When the glaciers receded after the last Ice Age, they left several vast depressions which filled with water and became the Great Lakes of North America. There are five of them.
Lake Superior is the northernmost and largest.
Lake Michigan is the only one entirely within the United States. It and
Lake Superior flow into
Lake Huron, which connects with
Lake Erie, the southernmost.
Niagara Falls joins
Lake Erie with
Lake Ontario, which flows out through the
Saint Lawrence River into the North Atlantic.
Major port cities include Chicago, Milwaukee, Duluth, Thunder Bay, Sault Ste. Marie, Cleveland, Erie, Buffalo, and Toronto.
