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Water levels on the Great Lakes

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.

Click to see larger image.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

Seasonal changes
In the fall and early winter, when the air above the lakes is cold and dry and the lakes are relatively warm, evaporation from the lakes is greatest. Consequently, water levels decline to their seasonal lows.

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.

Long-term changes
Long-term fluctuations occur over periods of consecutive years. Continuous wet and cold years will cause water levels to rise. Likewise, consecutive warm and dry years will cause water levels to decline. Over the last century, the range from extreme high to extreme low water levels has been nearly 4 feet for Lake Superior and between 6 and 7 feet for the other Great Lakes.

Graphic: Lake profile showing wind set-up.

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