Ten thousand years ago, glacial Lake Agassiz was the largest body of freshwater in North America. At its maximum extent, the lake covered an area of some 135,000 square miles and had a maximum depth of around 700 feet. Lake Agassiz was a product of widespread deglaciation that marked the closure of the last great Ice Age. By 11,700 years ago, the ice front had retreated far enough north to permit impoundment of glacial meltwater–incipient Lake Agassiz–in the extreme southern end of the Red River Valley of the North near the junction of present-day North Dakota, South Dakota, and Minnesota. As the ice sheet retreated the lake gradually expanded northward across the First Steppe (or Manitoba Plain) and over much of the adjoining Precambrian Shield. Initially, drainage flowed southward via the Minnesota and Mississippi Rivers. The level of the lake rose and fell, and its margins correspondingly expanded and contracted, several times in response to a sequence of ice frontal readvances and retreats that alternately opened and closed its outlets into the Lake Superior basin.

Migration made possible by extensive riverine connections with other drainages to the south, east, west, and northwest culminated in a varied fish population in Lake Agassiz. The lake's huge size, however, precluded human occupation of a sizeable portion of the northeastern Plains periphery, while adjacent areas to the south and west were inhabited by generations of hunters and gatherers.

The lake began its final phase of drainage just over 9,000 years ago. Around 1,000 years later a much-diminished Lake Agassiz discharged into ancestral Hudson Bay as the last vestiges of the once-formidable ice dam finally disintegrated.

See also PHYSICAL ENVIRONMENT: Glaciation.