The Great Lakes are usually listed as five separate lakes, but a map shows them better as one connected system. Water moves from the far west end of Lake Superior through a chain of lakes, rivers and straits, drops over Niagara Falls, and leaves through the St Lawrence River for the Atlantic. Reading the map in that order makes the geography of the region much easier to remember.
Why they matter
The scale is hard to grasp from a map alone. According to the US Environmental Protection Agency, the Great Lakes hold about 84 percent of North America’s surface fresh water and about 21 percent of the world’s supply of surface fresh water. The EPA describes them as the largest system of fresh surface water on Earth, with only the polar ice caps holding more fresh water.
The five lakes, upstream to downstream
- Lake Superior is the largest and sits at the top of the system, bordered by Minnesota, Wisconsin, Michigan and Ontario.
- Lake Michigan is the only one of the five entirely inside the United States, surrounded by Wisconsin, Illinois, Indiana and Michigan.
- Lake Huron, east of Michigan, includes Georgian Bay on the Canadian side.
- Lake Erie, the shallowest of the five, lies between Ontario and the states of Michigan, Ohio, Pennsylvania and New York.
- Lake Ontario, the smallest by surface area, is the last lake before the St Lawrence.
Lake Michigan and Lake Huron are joined by the wide Straits of Mackinac and sit at the same water level, so hydrologists often treat them as a single lake with two basins. On a map they look separate because the Lower Peninsula of Michigan sits between them.
The connecting waterways
The links between the lakes are where most of the region’s cities and ports grew up:
- The St Marys River carries water from Superior to Huron, with locks at Sault Ste. Marie.
- The St Clair River, Lake St Clair and the Detroit River link Huron to Erie, with Detroit and Windsor facing each other across the water.
- The Niagara River carries Erie into Ontario and drops over Niagara Falls on the way.
- The Welland Canal in Ontario lets ships bypass the falls.
- The St Lawrence River drains Lake Ontario past Montreal and Quebec City to the Gulf of St Lawrence and the Atlantic.
The border
The international boundary between the United States and Canada runs through four of the five lakes and along most of the connecting rivers. Eight US states touch the lakes: Minnesota, Wisconsin, Illinois, Indiana, Michigan, Ohio, Pennsylvania and New York. On the Canadian side, the shoreline belongs to the province of Ontario, and the St Lawrence then continues through Quebec. That split explains why so many of the region’s environmental agreements are joint US and Canadian efforts.
Peninsulas and islands worth spotting
A few landforms help you orient yourself on a map. The two peninsulas of Michigan, the mitten-shaped Lower Peninsula and the long Upper Peninsula, wrap around Lake Michigan and touch three lakes between them. The Bruce Peninsula in Ontario separates Georgian Bay from the main body of Lake Huron, and Manitoulin Island at its tip is the largest island in any freshwater lake in the world. Isle Royale, a US national park, sits in the north-west of Lake Superior close to the Canadian shore.
How the lakes shape the weather
The lakes are large enough to change the climate around them. In late autumn and winter, cold air moving over the warmer open water picks up moisture and drops it as snow on the downwind shores. These lake-effect snow belts are why towns east and south of the lakes, in places like western New York and Michigan’s Upper Peninsula, can get far more snow than cities a short drive away. In summer the effect reverses, and the water keeps nearby shores cooler than the inland areas around them.
Not to be confused with the African Great Lakes
The same name is used for the lakes of the East African Rift, including Victoria, Tanganyika and Malawi. They are a completely different system, covered in the map of East Africa. The North American Great Lakes were carved and filled by ice sheets, while the African ones sit in valleys opened by the rift.
Reading the lakes in context
The lakes sit at the heart of the map of North America, between the plains to the west and the Appalachians to the east. Their water reaches the sea through the St Lawrence, which makes them, in practice, an inland arm of the ocean described in the map of the Atlantic Ocean. And while the Rockies send their rivers to three different seas, as the map of the Rocky Mountains explains, everything in the Great Lakes basin ends up flowing through a single exit.
