The lake's modern-day remnants, the largest of which is Lake Winnipeg, dominate the geography of Manitoba. Forming around 11,700 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 (including 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.
The last major shift in drainage occurred about 8,500 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.
While mostly gone along with the ice sheet that fed it, Lake Agassiz left marks over a wide geographic area. Apparent beaches, 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 afore-mentioned 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.