The Great Lakes Basin was formed by glacier activity. The continental glaciers repeatedly moved over what is now the Great Lakes region as they thawed and melted. As the huge glaciers inched forward, they scoured the surface of the earth and leveled hills. Small valleys created by the river systems of the previous era were deepened and enlarged to form the basins for what are now the Great Lakes.

The Great Lakes and their connecting channels form the largest surface freshwater system on Earth. If you stood on the moon, you could see the lakes. The Great Lakes, in their current state, are one of the youngest natural features on the North American continent. Covering more than 94,000 square miles and draining more than twice as much land, these freshwater seas hold an estimated six quadrillion gallons of water, or about one-fifth of the world's surface freshwater supply and nine-tenths of the US supply. Lake Superior is the largest of the Great Lakes, and if you are measuring surface area, is the largest freshwater lake in the world.¹

Lake Victoria, located in Africa, is the world's second largest freshwater lake. This lake was formed quite differently from the Great Lakes. Its formation is connected to the development of the Great Rift Valley that you learned about in lesson 5. As you will recall, a rift valley is formed when two tectonic plates pull away from one another. The lake formed in a low-lying area that developed as the land stretched. The Nile River runs through portions of the Great Rift Valley and receives much of its water from Lake Victoria. The lake gets more than half of its water from rain; the rest of its water comes from streams and rivers that empty into it.