A map of the Great Lakes Basin showing the five sub-basins within. Left to right they are: Superior
, including Nipigon
's basin, (magenta); Michigan
(pale green); Erie
Though the five lakes lie in separate basins, they form a single, naturally interconnected body of fresh water, within the Great Lakes Basin. They form a chain connecting 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 are studded with 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 UK 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 Ontario and Michigan include in their boundaries portions of four of the lakes: Ontario does not border Lake Michigan, and 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.
||9,910 sq mi (25,700 km2)
||23,000 sq mi (60,000 km2)
||22,300 sq mi (58,000 km2)
||7,340 sq mi (19,000 km2)
||31,700 sq mi (82,000 km2)
||116 cu mi (480 km3)
||850 cu mi (3,500 km3)
||1,180 cu mi (4,900 km3)
||393 cu mi (1,640 km3)
||2,900 cu mi (12,000 km3)
||571 ft (174 m)
||577 ft (176 m)
||577 ft (176 m)
||246 ft (75 m)
||600.0 ft (182.9 m)
||62 ft (19 m)
||195 ft (59 m)
||279 ft (85 m)
||283 ft (86 m)
||483 ft (147 m)
||210 ft (64 m)
||748 ft (228 m)
||925 ft (282 m)
||804 ft (245 m)
||1,333 ft (406 m)
Bay City, MI
Owen Sound, ON
Port Huron, MI
Green Bay, WI
Traverse City, MI
Sault Ste. Marie, MI
Sault Ste. Marie, ON
Thunder Bay, ON
Relative elevations, average depths, maximum depths, and volumes of the Great Lakes.
||The area of each rectangle is proportionate to the volume of each lake. All measurements at Low Water Datum.
System profile of the Great 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, however, 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.
Primary connecting waterways
on Lake Michigan is in the western part of the lakes megalopolis, and the site of the waterway linking the lakes to the Mississippi River valley
on the Detroit River links the region's central metropolitan areas.
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 – currently at 577 feet (176 m) – rise and fall together, and the flow between Michigan and Huron frequently reverses direction.
Other 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 1,109 million years ago.
- Green Bay is an arm of Lake Michigan, along the south coast of the Upper Peninsula of Michigan and the east coast of Wisconsin. It is separated from the rest of the lake by the Door Peninsula in Wisconsin, the Garden Peninsula in Michigan, and the chain of islands between them, all of which were formed by the Niagara Escarpment.
- Lake Winnebago, connected to Green Bay by the Fox River, serves as part of the Fox–Wisconsin Waterway and is part of a larger system of lakes in Wisconsin known as the Winnebago Pool.
- Grand Traverse Bay is an arm of Lake Michigan on Michigan's west coast, being one of the largest natural harbors in the Great Lakes. The bay has one large peninsula and one major island known as Power Island. Its name is derived from Jacques Marquette's crossing of the bay from Norwood to Northport which he called La Grande Traversee.
- Georgian Bay is an arm of Lake Huron, extending northeast from the lake entirely within Ontario. The bay, along with its narrow westerly extensions of the North Channel and Mississagi Strait, is separated from the rest of the lake by the Bruce Peninsula, Manitoulin Island, and Cockburn Island, all of which were also formed by the Niagara Escarpment.
- Lake Nipissing, connected to Georgian Bay by the French River, contains two volcanic pipes, which are the Manitou Islands and Callander Bay. These pipes were formed by a violent, supersonic eruption of deep-origin. The lake lies in the Ottawa-Bonnechere Graben, a Mesozoic rift valley that formed 175 million years ago.
- Lake Simcoe, connected to Georgian Bay by the Severn River, serves as part of the Trent–Severn Waterway, a canal route traversing Southern Ontario between Lakes Ontario and Huron.
- Lake St. Clair, connected with Lake Huron to its north by the St. Clair River and with Lake Erie to its south by the Detroit River. Although it is 17 times smaller in area than Lake Ontario and only rarely included in the listings of the Great Lakes, proposals for its official recognition as a Great Lake are occasionally made, which would affect its inclusion in scientific research projects, etc., designated as related to "The Great Lakes".
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 in the world. The second-largest island is Isle Royale in Lake Superior. Both of these islands are large enough to contain multiple lakes themselves—for instance, Manitoulin Island's Lake Manitou is the world's largest lake on a freshwater island. Some of these lakes even have their own islands, like Treasure Island in Lake Mindemoya in Manitoulin Island
The Great Lakes also have several peninsulas between them, including the Door Peninsula, the Peninsulas of Michigan, and the Ontario Peninsula. Some of these peninsulas even contain smaller peninsulas, like the Keweenaw Peninsula, the Thumb Peninsula, the Bruce Peninsula, and the Niagara Peninsula. Population centers on the peninsulas include Grand Rapids, Michigan, Detroit, Michigan, London, Ontario, Hamilton, Ontario, and Toronto, Ontario.
Shipping connection to the ocean
The Saint Lawrence Seaway and Great Lakes Waterway make the Great Lakes accessible to ocean-going vessels. However, shifts in shipping to wider ocean-going container ships—which do not fit through the locks on these routes—have limited container shipping on the lakes. Most Great Lakes trade is of bulk material, and bulk freighters of Seawaymax-size or less can move throughout the entire lakes and out to the Atlantic. Larger ships are confined to working in the lakes themselves. Only barges can access the Illinois Waterway system providing access to the Gulf of Mexico via the Mississippi River. Despite their vast size, large sections of the Great Lakes freeze over in winter, interrupting most shipping from January to March. Some icebreakers ply the lakes, keeping the shipping lanes open through other periods of ice on the lakes.
The Great Lakes are also connected by the Chicago Sanitary and Ship Canal to the Gulf of Mexico by way of the Illinois River (from the Chicago River) and the Mississippi River. An alternate track is via the Illinois River (from Chicago), to the Mississippi, up the Ohio, and then through the Tennessee–Tombigbee Waterway (a combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf of Mexico. Commercial tug-and-barge traffic on these waterways is heavy.
Pleasure boats can also enter or exit the Great Lakes by way of the Erie Canal and Hudson River in New York. The Erie Canal connects to the Great Lakes at the east end of Lake Erie (at Buffalo, New York) and at the south side of Lake Ontario (at Oswego, New York).
In 2009, the lakes contained 84% of the surface freshwater of North America; if the water were evenly distributed over the entire continent's land area, it would reach a depth of 5 feet (1.5 meters). The source of water levels in the lakes is tied to what was left by melting glaciers when the lakes took their present form. Annually, only about 1% is "new" water originating from rivers, precipitation, and groundwater springs that drain into the lakes. Historically, evaporation has been balanced by drainage, making the level of the lakes constant. While the lake levels have been preserved, intensive human population growth only began in the region in the 20th century and continues today. At least two human water use activities have been identified as having the potential to affect the lakes' levels: diversion (the transfer of water to other watersheds) and consumption (substantially done today by the use of lake water to power and cool electric generation plants, resulting in evaporation).
The water level of Lake Michigan–Huron had remained fairly constant over the 20th century, but has nevertheless dropped more than 6 feet from the record high in 1986 to the low of 2013. One newspaper reported that the long-term average level has gone down about 20 inches because of dredging and subsequent erosion in the St. Clair River. Lake Michigan–Huron hit all-time record low levels in 2013; according to the US Army Corps of Engineers, the previous record low had been set in 1964. By April 2015 the water level had recovered to 7 inches (17.5 cm) more than the "long term monthly average".