Geography

The Great Lakes region contains not only the five main lakes themselves, but also numerous minor lakes and rivers, as well as approximately 35,000 islands.

Lake Michigan-Huron

Lakes Michigan and Huron are hydrologically a single lake, sometimes called Lake Michigan-Huron; they have the same surface elevation of

Rivers

• The St. Marys River connects Lake Superior to Lake Huron.
• The St. Clair River connects Lake Huron to Lake St. Clair
• The Detroit River connects Lake St. Clair to Lake Erie.
• The Niagara River, including Niagara Falls, connects Lake Erie to Lake Ontario.
• The St. Lawrence River connects Lake Ontario to the Atlantic Ocean

Other bodies of water

• Georgian Bay is a large bay located within Lake Huron, separated by the Bruce Peninsula and Manitoulin Island. It contains the majority of the islands of the Great Lakes, with a count of approximately 30,000.
• The Straits of Mackinac connects Lake Michigan to Lake Huron.
• The Welland Canal connects Lake Erie to Lake Ontario, bypassing the Niagara River which cannot be fully navigated due to the presence of Niagara Falls.
• Lake St. Clair is the smallest lake in the Great Lake system but due to its relatively small size (compared to the five "Great Lakes"), it is rarely, if ever, considered a Great Lake.

Islands

Dispersed throughout the Great Lakes are approximately 35,000 islands. The largest among them is Manitoulin Island in Lake Huron, the largest island in any inland body of water and home to the world's largest lake within a lake, Lake Manitou. The second-largest island is Isle Royale in Lake Superior. Both of these islands are large enough to contain multiple lakes themselves.

Connection to ocean and open water

The Saint Lawrence Seaway and Great Lakes Waterway opened the Great Lakes to ocean-going vessels. The move to wider ocean-going container ships — which do not fit through the locks on these routes — has limited shipping on the lakes. Despite their vast size, large sections of the Great Lakes freeze over in winter, interrupting most shipping. Some icebreakers ply the lakes.

The Great Lakes are also connected to the Gulf of Mexico by way of the Illinois River (from Chicago), to the Mississippi, to the Gulf. An alternate track is via the Illinois River (from Chicago), to the Mississippi, to the Ohio, up the Ohio, and then through the Tennessee-Tombigbee Waterway (combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf. Commercial tug-and-barge traffic on these waterways is heavy.