Glacial lake outburst flood

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Hubbard Glacier, Alaska squeezes towards Gibert Point on May 20, 2002. The glacier is close to sealing off Russell Fjord at top from Disenchantment Bay at bottom.

A glacial lake outburst flood (GLOF), also known as a jökulhlaup (a term that originated in Icelandic and is still used in that language) (a jökulhlaup is technically a sudden and often catastrophic flood that occurs during a volcanic eruption, but is also used to describe other sorts of glacial flooding), can occur when a lake contained by a glacier 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, or if a large enough portion of a glacier breaks off and massively displaces the waters in a glacial lake at its base.

[edit] Definition

In this Hubbard Glacier image from July 16, 2002, the glacier has closed off Russell Fjord from Disenchantment Bay. The waters behind the glacier rose 61 ft (18.6 m) in 10 weeks, creating a short lived Russell Lake.

A Jökulhlaup is a glacier outburst flood, principally from an ice-dammed lake. Jökulhlaup is an Icelandic term that has been adapted into the English language, and originally only referred to a glacier outburst flood triggered by a volcanic eruption, but now is accepted to describe any abrupt release of water dammed by a glacier.

Jökulhlaups have also been known to occur when a volcano erupts under a glacier. The ice over the volcano melts because of the heat, causing water to form a lake under the remaining ice-cap. Then the ice-cap collapses or the water breaks through the barrier in front of it, and there follows a more or less disastrous flooding of the land below the mountain. These events are similar to a type of lahar.

[edit] Monitoring

The Hubbard Glacier is overwhelmed on August 14, 2002 in the second largest GLOF in historical times.

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.^[1]

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 water stored.^[2]

[edit] Examples

[edit] Iceland

Remains of a steel bridge in Skaftafell National Park after a glacial outburst

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 the Icelandic, as the south of Iceland has very often been victim to such catastrophes. This was the case when in 1996, 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 45,000 cubic meters per second, and destroyed parts of the Hringvegur (road no. 1). After the flooding some icebergs 10 m 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 Grimsvotn 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.

[edit] Alaska

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.^[3]

[edit] Contiguous United States

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 over a half mile (.8 km). An estimated 650 million gallons of water were released in four days, raising the flow level of Dinwoody Creek from 200 ft³ per second to 900 ft³ per second, as recorded at a gauging station 17 miles (27 km) downstream. Debris from the flood was deposited more than 20 miles 32 km. 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.^[4]

[edit] Canada

In 1978 debris flows triggered by a jökulhlaup from Cathedral Glacier destroyed part of the Canadian Pacific railway track, derailed a freight train and buried parts of the Trans Canada Highway ^[5]

In 1994 a jökulhlaup occurred at Farrow Creek, British Columbia^[6]

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.^[7]

It has been suggested^[8] that Heinrich events during last glaciation could have been caused by gigantic jökulhlaups from a Hudson Bay lake dammed by ice at the mouth of Hudson Strait.

[edit] Bhutan

GLOFs occur with regularity in the valleys and low lying river plains of Bhutan.^[9] 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.^[10] 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.^[10]

[edit] England/France

The English Channel 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.^[11]

[edit] References

[edit] Printed media

• Post, A. and L.R. Mayo (1971) Glacier Dammed Lakes and Outburst Floods in Alaska. HYDROLOGIC INVESTIGATIONS ATLAS HA-455. Anchorage, Alaska U.S. Geological Survey, Denver CO. [1]

[edit] World Wide Web

[edit] External links

• United Nations Environmental Programme. Glacial Lake Outburst Flood Monitoring and Early Warning System. Retrieved on 2006-07-11.
• U.S. Geological Survey. Mount Rainier Glacier Hazards and Glacial Outburst Floods. Retrieved on 2006-07-11.
• Devaris, Aimee. Southeast Alaska Jökulhlaups. NOAA. Retrieved on 2006-07-11.
• Oswald, Liz. Jokulhlaup at Grasshopper Glacier, Wind River Mountains, Wyoming. Watershed Management Council. Retrieved on 2006-07-11.
• Schwarz, Terence; Todd Walter. Small-scale glacial lake outburst flood, Lemon Glacier, Alaska. Retrieved on 2006-07-11.
• U.S. Geological Survey, Terence; Todd Walter. Enlarged Views of 2002 Russell Fiord Closure and Russell Lake Outburst. Retrieved on 2006-07-11.
• The channeled scabland: a guide to the geomorphology of the Columbia Basin, Washington : prepared for the Comparative Planetary Geology Field Conference held in the Columbia Basin, June 5-8, 1978 / sponsored by Planetary Geology Program, Office of Space Science, National Aeronautics and Space Administration ; edited by Victor R. Baker and Dag Nummedal.