The advance and retreat of glaciers over millions of years scraped and scoured the Great Lakes basins until they attained their present form about 10,000 years ago. Forming the largest system of inland lakes in the world, the Great Lakes have a surface area of 94,200 mi² (244,000 km²) and a volume of more than 28 trillion yd⁴ (22,000 km³) of water, 20 percent of the world's surface freshwater.

Lake Superior, with more than 31,660 mi² (82,000 km²) of water, has the largest surface area of freshwater on earth. Lake Huron, the world's fifth largest lake, is at the same elevation and about the same size as Lake Michigan, the world's sixth largest lake. The two are joined by the narrow, deep Straits of Mackinac. Their accumulated waters empty into the St. Clair River which flows into the 460 mi²(1,190 km²) Lake St. Clair. The water continues its flow into the Detroit River before entering...