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In geomorphology, an outburst flood—a type of megaflood—is a high-magnitude, low-frequency catastrophic
flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
involving the sudden release of a large quantity of water. During the last deglaciation, numerous
glacial lake outburst flood 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 j ...
s were caused by the collapse of either ice sheets or glaciers that formed the dams of
proglacial lake In geology, a proglacial lake is a lake formed either by the damming action of a moraine during the retreat of a melting glacier, a glacial ice dam, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around th ...
s. Examples of older outburst floods are known from the geological past of the Earth and inferred from geomorphological evidence on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
.
Landslides Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...
, lahars, and
volcanic dam A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those create ...
s can also block rivers and create lakes, which trigger such floods when the rock or earthen barrier collapses or is eroded. Lakes also form behind glacial moraines or ice dams, which can collapse and create outburst floods.


Definition and classification

Megafloods are paleofloods (past floods) that involved rates of water flow larger than those in the historical record. They are studied through the sedimentary deposits and the erosional and constructional landforms that individual megafloods have created. Floods that are known to us through historical descriptions are mostly related to meteorological events, such as heavy rains, rapid melting of snowpacks, or combination of these. In the geological past of the Earth, however, geological research has shown that much larger events have occurred. In the case of outburst floods, such floods are typically linked to the collapse of a barrier which formed a lake. They fall in the following classification according to the mechanism responsible: *Collapse of glacier dams that impound proglacial lakes (
Missoula Floods The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the las ...
). *Rapid erosion, melting of ice sheets (
jökulhlaup A jökulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst flood ...
s). *Collapse of earthen barriers (landslides or glacial moraines). *Collapse of
volcanic dam A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those create ...
s created by lava flows, lahars, or pyroclastic flows. *Overtopping of earthen or rock barriers **Lake overtopping (e.g.,
Lake Bonneville Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. It was a pluvial lake that formed in response to an increase in precipitation and a decrease in evaporation as a result of cooler temperature ...
). **Ocean spilling over a dividing ridge into a landlocked basin (e.g., Zanclean flood and Black Sea flood). A smaller scale example would be the Pantai Remis landslide.


Examples

Examples where evidence for large ancient water flows has been documented or is under scrutiny include:


Overflow of lakes formed by landslides

An example is the lake overflow that caused one of the worst landslide-related disasters in history on June 10, 1786. A landslide dam on Sichuan's
Dadu River The Dadu River (), known in Tibetan as the Gyelmo Ngul Chu, is a major river located primarily in Sichuan province, southwestern China. The Dadu flows from the eastern Tibetan Plateau into the Sichuan Basin where it joins with the Min River, a t ...
, created by an earthquake ten days earlier, burst and caused a flood that extended downstream and killed 100,000 people.


Postglacial rebound

Postglacial rebound changes the tilt of ground. In lakes, this means that shores sink in the direction farther away from the former maximum depth of ice. When the lake rests against an
esker An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Ame ...
, water pressure increases with the increased depth. The esker may then fail under the load and burst, creating a new outflow.
Lake Pielinen Pielinen ( sv, Pielisjärvi) is the fourth largest lake of Finland, with a drainage basin area of equally distributed between eastern Finland and Russia. The creation of the lake and its outlet is attributed to a post-glacial isostatic rebound, w ...
in Finland is an example of this.


Tectonic basins


The Black Sea (around 7,600 years ago)

A rising sea flood, the proposed and much-discussed refilling of the freshwater glacial
Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...
with water from the Aegean, has been described as "a violent rush of salt water into a depressed fresh-water lake in a single catastrophe that has been the inspiration for the flood mythology" (Ryan and Pitman, 1998). The marine incursion, caused by the rising level of the Mediterranean, apparently occurred around 7,600 years ago. It remains an active subject of debate among geologists, with subsequent evidence discovered to both support and refute the existence of the flood, while the theory that it is the basis of later flood myths is not proven.


Persian Gulf Flood (24,000 to 14,000 years ago, or 12000 to 10000 years ago)

Flooding of this area scattered peoples to both sides of the gulf depression. It was an area fed by four rivers. Rose calls it the "Gulf Oasis" which may have been a demographic refuge fed by the Tigris, Euphrates, Karun, and Wadi Batin rivers. It was suggested to be an area of freshwater springs and rivers.


Glacial floods in North America (8,000 to 15,000 years ago)

In North America, during glacial maximum, there were no
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lak ...
as we know them, but "proglacial" (ice-frontage) lakes formed and shifted. They lay in the areas of the modern lakes, but their drainage sometimes passed south, into the Mississippi system; sometimes into the Arctic, or east into the Atlantic. The most famous of these proglacial lakes was
Lake Agassiz Lake Agassiz was a large glacial lake in central North America. Fed by glacial meltwater at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined. First postulated in 1823 by William H. Keating, i ...
. As ice-dam configurations failed, a series of great floods were released from Lake Agassiz, resulting in massive pulses of freshwater added to the world's oceans. The
Missoula Floods The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the las ...
of
Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...
and Washington states were also caused by breaking ice dams, resulting in the
Channeled Scablands The Channeled Scablands are a relatively barren and soil-free region of interconnected relict and dry flood channels, coulees and cataracts eroded into Palouse loess and the typically flat-lying basalt flows that remain after cataclysmic floods ...
.
Lake Bonneville Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. It was a pluvial lake that formed in response to an increase in precipitation and a decrease in evaporation as a result of cooler temperature ...
, a
pluvial lake A pluvial lake is a body of water that accumulated in a basin because of a greater moisture availability resulting from changes in temperature and/or precipitation. These intervals of greater moisture availability are not always contemporaneous ...
, burst catastrophically in the
Bonneville Flood The Bonneville flood was a catastrophic flooding event in the last ice age, which involved massive amounts of water inundating parts of southern Idaho and eastern Washington along the course of the Snake River. Unlike the Missoula Floods, whic ...
about 14,500 years ago, due to its water overflowing and washing away a sill composed of two opposing
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s which had blocked a gorge. Lake Bonneville was not a glacial lake, but glacial age climate change determined the lake level and its overflow. The first scientific report of a megaflood (Gilbert, 1890) describes this event. The last of the North American proglacial lakes, north of the present Great Lakes, has been designated Glacial Lake Ojibway by geologists. It reached its largest volume around 8,500 years ago, when joined with Lake Agassiz. But its outlet was blocked by the great wall of the glaciers and it drained by tributaries, into the Ottawa and
St. Lawrence River The St. Lawrence River (french: Fleuve Saint-Laurent, ) is a large river in the middle latitudes of North America. Its headwaters begin flowing from Lake Ontario in a (roughly) northeasterly direction, into the Gulf of St. Lawrence, connecting ...
s far to the south. About 8,300 to 7,700 years ago, the melting ice dam over Hudson Bay's southernmost extension narrowed to the point where pressure and its buoyancy lifted it free, and the ice-dam failed catastrophically. Lake Ojibway's beach terraces show that it was above sea level. The volume of Lake Ojibway is commonly estimated to have been about , more than enough water to cover a flattened-out Antarctica with a sheet of water deep. That volume was added to the world's oceans in a matter of months. The detailed timing and rates of change after the onset of melting of the great ice-sheets are subjects of continuing study.


The Caspian and Black Seas (around 16,000 years ago)

A theory proposed by Andrey Tchepalyga of the
Russian Academy of Sciences The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíyskaya akadémiya naúk'') consists of the national academy of Russia; a network of scientific research institutes from across ...
dates the flooding of the Black Sea basin to an earlier time and from a different cause. According to Tchepalyga,
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
beginning from about 16,000 BP caused the melting of the Scandinavia Ice Sheet, resulting in massive river discharge that flowed into the
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia ...
, raising it to as much as above normal present-day levels. The Sea of Azov rose so high that it overflowed into the Caspian Sea. The rise was extremely rapid and the Caspian basin could not contain all the floodwater, which flowed from the northwest coastline of the Caspian Sea, through the Kuma-Manych Depression and
Kerch Strait The Kerch Strait, uk, Керченська протока, crh, Keriç boğazı, ady, Хы ТӀуалэ is a strait in Eastern Europe. It connects the Black Sea and the Sea of Azov, separating the Kerch Peninsula of Crimea in the west f ...
into the Black Sea basin. By the end of the Pleistocene this would have raised the level of the Black Sea by some below its present-day level, flooding large areas that were formerly available for settlement or hunting. Tchepalyga suggests this may have formed the basis for legends of the great
Deluge A deluge is a large downpour of rain, often a flood. The Deluge refers to the flood narrative in the Biblical book of Genesis. Deluge may also refer to: History *Deluge (history), the Swedish and Russian invasion of the Polish-Lithuanian Com ...
.


Red Sea floods

The barrier across
Bab-el-Mandeb The Bab-el-Mandeb ( Arabic: , , ) is a strait between Yemen on the Arabian Peninsula, and Djibouti and Eritrea in the Horn of Africa. It connects the Red Sea to the Gulf of Aden. Name The strait derives its name from the dangers a ...
, between Ethiopia and Yemen, seems to have been the source of outbreak flooding similar to that found in the Mediterranean. The
Lake Toba Lake Toba ( id, Danau Toba) ( Toba Batak: ᯖᯀᯬ ᯖᯬᯅ; romanized: ''Tao Toba'') is a large natural lake in North Sumatra, Indonesia, occupying the caldera of a supervolcano. The lake is located in the middle of the northern part of the i ...
event, approximately between 69,000 and 77,000 years ago, caused a massive drop in sea levels, exposing the barrier and enabling modern ''Homo sapiens'' to leave Africa via a route other than Sinai. The finding of saline
evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
s on the floor of the Red Sea confirms that this dam has functioned at various periods in the past. Rising sea levels during the Flandrian transgression (and in earlier interglacial periods) suggest that this area may have been subject to outburst flooding.


English Channel floods

Originally there was an isthmus across the Strait of Dover. During an earlier glacial maximum, the exit from the
North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea, epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the ...
was blocked to the north by an ice dam, and the water flowing out of rivers backed up into a vast lake with freshwater glacial melt on the bed of what is now the North Sea. A gently upfolding chalk ridge linking the Weald of Kent and
Artois Artois ( ; ; nl, Artesië; English adjective: ''Artesian'') is a region of northern France. Its territory covers an area of about 4,000 km2 and it has a population of about one million. Its principal cities are Arras (Dutch: ''Atrecht'') ...
, perhaps some 30 metres (100 feet) higher than the current sea level, contained the glacial lake at the Strait of Dover. At some time, probably around 425,000 years ago and again around 225,000 years later the barrier failed or was overtopped, loosing a catastrophic flood that permanently diverted the
Rhine ), Surselva, Graubünden, Switzerland , source1_coordinates= , source1_elevation = , source2 = Rein Posteriur/Hinterrhein , source2_location = Paradies Glacier, Graubünden, Switzerland , source2_coordinates= , so ...
into the English Channel and replacing the "Isthmus of Dover" watershed by a much lower watershed running from East Anglia east then southeast to the
Hook of Holland Hook of Holland ( nl, Hoek van Holland, ) is a town in the southwestern corner of Holland, hence the name; ''hoek'' means "corner" and was the word in use before the word ''kaap'' – "cape", from Portuguese ''cabo'' – became Dutch. The English t ...
and (as at modern sea level) separated Britain from the continent of Europe; a
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
study of the sea bed of the English Channel published in ''Nature'', July 2007, revealed the discovery of unmistakable marks of a megaflood on the English Channel seabed: deeply eroded channels and braided features have left the remnants of streamlined islands among deeply gouged channels where the collapse occurred.


The refilling of the Mediterranean Sea (5.3 million years ago)

A catastrophic flood refilled the Mediterranean Sea 5.3 million years ago, at the beginning of the
Zanclean The Zanclean is the lowest stage or earliest age on the geologic time scale of the Pliocene. It spans the time between 5.332 ± 0.005 Ma (million years ago) and 3.6 ± 0.005 Ma. It is preceded by the Messinian Age of the Miocene Epoch, and ...
age that ended the
Messinian salinity crisis The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (d ...
. The flood occurred when Atlantic waters found their way through the Strait of Gibraltar into the desiccated Mediterranean basin, following the Messinian salinity crisis during which it repeatedly became dry and re-flooded, dated by consensus to before the emergence of modern humans. The Mediterranean did not dry out during the most recent
glacial maximum An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
. Sea level during glacial periods within the Pleistocene is estimated to have dropped only about 110 to 120 metres (361 to 394 ft). In contrast, the depth of the Strait of Gibraltar where the Atlantic Ocean enters ranges between .


See also

* * * * * * , formerly of Alaska * , formerly of California * ( megaflood) ** ** * * *


References


External links

*
Catastrophic floods in the English Channel - Inundations
*{{cite book , doi=10.1007/978-90-481-2642-2_196 , chapter=Glacier Lake Outburst Floods , title=Encyclopedia of Snow, Ice and Glaciers , pages=381–99 , series=Encyclopedia of Earth Sciences Series , year=2011 , last1=Iturrizaga , first1=Lasafam , isbn=978-90-481-2641-5 Bodies of water