This Prehistoric Lake Was Once Larger Than All The Great Lakes

During the past ice ages, North America looked different - very different. Not only was there the Bering land bridge (the remnants of which one can still see today) but Canada was largely under a sheet of ice. At this time there was a massive glacial lake - Lake Agassiz that was fed by glacial meltwater as the last glacial period came to an end.

Lake Agassiz was larger than all the Great Lakes combined. It covered much of central North America between 30,000 and 10,000 years ago. To learn about the impact of a modern lake forming and then disappearing and its massive ecological impact, read about the strange story of the Salton Sea in California.

Size and Formation of Lake Agassiz

During the Ice Age, much of Canada and the northern United States were covered with a massive ice sheet. As this began to melt, Lake Agassiz formed from the meltwater forming an immense proglacial lake.

When: Between 30,000 and 10,000 Years Ago
Duration: Lake Agassiz Lasted for around 4,500 Years (Depending on The Stage Being Dated From)
First Postulated: In 1823 by William H. Keating

This lake grew to cover much of southeastern Manitoba, northwestern Ontario, eastern North Dakota, northern Minnesota, and eastern Saskatchewan. At its greatest extent, it may have covered as much as 170,000 square miles - that is larger than any other lake in the world today. It's even larger than the Caspian Sea and around the same size as the Black Sea.

Size: Up to 170,000 Square Miles or 440,000 Square Kilometers At Its Greatest Extent

Over the course of its history, Lake Agassiz was constantly changing. It would drain in different directions at different times. Sometimes it would drain north through the Mackenzie River to the Arctic Ocean and at other times down through the Mississippi River to the gulf.

Winnipeg: At Times Winnipeg Was Below 213 Meters of Water

The lake contracted and expanded with the return or melting of the ice sheets covering North America.

Draining of Lake Agassiz And Global Impact

The last time it was refilled was around 10,000 years ago. Later, when the last of the remaining Hudson Bay ice melted, Lake Agassiz was then free to drain almost completely into the Hudson Bay.

So much water was in Lake Agassiz that it is estimated when it drained it rose the global sea levels by between 0.8 and 2.8 meters or 2.6 to 9.2 feet. The drainage of the lake was a major event and it significantly impacted the climate and maybe early human civilization.

Sea Level: It Rose Global Sea Levels by between 0.8-2.8 Meters or 2.6-9.2 Feet

So much freshwater poured into the ocean that it is thought to have at least temporarily cooled the earth and disrupted oceanic circulation. It has even been suggested to have been the trigger for the Younger Dryas stadial.

Ocean Currents: It May Have Impacted Ocean Currents
Younger Dryas: It May have Cooled The Planet and Triggered The Younger Dryas

Some scholars have gone so far as to say that its drainage may have positively affected agriculture in Europe.

Remnants of the Lake And Traces Today

Today there are many signs and tell-tale signs that this lake once existed all over the region if one knows what to look for. These include carved-out valleys where it drained, ancient shorelines, ancient beaches, ancient dunes, and more.

Carved River Valley: The valley that the Minnesota River flows through was carved by the outflow of Lake Agassiz through the outlet of River Warren. It carved a valley about 1.2 to 3.1 miles wide and between 100 feet and 125 feet deep.

Remnant Lakes: Today there are numerous lakes in Lake Agassiz's basin. The Great Lakes of Manitoba may be thought of as the remains of this great lake including:

Lake Winnipeg
Lake Manitoba
Lake Winnipegosis
Cedar Lake
Lake Dauphin
Beach Ridges and Low Cliffs:

As the lake rose and fell several times, it resulted in the opening and closing of various drainage channels. Whenever the lake stabilized for a time, its waves created low cliffs and beach ridges, these are still visible on the landscape today.

The native people living in the region at the time were also affected by the lake. The post-glacial beaches were elevated and well-drained and so they would use them as campsites, lookouts for spotting game, burial grounds, and more. Today these are often significant archeological sites.

The Agassiz Dunes SNA: One place in the USA where one can see the dune fields created by the lake is the Agassiz Dunes SNA. They are made up of sediments and originally accumulated as a delta and have been sculpted by the wind over the centuries.