Introduction: Lake Superior, Lake Huron, Lake Michigan, Lake Ontario and Lake Erie make up the Great Lakes system. The Great Lakes contain the largest supply of freshwater in the world, holding approximately 18% of the world’s supply of freshwater, and 84% of North America’s supply of freshwater. They are located along the southern boundary of the Canadian Shield and cover more than 94 thousand square miles (245 thousand square kilometres). As five massive lakes, they are the foundation for billions of dollars in manufacturing, shipping, trade, fishing, forestry, agriculture, energy, and tourism. The Great Lakes basin support a diverse ecosystem and job opportunities that are essential to the sustainability of the region. History; How were the Great Lakes formed?: The five Great Lakes exist because of ice and glacial movement. More than a million years ago, enormous glaciers covered the land. Their weight depressed the soil where rivers had existed before the glaciers crept southward from the North Pole. About 20 thousand years ago, when the Earth’s climate warmed, the last glacial continental ice sheet retreated toward the north. The glacier was up to 2 miles thick and it was so powerful and heavy that it scraped the Earth’s surface to create lake basins. Meltwater from the retreating glacier filled the created basins. Roughly 3,500-4,000 years ago, the Great Lakes reached their modern levels and area. The Lake System: In addition to the five interconnecting 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 St. Marys River, St. Clair River, Detroit River and Niagara River. St. Marys River flows for 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. Based on the International Great Lakes Datum of 1985, the average elevation in the Great Lakes range from 597 ft (182 metres) on Lake Superior to 242 feet (74 meters) on Lake Ontario.Why is it Important? Why Care? Recently, the water levels of the Great Lakes have been declining. This is due to greater evaporation caused by warmer than normal temperatures. Milder winters and lower than average snowfall have also contributed to the lake’s declining water levels. We need to take action on this issue soon as 40 billion citizens of Canada and US depend on the Great Lakes as a direct source for drinking water. Water is one of the most important substances on Earth, as plants, animals and human depend on it to survive. So, not only is it affecting us, but also the ecosystem and biodiversity around us.