The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes spanning the Canada–United States border. The five lakes are Superior, Michigan, Huron, Erie, and Ontario (though hydrologically, Michigan and Huron are a single body of water, joined at the Straits of Mackinac). The Great Lakes Waterway enables modern travel and shipping by water among the lakes. The lakes connect ultimately to the Atlantic Ocean via the Saint Lawrence River as their primary drainage outflow. The lakes are also connected to the Mississippi River basin through the Illinois Waterway.
The Great Lakes are the largest group of freshwater lakes on Earth by total area and the second-largest by total volume. They contain more than 20% of the world's surface fresh water by volume. The total surface is 94,250 square miles (244,106 km2), and the total volume (measured at the low water datum) is 5,439 cubic miles (22,671 km3), slightly less than the volume of Lake Baikal (5,666 cu mi or 23,615 km3, 22–23% of the world's surface fresh water). Because of their sea-like characteristics, such as rolling waves, sustained winds, strong currents, great depths, and distant horizons, the five Great Lakes have long been called inland seas. Depending on how it is measured, by surface area, either Lake Superior or Lake Michigan–Huron is the second-largest lake in the world and the largest freshwater lake. Lake Michigan is the largest lake, by surface area, that is entirely within one country, the United States.
The Great Lakes began to form at the end of the Last Glacial Period around 14,000 years ago, as retreating ice sheets exposed the basins they had carved into the land, which then filled with meltwater. The lakes have been a major source for transportation, migration, trade, and fishing, serving as a habitat to many aquatic species in a region with much biodiversity. The surrounding region is called the Great Lakes region, which includes the Great Lakes megalopolis. Major cities within the region include, on the American side, from east to west, Rochester, Buffalo, Cleveland, Detroit, Chicago, Milwaukee, and Duluth; and, on the Canadian side, Toronto, Mississauga and Hamilton.
Though the five lakes lie in separate basins, they form a single, naturally interconnected body of fresh water, within the Great Lakes Basin. As a chain of lakes and rivers, they connect the east-central interior of North America to the Atlantic Ocean. From the interior to the outlet at the Saint Lawrence River, water flows from Superior to Huron and Michigan, southward to Erie, and finally northward to Lake Ontario. The lakes drain a large watershed via many rivers and contain approximately 35,000 islands. There are also several thousand smaller lakes, often called "inland lakes", within the basin.
The surface area of the five primary lakes combined is roughly equal to the size of the United Kingdom, while the surface area of the entire basin (the lakes and the land they drain) is about the size of the United Kingdom and France combined. Lake Michigan is the only one of the Great Lakes that is entirely within the United States; the others form a water boundary between the United States and Canada. The lakes are divided among the jurisdictions of the Canadian province of Ontario and the U.S. states of Michigan, Wisconsin, Minnesota, Illinois, Indiana, Ohio, Pennsylvania, and New York. Both the province of Ontario and the state of Michigan include in their boundaries portions of four of the lakes. The province of Ontario does not border Lake Michigan, and the state of Michigan does not border Lake Ontario. New York and Wisconsin's jurisdictions extend into two lakes, and each of the remaining states into one of the lakes.
As the surfaces of Lakes Superior, Huron, Michigan, and Erie are all approximately the same elevation above sea level, while Lake Ontario is significantly lower, and because the Niagara Escarpment precludes all natural navigation, the four upper lakes are commonly called the "upper great lakes". This designation is not universal. Those living on the shore of Lake Superior often refer to all the other lakes as "the lower lakes", because they are farther south. Sailors of bulk freighters transferring cargoes from Lake Superior and northern Lake Michigan and Lake Huron to ports on Lake Erie or Ontario commonly refer to the latter as the lower lakes and Lakes Michigan, Huron, and Superior as the upper lakes. This corresponds to thinking of lakes Erie and Ontario as "down south" and the others as "up north". Vessels sailing north on Lake Michigan are considered "upbound" even though they are sailing toward its effluent current.
The St. Marys River, including the Soo Locks, connects Lake Superior to Lake Huron, via the North Channel.
The Straits of Mackinac connect Lake Michigan to Lake Huron (the two are hydrologically one lake).
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 Welland Canal, bypassing the Niagara River, connects Lake Erie to Lake Ontario.
The Saint Lawrence River and the Saint Lawrence Seaway connect Lake Ontario to the Gulf of Saint Lawrence, which connects to the Atlantic Ocean.
Secondary connecting waterways
The Chicago River and Calumet River systems connect the Great Lakes Basin to the Mississippi River System through human-made alterations and canals.
Lakes Huron and Michigan are sometimes considered a single lake, called Lake Michigan–Huron, because they are one hydrological body of water connected by the Straits of Mackinac. The straits are five miles (8 km) wide and 120 feet (37 m) deep; the water levels rise and fall together and the flow between Michigan and Huron frequently reverses direction.
Large bays and related significant bodies of water
Lake Nipigon, connected to Lake Superior by the Nipigon River, is surrounded by sill-like formations of mafic and ultramafic igneous rock hundreds of meters high. The lake lies in the Nipigon Embayment, a failed arm of the triple junction (centered beneath Lake Superior) in the Midcontinent Rift System event, estimated at 1.1 billion years ago.