Map showing the large modern lakes in black which are the remnants of the largest single body of fresh water ever to cover the North American continent, Lake Agassiz. This body of water formed as the retreating Wisconsin ice sheet released large quantities of meltwater. Lake Agassiz discharged first down the Minnisota River which joins the Mississippi River at Minneapolis and St. Paul. Later, however, the ice retreated farther north and uncovered a lower outlet to Hudson Bay (Zumberge 1958, p. 355).