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Megafloods


Megafloods
North-west America hosts some of the strangest and most spectacular landforms on our planet. Scarred into the black rock are giant channels carved out by water – the largest is the Grand Coulee in Washington at 97 kilometres (60 miles) long.

In 1922, geologist J Harlen Bretz began investigating how these channels formed. He initially attributed them to the slow action of rivers, but the more he looked into it, the more unusual landforms he discovered. Among them were piles of gravel some ten storeys high and hills shaped like boat prows.
These gargantuan features were best explained by water tearing through the landscape – an unimaginably massive megaflood. The prow-shaped hills pointed in the direction of flow, while the gravel was dropped when the floodwaters receded. Bretz tracked the source of this colossal torrent – possibly the largest flood in history – to the glacial Lake Missoula. During the last ice age, this lake formed behind a 610-metre (2,000-foot)-tall wall of ice. When this dam failed, around 15,000 years ago, the lake emptied in just 48 hours and the waters carved the Grand Coulee. They left ripples, like those on a streambed, but a monstrous nine metres (30 feet) high. The process would repeat itself over the next 2,000 years, carving out more colossal landmarks in North America.
Lake Missoula might be the biggest known megaflood, but it’s by no means the only one. Indeed, during the last 1.8 million years, at least 27 gigantic freshwater floods have shaped our planet, carrying more than 100,000 cubic metres (3.5 million cubic feet) of water every second – equivalent to over 30 Niagara Falls! We know less about giant floods from further back in Earth’s history. “As you go back in time, you don’t have the landforms preserved,” says ice-age geologist Professor Philip Gibbard.

Water escaping from natural dams or glaciers is responsible for many of the biggest floods, including Lake Missoula. Gibbard continues: “You get substantial flooding if a major dam floods and releases water.” Among them is the megaflood that turned Britain into an island around 450,000 to 200,000 years ago, when sea levels were lower than today. A gigantic lake formed in what is today’s North Sea behind a chalk ridge that once connected Britain to France by land. When the lake punched through the natural dam, floodwaters gouged a huge valley into the English Channel seabed (see the thermal imagery on the opposite page).
Other massive floods occurred when sea levels rose after an arid spell. For instance, around 5.3 million years ago, Atlantic waters spilled into the dried-up Mediterranean; the ocean eroded a channel through the Strait of Gibraltar, filling the sea in as little as two years.
Megafloods can also have a massive impact on the climate. The Lake Agassiz flood, for example, is blamed for a cold spell 12,900 years ago that sent North America’s large mammals, like the woolly mammoth, extinct. Floodwaters poured into the North Atlantic, interfering with the ocean circulation that brings warm water to the poles, known as the Gulf Stream.
Fortunately floods as large as Lake Agassiz or Missoula are unlikely to happen today. The biggest deluges were associated with giant ice sheets that swathed the northern hemisphere during the last ice ages. The ice released torrents of meltwater and trapped huge lakes behind ice dams, which gradually succumbed to global warming. Only the Greenland and Antarctic ice sheets, and a few ice caps, remain.
But that doesn’t count megafloods out altogether. Among the 27 known big freshwater floods in the last 1.8 million years, eight occurred after 1900. According to Gibbard: “We could experience a megafl ood today. Look no further than Iceland where we see jökulhlaup.” Jökulhlaup, or glacier bursts, occur when a volcano erupts under an ice cap, such as the 1996 Vatnajökull eruption in south-east Iceland.
A torrent of meltwater flooded from beneath the ice, wiping out anything that stood in its way, including bridges, roads and power lines. It’s estimated that peak flows reached 50,000 cubic metres (1.8 million cubic feet) of water per second in this massive jökulhlaup.
Another cause of giant floods, says Gibbard, is where “you have a volcanic lake, and the volcano erupts and breaches the barrier holding the water in the crater. You can get a deadly mudflow, sweeping away everything in its path.” An example is Lake Taupo, New Zealand’s largest lake, which fl ooded during an eruption some 1,800 years ago.
Debris from volcanoes can also block rivers. An eruption in Mount St Helens about 2,500 years ago caused Spirit Lake to empty catastrophically. More than 260,000 cubic metres (9.2 million cubic feet) of water per second flooded downstream. This was like a bathtub overflowing by comparison to Lake Missoula, though, which emptied at an incomprehensible 17 million cubic metres (600 million cubic feet) per second. Similar floods would have been likely after the 1980 eruption of Mount St Helen’s if mitigation measures weren’t undertaken and the lake had not been drained via a pipeline.
So how do these megafloods compare to more common flooding events, like we’ve seen in recent months? “They’re orders of magnitude greater,” explains Gibbard: “We’re talking stuff that’s incredibly dramatic. Much larger scale than conventional flooding of rivers.”
Rivers bursting their banks are the most common cause of flooding today. Excessive rain or rapid snowmelt can overfill a river channel, causing water to spill out over low-lying land.
Waterlogged or parched soil can also cause water levels to rise rapidly. Even a little rainfall cannot be soaked up by the ground so rush straight into the river, causing a flood.
Human activity – such as building hard tarmac roads – is making river flooding even worse, warns Gibbard. “The inevitable consequence of covering the landscape with impermeable materials is like wrapping it in a polythene bag. [More and more] water runs straight into our rivers.”
With sea levels predicted to rise dramatically this century – and climate change likely to boost storm power and frequency – the risk of coastal flooding is also on the rise.
Coastal flooding occurs when a storm blows seawater inland or when a tsunami – a giant wave usually generated by an oceanic earthquake – hits the shore.
The big question is, can we predict future megafloods? “Not easily,” says Gibbard. “We know volcanoes erupt under ice caps in Iceland.
People are trying to predict volcanic activity, but we’re not there yet.” The next ‘megaflood’ might come from a man-made dam rupturing during an earthquake. Gibbard explains, “Anywhere where water is stored on less-than-fi rm ground might go. The sheer height of water behind those barrages must be enormous.”
It’s a real risk. Out of 85,000 American dams, over 4,400 are considered liable to failure. Among them is the Lake Isabella Dam in California. A strong earthquake could send around 700 million cubic metres (2.5 billion cubic feet) of water tumbling downriver. For now though the best we can do is never to underestimate the mighty power of water.

Megafloods