<P> 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 a large number of aquatic species in a region with much biodiversity . </P> <P> The surrounding region is called the Great Lakes region, which includes the Great Lakes Megalopolis . </P> <P> 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 located 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 . </P> <Table> <Tr> <Th> </Th> <Th> Lake Erie </Th> <Th> Lake Huron </Th> <Th> Lake Michigan </Th> <Th> Lake Ontario </Th> <Th> Lake Superior </Th> </Tr> <Tr> <Th> Surface area </Th> <Td> 9,910 sq mi (25,700 km) </Td> <Td> 23,000 sq mi (60,000 km) </Td> <Td> 22,300 sq mi (58,000 km) </Td> <Td> 7,340 sq mi (19,000 km) </Td> <Td> 31,700 sq mi (82,000 km) </Td> </Tr> <Tr> <Th> Water volume </Th> <Td> 116 cu mi (480 km) </Td> <Td> 850 cu mi (3,500 km) </Td> <Td> 1,180 cu mi (4,900 km) </Td> <Td> 393 cu mi (1,640 km) </Td> <Td> 2,900 cu mi (12,000 km) </Td> </Tr> <Tr> <Th> Elevation </Th> <Td> 571 ft (174 m) </Td> <Td> 577 ft (176 m) </Td> <Td> 577 ft (176 m) </Td> <Td> 246 ft (75 m) </Td> <Td> 600.0 ft (182.9 m) </Td> </Tr> <Tr> <Th> Average depth </Th> <Td> 62 ft (19 m) </Td> <Td> 195 ft (59 m) </Td> <Td> 279 ft (85 m) </Td> <Td> 283 ft (86 m) </Td> <Td> 483 ft (147 m) </Td> </Tr> <Tr> <Th> Maximum depth </Th> <Td> 210 ft (64 m) </Td> <Td> 748 ft (228 m) </Td> <Td> 925 ft (282 m) </Td> <Td> 804 ft (245 m) </Td> <Td> 1,333 ft (406 m) </Td> </Tr> <Tr> <Th> Major settlements </Th> <Td> Buffalo, NY Erie, PA Cleveland, OH Lorain, OH Toledo, OH Sandusky, OH </Td> <Td> Alpena, MI Bay City, MI Owen Sound, ON Port Huron, MI Sarnia, ON </Td> <Td> Chicago, IL Gary, IN Green Bay, WI Sheboygan, WI Milwaukee, WI Kenosha, WI Racine, WI Muskegon, MI Traverse City, MI </Td> <Td> Hamilton, ON Kingston, ON Mississauga, ON Oshawa, ON Rochester, NY Toronto, ON </Td> <Td> Duluth, MN Marquette, MI Sault Ste . Marie, MI Sault Ste . Marie, ON Superior, WI Thunder Bay, ON </Td> </Tr> </Table>

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