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Our lake shaped world
Apr 09, 2010

Ultimately, glacial Lake Agassiz was a blessing for Manitoba, providing today’s province with thousands of kilometres of beachfront on which to erect cottages as well as numerous lakes that are a resource for commercial fishing and recreational activities. But twice in its existence the lake abruptly drained to catastrophically alter the Earth’s climate.

Over the years, Manitoba scientists have found evidence of massive flood events in the form of huge rocks and gravel deposits showing the path of flooding toward the east and the north toward Hudson Bay. It has long been known that the lake increased and decreased in size according to the ebb and flow of the ice sheet holding it back.

It is only recently that scientists have found evidence of a previously unknown flood path and the extraordinary size of Lake Agassiz. In fact, the new evidence reveals a lake that is far greater in extent than previously realized, covering an area encompassing a million square kilometres — the world’s largest lake known to have existed.

A British-Canadian team reported in March in the scientific journal Nature that 1,000 years after the last Ice Age ended due to global warming, cold freshwater poured into the Arctic Ocean, plunging the world into a cooling period known as the Younger Dryas. The scientists said they had found the “missing flood path” of the event that occurred 13,000 years ago. The evidence was scoured-out river valleys and giant boulders which indicated the flood path went through the Mackenzie, Athabasca and Clearwater river systems into the Arctic Ocean. Essentially, the lake had spilled forth around the western edge of the ice sheet where it had been weakened by melting.

“You know something big happened when you look at the size of the boulders in that gravel,” said University of Manitoba geologist James Teller, who was the co-author of the Nature article.

The freshwater from Lake Agassiz poured into the Arctic Ocean suddenly disrupting climate-warming currents in the Atlantic, similar to the scenario played out in the modern climate-change movie, The Day After Tomorrow. But while the Hollywood film was fantasy rather than reality, what happened 13,000 years ago was every bit as disastrous for the Earth’s climate.

Named after the Arctic wildflower Dryas octopetala that spread across Northern Europe when the big chill set in, the Younger Dryas was a time of a temperature drop of 10°C or more in the space of a few short decades. In Europe, tundra replaced forest that had finally gained a foothold with the end of the Ice Age.

Scientists had earlier suggested a massive flooding brought on by the abrupt draining of Lake Agassiz after the glacial ice dam was breached heading to the St. Lawrence River Valley. None had really suspected the Mackenzie Valley was the path of the flood.

Led by Julian Murton of the University of Sussex in Brighton, U.K., the team dated sand, gravel and boulders from  eroded surfaces in the Mackenzie River Delta and the Athabasca Valley. 

The huge gravel bars on the Mackenzie River Delta were observed by Murton, who brought samples back to England, which in a lab analysis showed the deposits were 13,000 years old.

“Geomorphic observations and chronology clearly indicate a northwestern flood route down the MackenzieValley,” said Teller.

The flood is estimated to have sent a volume of several thousand cubic kilometres of freshwater into the Arctic Ocean.

“We’re pretty sure that the water, had it flooded the northern Atlantic, would have been capable of slowing thermocline ocean circulating (the circulation of warm water from the tropics to northern regions) and producing the Younger Dryas cooling.”

The Earth had been in a warming period, only to wake up to the assault of cooling reminiscent of being in the grasp of another Ice Age.

When the ice sheet again retreated in North America, meltwater reformed Lake Agassiz to a massive depth and breadth.

When freshwater from the lake burst forth again 8,400 years ago, the effect was dramatic for humans living thousands of kilometres away. 

In a low-lying plain in Turkey, farming had taken root for the first time in the region, feeding an estimated 120,000 people. Agricultural practices arose in the Fertile Crescent of the Middle East and within a couple of thousand years had spread and flourished in northern Turkey. 

Researchers have worked out that the freshwater from Lake Agassiz burst forth from the bottom of the ice sheet which had been holding it back, spilling its massive volume into Hudson Bay and sending a surge that swept across the Atlantic. The surge separated the British Isles from mainland Europe and then raged into the Mediterranean Sea. The sea was so inundated with water that it forced a breach in the vicinity where today’s Bosporus Strait is found. 

At first, water overflowed the breach in a trickle, but soon it turned into a torrent, creating a vast waterfall that filled the basin resting below sea level, similar to filling a gigantic bathtub. 

Within a generation — the estimate is about 34 years given the volume of water involved — the region that had supported  tens of thousands of early farmers was turned into the Black Sea. It was a flood of Biblical proportion, displacing the people who had gained a livelihood from growing grain and raising animals from their Garden of Eden.

Even when Lake Agassiz covered large parts of eastern Manitoba, prairies formed in the western region of the province, attracting aboriginal hunters who pursued the big-game animals that browsed on the new ly-created grass-covered plain. When lake Agassiz finally drained into Hudson Bay,  lakes Manitoba and Winnipeg remained as witnesses to its previous expanse.

While we now applaud the legacy of Lake Agassiz for its blessings to our  quality of life, its past provides a warning of how the Earth’s climate can dramatically change within the course of a few decades. While there is no longer a Lake Agassiz to spill forth and create a catacylsmic climactic event, global warming and the rapid melting of the Greenland and polar ice caps could have the same effect.