The Great Lakes are an iconic freshwater system that makes up more than 20 percent of the world’s available surface freshwater. In Michigan, standing anywhere in the state, you are within 85 miles of one of the Great Lakes. We also have 3,288 miles of Great Lakes coastline, the most freshwater shoreline in the world.
The present day lakes are the result of glaciers advancing and retreating over thousands of years. A lake that is created by a glacier is referred to as a glacial lake. It is created by the large ice sheets eroding the land and then filling the spaces when the glacier melts. The ice sheets are on the Earth’s crust, which is “floating” on the Earth’s mantle, the molten layer beneath the Earth’s crust. The weight of the glaciers pushes the Earth’s crust down. As the glaciers retreat, the Earth’s crust slowly rebounds (centimeters per century). This process is known as isostatic rebound. The land that is eroded is deposited by the glaciers as they advance and retreat. These formations, known as moraines, make up the boundaries of the present day Great Lakes. This long process has resulted in various lake stages within the Great Lakes basin. Some of these prehistoric lakes include Lake Algonquin, Lake Nippising and Lake Chippewa.
Lake Algonquin was a large glacial lake that was present roughly 11,000 years ago when the Laurentide Ice Sheet was retreating north from the Great Lakes region. Lake Algonquin covered an area of approximately 100,000 square miles with maximum depths of 1,500 feet. Geologists have determined that the shoreline steadily rose as it moved north. Lake Nippising was created as the Wisconsin ice sheet receded north and represents a high point in the Great Lakes water levels. Lakes Michigan, Huron and Superior were combined into one body of water. Isostatic rebound raised the North Bay to levels of two previously existing lakes. Over 1,600 years, erosion slowly lowered the lake levels to present day Lake Superior. Lake Chippewa was a lower lake stage of Lake Michigan, created when the retreating ice sheet uncovered the St. Lawrence Seaway, a sea level outlet. It was the result of significantly lower lake levels, referred to as the Chippewa low levels. There is now a gorge that was carved by the Lake Chippewa outflow that lies submerged below the straits of Mackinac.
The present day Great Lakes have six quadrillion gallons of water, with only the polar ice caps and Lake Baikal containing more. Lake Superior has the deepest point, at a depth of 1,333 feet. The Great Lakes are also unique in that each lake’s basin is linked, forming a continuous drainage basin, the length of which is about 2,212 miles. This continuous path is important for shipping because it provides direct access to the Atlantic Ocean via the St. Lawrence Seaway.
Emily Shaw, Contributing Writer