The Great Lakes receive their water supply from precipitation, which increases from west to east and from north to south. The average annual rainfall in the Lake Superior basin is 30 inches (760 millimeters), 31 inches in Lakes Huron and Michigan, and 34 and 36 inches in Lakes Erie and Ontario, respectively.
Where does water in the Great Lakes originate?
How were the Great Lakes of North America formed?
The present configuration of the Great Lakes basin is the result of the movement of massive glaciers through North America, a process that began about one million years ago during the Pleistocene Epoch. The glaciers undoubtedly scoured existing valleys, widening and deepening them and radically changing the drainage of the area.
