Managing Great Lakes water levels
Why water levels in the Great Lakes change, why it matters and how they’re being managed.
The Great Lakes
For Ontarians the Great Lakes are a vital source of:
- drinking water
- employment and economic opportunities (e.g. commercial fisheries, tourism, shipping, energy generation etc.)
- recreational opportunities (e.g. fishing, boating, cruising)
- important wildlife (mammals, fish, plants and birds) and wildlife habitats
Why water levels fluctuate
The water levels in the Great Lakes naturally vary, responding monthly, seasonally and annually to a variety of factors. They are usually lowest in the winter and highest in the middle of summer. Water levels are influenced by:
- natural factors, including precipitation, evaporation, winds, runoff from rivers or streams and inflow from upstream lakes
- human factors, including dredging, water withdrawals, water diversion and regulating structures such as dams
Natural fluctuations are essential for ecological function, but high and low levels may impact local economies and communities as well as local habitat and species.
High water levels may affect:
- shoreline alteration
- changes to wetland diversity
Low levels may affect:
- boating access
- private and municipal water supplies
- strand wetlands and habitat
Learn more about historical trends in lake levels
It’s important to ensure the Great Lakes are protected and available for future generations to enjoy.
The Great Lakes are international boundary waters, so managing them responsibly involves cooperation among many different parties.
Over the years, agreements have been signed between:
- Canada and the U.S.
- Ontario, Quebec and 8 U.S. Great Lakes states (Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania and Wisconsin)
Under these agreements:
- the International Joint Commission (IJC), created by the Boundary Treaty of 1909, the Canadian and U.S. governments have ultimate responsibility for managing water levels in the Great Lakes
- Ontario, Quebec and the 8 Great Lakes states notify and consult one another on major proposed water diversions, withdrawals or consumption (Great Lakes Charter 1985) and the regulation of water used within the Great Lakes Basin based on common environmental standards (Great Lakes St. Lawrence River Basin Sustainable Water Resources Agreement 2005)
Learn more about Ontario’s role in Great Lakes water management