The Great Lakes-St. Lawrence River system is a dynamic environment, still evolving over time. Ever since the last glaciers retreated more than 10,000 years ago, Great Lakes water levels and outflows have varied dramatically. The Great Lakes affect human activities and all aspects of the natural environment, from weather and climate, to wildlife and habitat. Unlike oceans, where tides are constant and predictable, water levels on the Great Lakes can vary significantly in frequency and magnitude making them difficult to accurately predict.

A joint publication of the U.S. Army Corps of Engineers and the Great Lakes Commission, which provides a comprehensive overview of hydrologic and hydraulic factors affecting the Great Lakes – St. Lawrence River watershed and human uses of the resource.