Lake Agassiz was an immense lake—bigger than all of the present-day Great Lakes combined—in the center of North America, which was fed by glacial runoff at the end of the last ice age. First postulated in 1823 by William Keating, it was named after Louis Agassiz in 1879 after he was the first to realize it was formed by glacial action.

The lake's modern-day remnants, the largest of which is Lake Winnipeg, dominate the geography of Manitoba. Forming around 13,000 years ago, the lake came to cover much of Manitoba, western Ontario, northern Minnesota, northern North Dakota, and Saskatchewan. At its greatest extent it may have covered as much as 440,000 square kilometers, larger than any lake currently in the world, as well as the Caspian Sea.

The lake drained at various times south into the Minnesota River (part of the Mississippi River system), into the Great Lakes, or west through the Yukon Territory and Alaska. A return of the ice for some time offered a reprieve, and after retreating north of the Canadian border about 9,900 years ago it refilled. These events had significant impact on climate, sea level and possible early human civilizations in the Younger Dryas event. Climatologists believe that a major outbreak of Lake Agassiz in about 11000 BC drained through the Great Lakes and Saint Lawrence River into the Atlantic Ocean.

The last major shift in drainage occurred about 8,400 years ago, when the lake took up its current watershed, that of Hudson Bay. The lake drained nearly completely over the next 1,000 years or so, leaving behind Lake Winnipeg, Lake Winnipegosis, Lake Manitoba, and Lake of the Woods, among others. These lakes are still shrinking slowly, due to isostatic rebound. Most of the lake drainage may have occurred in a very short time - perhaps as little as one year - and is believed to be responsible for the "8.2 kyr event" visible in and other climate records. Note that there is a slight incompatibility in the 8.2 vs 8.4 kyr datings.

While mostly gone along with the ice sheet that fed it, Lake Agassiz left marks over a wide geographic area. Apparent beaches, kilometres or miles from any water, can be found in many locations—these mark the former boundaries of the lake. Several modern river valleys, including the Red River, the Assiniboine River and the aforementioned Minnesota River, were originally cut by water entering or leaving the lake. The Red River Valley agricultural region also exists because of the silt that sank to the bottom of the lake.