The Great Lakes are a group of five large lakes in North America at the US and Canadian borders. These five lakes have 246,744 km2 (they are the largest freshwater area in the world and they hold around 20% of the total freshwater area on Earth). The Great Lakes include lakes: Superior or Upper Lake, Michigan, Huron, Erie, and Ontario. Only Michigan Lake is located across the USA. Their origins are associated with Pleistocene and post-glacial tectonic movements. All the lakes except Erie are depressurized. In the winter period, parts are frozen. The lakes are interconnected by watercourses, canals in addition to strains. With the river St. Lawrence they are are connected to the Atlantic Ocean. After the construction of the waterway St. Lawrence (in 1959), it was possible to go by boats through the ocean up to Duluth (at Superior); the total length of the waterway is 1873 km.
The lakes link to agriculture and mining areas with the mining areas of developed centers, with big cities (Chicago, Detroit, Cleveland, Toronto etc.) and export ports to the east coast. Iron ore and wheat are transported. The total length of the coast is 15,450 km (8,700 km belongs to the USA and 6,750 km to Canada). Large hydro power plants are built between Lake Erie and Ontario at the river side Niagara Falls. The lakes have been endangered lately and have become very polluted because of the release of harmful substances and waste water. That has indeed jeopardized the fish population. In 1787 lakes were declared as national sailing waters. The first steamship lines were introduced in 1818 at Lake Erie. Channels connecting lakes to each other, and connecting with New York (Atlantic), Hudson and the Mississippi-Ohio water system, were built in the period 1825-55.
The Great Lakes age is not yet definitely determined. It is estimated to range from 7,000 to 32,000 years. The water began to fill the glacial pool as soon as the ice receded, some 14,000 years ago. It is generally accepted that Lake Erie reached the present level about 10,000 years ago, Lake Ontario about 7,000 years ago, and the Huron, Michigan, and Superior lakes some 3,000 years ago. This configuration of the Great Lakes Basin is the result of the movement of huge glaciers through the middle of the continent, which is a process that began a million years ago.