Why the Low Lake Levels?

Overall, the Great Lakes had 4% above average precipitation this year through August, according to the U.S. Army Corps of Engineers. So what caused this summer’s low water levels on the Great Lakes?

The principal reasons may be a combination of factors that are very hard to measure. The Great Lakes Basin had some prolonged periods of below average precipitation, such as April-December in 1997 and 1998, which probably accounts for some of the decline.

In 1998, El Nino is thought to have caused a milder winter, warmer waters, much less ice cover, and much less snow accumulation than average, which causes much greater than average evaporation of lake water.

Evaporation of lake water is tough to measure and is typically assumed to be the difference between the water flowing into and out of the system and the change in storage within the lakes that causes the rise or fall of lake levels. Evaporation is the large value that reconciles the hydrologic balance sheet on each lake. All of the lakes continue in their seasonal decline as cool air blowing over warmer waters hastens evaporation.

The low water levels continue to cause problems for commercial vessels engaged in the rush before the December closings of the locks and for some late season recreational vessels and marinas.

For more information, see the Lake Level Alert posted by UW Sea Grant Coastal Engineer Phil Keillor

Philip Keillor
Coastal Engineering Specialist

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Last updated 28 March 2000 by Karl
All contents copyright 1999 University of Wisconsin Sea Grant Institute

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