A glacial lake larger than all modern Great Lakes combined catastrophically drained multiple times. One drainage event 13,000 years ago may have shut down Atlantic circulation and triggered the Younger Dryas cold snap. A lake that changed global climate.
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A basin of continental scale
At the end of the last ice age, the retreating Laurentide Ice Sheet acted as a colossal dam. Meltwater pooled behind it, forming a proglacial lake of immense proportions: Lake Agassiz. First forming around 13,000 years ago, it came to cover large parts of Manitoba, Saskatchewan, Ontario, Minnesota, and North Dakota. At its maximum extent, Lake Agassiz covered up to 440,000 square kilometers, an area larger than any modern lake, including the Caspian Sea. Its deepest sections reached over 200 meters.
The lake's existence was dynamic, defined by the shifting position of the ice dam. Over about 5,000 years, its level and drainage routes changed dramatically. Initially, the lake drained south through the Traverse Gap into what became the Minnesota and Mississippi Rivers, carving a valley up to five miles wide. As the ice sheet retreated and advanced, new outlets opened and closed. At times, water flowed east toward the future Great Lakes or northwest toward the Arctic Ocean. The landscape of southern Manitoba, now a flat plain of fertile clay soil, was once the floor of this vast lake, with Winnipeg situated under more than 150 meters of water. The ancient shorelines are still visible today as a series of low, sandy ridges.
Floods that changed the world
The history of Lake Agassiz is punctuated by a series of catastrophic outburst floods. These were not gradual drainages but sudden, massive releases of water as ice dams failed or new, lower outlets became exposed. One of the most significant events occurred around 12,900 years ago. An outburst of an estimated 9,500 cubic kilometers of fresh water likely flowed into the North Atlantic. This sudden influx of cold, fresh water would have disrupted the Atlantic Meridional Overturning Circulation (AMOC), an ocean current system that transports warm water northward. The shutdown of this current explains the Younger Dryas, an abrupt return to near-glacial conditions in the Northern Hemisphere that lasted for over a thousand years.
Another major event took place around 8,200 years ago. The final collapse of the ice dam over Hudson Bay triggered the lake's near-complete drainage. This event released an estimated 163,000 cubic kilometers of water, raising global sea levels by 0.8 to 2.8 meters. This flood, routed into the Labrador Sea, caused the "8.2 ka event," a distinct, century-scale cold snap recorded in ice cores and other paleoclimatic records. The peak discharge of some of these floods was immense, with estimates ranging from 0.2 to 5 million cubic meters per second, dwarfing the flow of all modern rivers combined.
💡Fun Facts
Lake Agassiz was named in 1879 after Louis Agassiz, a Swiss-American scientist who was a founder of glaciology and first proposed the theory of a past ice age.
The fertile "gumbo" soil of the Red River Valley is composed of fine silty clay that settled at the bottom of the ancient lake.
Remnants of Lake Agassiz still exist today, including Lake Winnipeg, Lake Manitoba, Lake of the Woods, and Red Lake.
Ancient beach ridges from the lake's former shorelines are still visible in Manitoba and Minnesota, and now support some of the last remnants of native tallgrass prairie.
