacts in Nepal, India and China". 14 March. 2005
A major GLOF was reported in 1978 in the valley of the Shaksgam River in the Karakoram, a part of historic Kashmir, ceded by Pakistan to China.
Europe Iceland The most famous are the immense jökulhlaup released from the Vatnajökull Ice Cap in Iceland. It is not by chance that the term jökulhlaup (jökull = glacier, hlaup = run (n.)/runningIcelandic online dictionary) comes from Icelandic, as the south of Iceland has frequently been the victim of such catastrophes. This was the case in 1996, when a volcano north of the Grímsvötn lake belonging to the Vatnajökull glacier erupted, filling Grímsvötn, and then the river Skeiðará flooded the land in front of Skaftafell, now part of Vatnajökull National Park. The jökulhlaup reached a flow rate of 50,000 cubic metres per second, and destroyed parts of the Hringvegur (Ring Road or Iceland Road #1). The flood carried ice floes that weighed up to 5000 tons with icebergs between 100 and 200 tons striking the Gigjukvisl Bridge of the Ring Road (the ruins are well marked with explanatory signs today as a popular tourist stop). The tsunami released was up to high and wide. The flood carried with it 185 million tons of silt.Benediktsson, Stefán; Helgadóttir, Sigrún. "The Skeiđarđá River in Full Flood 1996," Skaftafell National Park: Environment and Food Agency, UST, March 2007. The jökulhlaup flow made it for several days the second largest river (in terms of water flow) after the Amazon.
After the flooding, some icebergs high could be seen on the banks of the river where the glacier run had left them behind (see also Mýrdalsjökull). The peak water release from a lake that develops around the Grímsvötn Volcanic Crater in the center of the Vatnajökull ice cap generates flows that exceed the volume of the Mississippi River. The outbursts have occurred in 1954, 1960, 1965, 1972, 1976, 1982, 1983, 1986, 1991 and 1996. In 1996, the eruption melted of ice and yielded an outburst of per second at peak flow.
Strait of Dover The Strait of Dover is thought to have been created around 200,000 years ago by a catastrophic GLOF caused by the breaching of the Weald-Artois Anticline, which acted as a natural dam that held back a large lake in the Doggerland region, now submerged under the North Sea. The flood would have lasted several months, releasing as much as one million cubic metres of water per second. The cause of the breach is not known but may have been caused by an earthquake or simply the build-up of water pressure in the lake. As well as destroying the isthmus that connected Britain to continental Europe, the flood carved a large bedrock-floored valley down the length of the English Channel, leaving behind streamlined islands and longitudinal erosional grooves characteristic of catastrophic megaflood events."Catastrophic flooding origin of shelf valley systems in the English Channel". Sanjeev Gupta, Jenny S. Collier, Andy Palmer-Felgate & Graeme Potter. Nature 448, 342–345 (19 July 2007)
Swiss Alps