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2 | Three types of water level fluctuations
Water levels on the Great Lakes change seasonally each year and can vary dramatically over longer periods. Short-term changes are generally of greater magnitude than the monthly averages.Short-term changes
Some water level fluctuations are not a function of changes in the amount of water in the lakes, but are due to winds or changes in barometric pressure. These short-term fluctuations, often due to storms or ice jams, can last from a couple hours to several days and can be very dramatic.
One such phenomenon, known as wind set-up or storm surge, occurs when sustained high winds from one direction push the water level up at one end of a lake, which makes the level drop by a corresponding amount at the opposite end. Changes in barometric pressure can add to this effect. When the wind abruptly subsides or barometric pressure changes rapidly, the water often oscillates until it stabilizes again. This oscillation is called seiche (pronounced "sayshe"). These events are most common on Lake Erie due to its east-west orientation in an area of prevailing westerly winds and its generally shallow western end.
See also: Great Lakes Facts and Figures
As the snow melts in the spring, runoff to the lakes increases. Evaporation from the lakes is also least in the spring and summer when the air above the lakes is warm and moist and the lakes are cold. With more water entering the lakes than leaving, water levels rise to their peak in the summer. In the early fall, evaporation and outflows again begin to exceed the amount of water entering the lakes.
The range of seasonal water level fluctuations on the Great Lakes averages about 12 to 18 inches from winter lows to summer highs. Seasonal rises begin earlier on the more southern lakes that experience a slightly warmer climate. Lake Superior, the northernmost lake, is generally the last to peak, usually in August or September.
Graphic: Lake profile showing wind set-up.