The Great Lakes are a living, breathing system. They are dynamic and constantly changing, evolving over time. So, it makes sense that water levels continuously rise and fall and have been fluctuating dramatically since the formation of the Great Lakes.1
Not only do water levels change over centuries, but they also fluctuate between seasons and within a single day. Along the shoreline, the water level may appear to change quite significantly throughout the duration of a day. Most of these changes give the lakes the appearance of a tide, but are actually caused by seiches. Learn more about Great Lakes and their tide-like patterns.2
Annual or seasonal fluctuations in water level are primarily caused by precipitation and nearby runoff. Water levels in the lakes can increase drastically if there is a significant amount of precipitation or land runoff. They can decrease drastically with low levels of precipitation and increased evaporation. Lakes Superior, Erie, and Ontario have all set record-high water levels in 2019 and lakes Michigan and Huron may set record levels as well.3
In the spring months, the snow and ice begins to melt and drain into the lakes. By June and July, water levels are at their highest. When August hits, the weather warms along with the lakes’ surface water. This results in increased evaporation and water levels begin to fall once again.4
Naturally fluctuating water levels are a sign of a healthy lake. Wetlands benefit from periodic high and low water levels, which create conditions for healthier and more diverse plant habitats, and improve water quality so wildlife can thrive.5
The water levels within each Great Lake are determined by how much water flows in and out of the lake. These are influenced by long-term natural trends in precipitation and evaporation. The levels of lakes Superior, Michigan, Huron, and Ontario are also influenced by dams in the St. Marys River and St. Lawrence River that control the outflows from lakes Superior and Ontario. The International Joint Commission (IJC) regulates the outflows to balance how water levels affect upstream and downstream interests in Canada and the United States. To learn more about how water flows into the lakes, read about the Great Lake Watersheds and the IJC's regulation of Lake Superior and Lake Ontario.6
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