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.