A glacial lake outburst flood (GLOF) can occur when a lake contained by a glacier (called Jökulhlaup if it was a subglacial lake, marginal lake drainage if it was dammed between ice and the ground) or a terminal moraine dam fails. This 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 if a large enough portion of a glacier breaks off and massively displaces the waters in a glacial lake at its base.
A glacial lake outburst flood is created 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 marginal lake, and a water body that is capped by the glacier is called a sub-glacial lake. When the former bursts it may also be called a marginal lake drainage, and when the latter bursts it may be called a jökulhlaup.
A jökulhlaup is thus a sub-glacial outburst flood. Jökulhlaup is an Icelandic term that has been adapted into the English language, and originally only referred 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 lakes come in various sizes, but may hold millions to
hundreds millions of cubic meters of water. Catastrophic failure of
the containing ice or glacial sediment can release this water over
a timespan of minutes to days. Peak flows as high as 15,000 cubic
meters per second have been recorded in these events, suggesting
that the v-shaped canyon of a normally small mountain stream could
suddenly develop an extremely turbulent and fast-moving torrent
some 50 meters deep. On a downstream floodplain, it suggests a
somewhat slower inundation spreading as much as 10 kilometers wide.
Both scenarios are horrific threats to lives, property and
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 GLOFs situations that have
been identified worldwide. The Tsho Rolpa glacier lake is located
in the Rolwaling Valley, about 110 kilometers (68 miles)
northeast of Kathmandu, Nepal, at an altitude of
4580 m (15,026 ft). The lake is dammed by a 150 m
(492 ft) 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 90 to 100 million m³ (117 to 130 million yd³) of
The most famous are the immense jökulhlaup released from the Vatnajökull Ice Cap in Iceland. It is not by chance that the term jökulhlaup comes from Icelandic, as the south of Iceland has very often been the victim of such catastrophes. This was the case in 1996, when the volcano under the Grímsvötn lakes belonging to the Vatnajökull glacier erupted, and the river Skeiðará flooded the land in front of Skaftafell National Park. The jökulhlaup reached a flow rate of 50,000 cubic meters 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 100-200 tons striking the Gigjukvisl Bridge of the Ring Road (the ruins are well marked with explanatory signs today as a popular tourist stop). The tidal wave released was up to 4 meters (over 13 feet) high and 600 meters (well over 1/3 mile) wide. The flood carried with it 185 million tons of silt. The jökulhlaup flow made it for several days the 2nd largest river (in terms of water flow) after the Amazon.
After the flooding, some icebergs 10 metres (33 ft) 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 3 cubic kilometres (0.72 cu mi) of ice and yielded an outburst of 6,000 cubic metres (7,800 cu yd) per second at peak flow.
Some jökulhlaups release annually. Lake George near the Knik River had large annual outbreaks from 1918 to 1966. Since 1966 the Knik Glacier has retreated and an ice-dam is no longer created. Lake George might resume annual floods if the glacier thickens again and blocks the valley (Post and Mayo, 1971). Almost every year, GLOFs occur in two locations in southeastern Alaska, one of which is Abyss Lake. The releases associated with the Tulsequah Glacier near Juneau often inundate a nearby airstrip. About 40 cabins could potentially be affected and a few have been damaged by the larger floods. Events from Salmon Glacier near Hyder have damaged roads near the Salmon River.
Immense prehistoric GLOFs, known as the Missoula Floods, occurred in North American's Columbia River watershed towards the end of the last ice age. They were the result of periodic breaches of ice dams in present day Montana, resulting in the draining of a body of water now known as Glacial Lake Missoula.
Glacial River Warren drained Glacial Lake Agassiz during the Wisconsinian glaciation, in whose bed the now mild Minnesota River flows. This river seasonably drained glacial meltwater into what is now the Upper Mississippi River. The region now termed the Driftless Area of North America was contemporaneously also subject to glacial outburst floods from Glacial Lake Grantsburg, and Glacial Lake Duluth during all three phase of the last ice age.
Between September 6 to 10, 2003, a GLOF occurred from Grasshopper Glacier in the Wind River Mountains, Wyoming. A proglacial lake at the head of the glacier burst through a glacial dam, and water from the lake carved a trench down the center of the glacier for more than 0.8 kilometres (0.5 mi). An estimated 2,460,000 cubic metres (650,000,000 US gal) gallons of water were released in four days, raising the flow level of Dinwoody Creek from 5.66 cubic metres (200 cu ft) per second to 25.4 cubic metres (900 cu ft) per second, as recorded at a gauging station 27 kilometres (17 mi) downstream. Debris from the flood was deposited more than 32 kilometres (20 mi) along the creek. The GLOF has been attributed to the rapid retreat of the glacier, which has been ongoing since the glacier was first accurately measured in the 1960s.
In 1994 a jökulhlaup occurred at Farrow Creek, British Columbia
In 2003 a jökulhlaup drained into Lake Tuborg on Ellesmere Island, and the events and its aftermath were monitored. The ice-dammed lake drained catastrophically by floating its ice dam. This is an extremely rare occurrence in the Canadian High Arctic, where most glaciers are cold based, and ice-dammed lakes typically drain slowly by overtopping their dams.
GLOFs occur with regularity in the valleys and low lying river plains of Bhutan. 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 90 kilometers upstream from Punakha Dzong caused massive flooding on the Pho Chhu River, damaging the dzong and causing casualties.
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.
Even though GLOF events have been occurring in Nepal for many decades, the Dig Tsho glacier outburst, which took place in 1985, 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 recent study done by ICIMOD and UNEP (UNEP, 2001) reported 27 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.
Thulagi glacier The Thulagi glacier, which is 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 KfW, Frankfurt, 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 that even assuming the worst case, a disastrous outburst of the lake can be excluded in the near future.
Longbasaba and Pida lakes are two moraine-dammed lakes at an altitude of about 5700 m in the Chinese 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 the Tibet Autonomous Region (HDTAR) 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.