The Great Lakes, bodies of fresh water with many features, are connected to each other and to the world ocean
- The Great Lakes are a dominant physical feature of North America and form part of the political boundary between the United States and Canada.
- The Great Lakes system includes five Great Lakes (Superior, Huron, Michigan, Erie and Ontario), Lake St. Clair and the connecting channels, along with many harbors and bays. Each lake has distinctive basin features, circulation and ecology.
- The Great Lakes contain nearly 20 percent of the world’s fresh surface water and have a coastline longer than the East coast of the United States. Most of North America’s fresh surface water (95%) is in the Great Lakes.
- The Great Lakes, their respective watersheds and waterways, and the ocean are all connected. Within the Great Lakes system, water flows from Lake Superior and Lake Michigan to Lake Huron, through Lake St. Clair into Lake Erie, over Niagara Falls and into Lake Ontario before flowing through the St. Lawrence River into the ocean. Rivers and streams transport nutrients, dissolved gases, salts and minerals, sediments and pollutants from watersheds into the Great Lakes.
The Great Lakes are an integral part of the water cycle and are connected to the region’s watersheds and water systems. Changes in water systems affect the quality, quantity and movement of water, including retention time.
- Water currents circulate within the Great Lakes and are powered by wind, waves, energy from the sun and water density differences. The shape of a lakebed and its geographic orientation, the direction of the prevailing winds, the shores and the structures along the shores influence the path of circulation. Circulation between the lakes is driven by gravity.
- Lake level is the height of the Great Lakes relative to sea level. Lake level changes are caused by variations in precipitation, evaporation, runoff, and snow melt, as well as wind and waves. While tides are typically not discernable in the Great Lakes, seiches are common in the Lakes.
- The Great Lakes stratify in the summer and in winter under ice cover, forming distinct layers based on water temperature differences. Turnover occurs in the spring and fall when cooler weather minimizes temperature differences and the layers mix. Turnover is the main way that oxygen and nutrient-poor water in the deeper areas of the lakes can be mixed with oxygen and nutrient-rich surface water.
- Although the Great Lakes are large, they are finite and their resources are limited.
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