Great Lakes

Great Lakes

The Great Lakes are a system of five large freshwater lakes in central North America—Lake Erie, Lake Huron, Lake Michigan, Lake Ontario, and Lake Superior—that drain into the Atlantic Ocean via the St. Lawrence Seaway. Combined, the Great Lakes constitute the largest surface area of unfrozen fresh water in the world: 94,850 mi² (245,660 km²), an area larger than the United Kingdom. Except for Lake Michigan, which is wholly contained in the United States, the Great Lakes form a natural segment of the U.S.-Canadian border.

Lake Superior is the largest of the five lakes by almost 10,000 mi² (41,682 km²), and has the greatest average (and maximum) depth. As a result, Lake Superior contains slightly more water than all the other Great Lakes combined—almost 3,000 mi³ (12,504 km³). The deepest parts of all the Great Lakes except Lake Erie are below sea level; in Lake Superior's case, over 600 ft (183 m) below.

Lake Superior has an average depth of 487 ft (148 m), a maximum depth of 1,302 ft (397 m) and covers 31,820 mi²(82,413 km²). Lake Huron has an average depth of 195 ft (59m), a maximum depth of 750 ft (229 m), and covers 23,010 mi² (59,596 km²). Lake Michigan, covering 22,400 mi²(58,016 km²), has an average depth of 276 ft (84 m), and a maximum depth of 923 ft (281 m). Lake Erie has an average depth of 62 ft (19 m), a maximum depth of 210 ft (64 m), and covers approximately 9,930 mi² (25,719 km²). Lake Ontario, the smallest of the Great Lakes in terms of surface area (7,520 mi²/19,477 km²), has an average depth of 62 ft (19 m) but reaches a maximum depth of 778 ft (237 m).

The Great Lakes drain 295,800 mi² (766,118 km²) of watershed (counting the surfaces of lakes themselves), or about 3% of the continent. Half the water entering the lakes evaporates; the rest flows from lake to lake, west to east, until it reaches Lake Ontario and then the St. Lawrence River.

By geological standards, the Great Lakes formed very recently. Prior to the beginning of the ice ages of the Pleistocene Epoch—about 1 million years ago—river valleys drained through the areas now occupied by the five lakes. As the ice-sheets flowed southward they favored these preexisting channels, scouring them and so increasing their depth. The latest glacial episode was the Wisconsin Glaciation , which ended about 18,000 years ago. When melting removed the glacier's enormous weight, the land began to rise. (It is still rising, at about .12 in / 3 mm per year.) This rising of the land, along with deposition of glacial sediments (moraines ), blocked all drainage from the Great Lakes area except eastward via the St. Lawrence. Lake Superior is the only Great Lake not formed by glacial scouring and deposition of moraines. Lake Superior's basin, although somewhat enlarged by glacial scouring, is the trough of a V-shaped fold in the rock termed a syncline .

But the Wisconsin glaciation did not simply advance to a most southerly limit, then retreat in an orderly way. It advanced and retreated several times over thousands of years in a three-steps-northward, two-steps-southward fashion. These oscillations partially uncovered and recovered the Great Lakes basins, forming a series of lakes corresponding partly to the modern ones. At one point, a superlake submerged what are today the basins of Superior, Michigan, and Huron. The history of these fluctuations can be traced primarily by the many abandoned beaches that are today found far above water level (often hundreds of feet above). Each abandoned beach records a lake stage or period during which the water level was stable long enough to form a beach. From these and other data, it is known that Erie reached its present level about 10,000 years ago; Ontario about 7,000 years ago; and Superior, Michigan, and Huron only about 3,000 years ago.

Human activity has significantly altered the chemistry and ecology of the lower four lakes, which are ringed by such cities as Buffalo, Chicago, Cleveland, Detroit, Gary, Milwaukee, Rochester, Toledo, and Toronto. Sewage and industrial effluents have burdened these lakes increasingly for over a century. (Chicago and several other cities, however, now divert their sewage southward, away from the lakes.) Lake Superior has been less affected by pollution, having no major settlements on its shores.

Another detrimental side-effect of human activity is the introduction into the lake ecosystem, both deliberate and accidental, of non-native species. The sea lamprey (1930s), alewife (probably 1940s), and zebra mussel (1980s) have been particularly destructive to the native lake fauna. Alewives are now the most abundant fish species in the lakes. They suffer intermittent mass die-offs, wash up on the beaches by the millions, and must be removed using bulldozers and trucked away.

See also Drainage basins and drainage patterns; Glacial land-forms; Syncline and anticline; Water pollution and biological purification