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geomorphology Geomorphology () is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand wh ...
, an outburst flood—a type of megaflood—is a high-magnitude, low-frequency catastrophic
flood A flood is an overflow of water (list of non-water floods, 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 of significant con ...
involving the sudden release of a large quantity of water. During the last
deglaciation Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice shee ...
, 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 lakes. 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. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
. Landslides,
lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...
s, and volcanic dams 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
landform A landform is a land feature on the solid surface of the Earth or other planetary body. They may be natural or may be anthropogenic (caused or influenced by human activity). Landforms together make up a given terrain, and their arrangement ...
s 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). *Rapid erosion, melting of ice sheets ( jökulhlaups). *Collapse of earthen barriers (landslides or glacial moraines). *Collapse of volcanic dams created by lava flows, lahars, or pyroclastic flows. *Overtopping of earthen or rock barriers **Lake overtopping (e.g., Lake Bonneville). **Ocean spilling over a dividing ridge into a landlocked basin (e.g.,
Zanclean flood The Zanclean flood or Zanclean deluge is theorized to have refilled the Mediterranean Sea 5.33 million years ago. This flooding ended the Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean, although it i ...
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, 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, water pressure increases with the increased depth. The esker may then fail under the load and burst, creating a new outflow. Lake Pielinen in Finland is an example of this.


Tectonic basins


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 sea, marginal Mediterranean sea (oceanography), mediterranean sea 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 bound ...
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 myth A flood myth or a deluge myth is a myth in which a great flood, usually sent by a deity or deities, destroys civilization, often in an act of divine retribution. Parallels are often drawn between the flood waters of these Mythology, myths and the ...
s is not proven.


Persian Gulf Flood (24,000 to 14,000 years ago, or 12,000 to 10,000 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 (15,000 to 8,000 years ago)

In
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
, 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 spanning the Canada–United States border. The five lakes are Lake Superior, Superior, Lake Michigan, Michigan, Lake Huron, H ...
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 proglacial lake that existed in central North America during the late Pleistocene, fed by meltwater from the retreating Laurentide Ice Sheet at the end of the last glacial period. At its peak, the lake's area wa ...
. 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 of
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while 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, a pluvial lake, burst catastrophically in the Bonneville Flood 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 Semi-arid climate, semiar ...
s which had blocked a
gorge A canyon (; archaic British English spelling: ''cañon''), gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tend ...
. 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 Ottawa is the capital city of Canada. It is located in the southern Ontario, southern portion of the province of Ontario, at the confluence of the Ottawa River and the Rideau River. Ottawa borders Gatineau, Gatineau, Quebec, and forms the cor ...
and
St. Lawrence River The St. Lawrence River (, ) is a large international river in the middle latitudes of North America connecting the Great Lakes to the North Atlantic Ocean. Its waters flow in a northeasterly direction from Lake Ontario to the Gulf of St. Lawren ...
s far to the south. About 8,300 to 7,700 years ago, the melting ice dam over
Hudson Bay Hudson Bay, sometimes called Hudson's Bay (usually historically), is a large body of Saline water, saltwater in northeastern Canada with a surface area of . It is located north of Ontario, west of Quebec, northeast of Manitoba, and southeast o ...
'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; ''Rossíyskaya akadémiya naúk'') consists of the national academy of Russia; a network of scientific research institutes from across the Russian Federation; and additional scientific and social units such ...
dates the flooding of the Black Sea basin to an earlier time and from a different cause. According to Tchepalyga,
global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
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, described as the List of lakes by area, world's largest lake and usually referred to as a full-fledged sea. An endorheic basin, it lies between Europe and Asia: east of the Caucasus, ...
, raising it to as much as above normal present-day levels. The
Sea of Azov The Sea of Azov is an inland Continental shelf#Shelf seas, shelf sea in Eastern Europe connected to the Black Sea by the narrow (about ) Strait of Kerch, and sometimes regarded as a northern extension of the Black Sea. The sea is bounded by Ru ...
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 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 from the Taman Peninsula of Russia's Krasnodar Krai in the east. The strait is to wide and up ...
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 or Le Déluge may also refer to: History *Deluge (history), the Swedish and Russian invasion of the Polish-L ...
.


Red Sea floods

The barrier across Bab-el-Mandeb, between Ethiopia and Yemen, seems to have been the source of outbreak flooding similar to that found in the Mediterranean. The Lake Toba 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 oce ...
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 An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene i ...
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 The Strait of Dover or Dover Strait, historically known as the Dover Narrows, is the strait at the narrowest part of the English Channel, marking the boundary between the Channel and the North Sea, and separating Great Britain from continental ...
. During an earlier glacial maximum, the exit from the
North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...
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 ( , ; ; Picard: ''Artoé;'' 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 include Arras (Dutch: ...
, perhaps some 30 metres (100 feet) higher than the current sea level, contained the
glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10,0 ...
at the
Strait of Dover The Strait of Dover or Dover Strait, historically known as the Dover Narrows, is the strait at the narrowest part of the English Channel, marking the boundary between the Channel and the North Sea, and separating Great Britain from continental ...
. 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 The Rhine ( ) is one of the List of rivers of Europe, major rivers in Europe. The river begins in the Swiss canton of Graubünden in the southeastern Swiss Alps. It forms part of the Swiss-Liechtenstein border, then part of the Austria–Swit ...
into the English Channel and replacing the "Isthmus of Dover" watershed by a much lower watershed running from
East Anglia East Anglia is an area of the East of England, often defined as including the counties of Norfolk, Suffolk and Cambridgeshire, with parts of Essex sometimes also included. The name derives from the Anglo-Saxon kingdom of the East Angles, ...
east then southeast to the
Hook of Holland Hook of Holland (, ) is a coastal village in the southwestern corner of Holland, hence the name; ''hoek'' means "corner" and was in use before the word ''wikt:kaap#Dutch, kaap'' – "cape". The English translation using Hook is a false cognate of 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 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.


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 f ...
age that ended the
Messinian salinity crisis In the Messinian salinity crisis (also referred to as the Messinian event, and in its latest stage as the Lago Mare event) the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (drying-up) throughout the latter part of ...
. The flood occurred when Atlantic waters found their way through the
Strait of Gibraltar The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Europe from Africa. The two continents are separated by 7.7 nautical miles (14.2 kilometers, 8.9 miles) at its narrowest point. Fe ...
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. 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 The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Europe from Africa. The two continents are separated by 7.7 nautical miles (14.2 kilometers, 8.9 miles) at its narrowest point. Fe ...
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