Lake Superior, Lake Huron, Lake Michigan, Lake Ontario, and Lake Erie make up the Great Lakes system. The Great Lakes contain the largest freshwater supply in the world, holding approximately 18% of the world's freshwater supply and 84% of North America's freshwater supply. They are located along the southern border of the Canadian Shield and cover more than 94,000 square miles (245,000 square kilometers). Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Like five huge lakes, they are the basis for billions of dollars in manufacturing, shipping, trade, fishing, forestry, agriculture, energy and tourism. The Great Lakes Basin supports a diverse ecosystem and job opportunities essential to the region's sustainability. How were the Great Lakes formed? The five Great Lakes exist because of ice and glacial movement. More than a million years ago, huge glaciers covered the earth. Their weight depressed the land where rivers existed before glaciers moved southward from the North Pole. About 20,000 years ago, as the Earth's climate warmed, the last continental ice sheet retreated northward. The glacier was up to 2 miles thick and was so powerful and heavy that it scraped the earth's surface creating lake basins. The melt water from the retreating glacier filled the created basins. About 3,500 to 4,000 years ago, the Great Lakes reached modern levels and area. The Lake System: In addition to the five interconnected lakes, the Great Lakes system also includes Lake St. Clair. Lake St. Clair is a small lake located between Lake Erie and Lake Huron. Connecting these lakes are the St. Marys River, the St. Clair River, the Detroit River, and the Niagara River. The St. Marys River flows 60 miles from Lake Superior to Lake Huron. Connecting Lake Huron to Lake Erie are the St. Clair River and the Detroit River. Together, these two rivers flow for 89 miles between the two lakes. The Niagara River flows from Lake Erie to Lake Ontario. According to the 1985 Great Lakes International Data, the average elevation in the Great Lakes ranges from 597 feet (182 meters) on Lake Superior to 242 feet (74 meters) on Lake Ontario. Please note: this is just an example. Get a custom paper now from our expert writers. Get a Custom Essay Why is this important? Why worry? Recently, water levels in the Great Lakes have declined. This is due to increased evaporation caused by warmer than normal temperatures. Milder winters and below-average snowfall have also contributed to the lake's declining water levels. We must act quickly on this issue as 40 billion citizens of Canada and the United States depend on the Great Lakes as a direct source of drinking water. Water is one of the most important substances on Earth, as plants, animals and humans depend on it to survive. So, it not only affects us, but also the ecosystem and biodiversity around us.