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Name: David
Status: Other
Grade: Other
Location: WI
Country: United States
Date: September 2007
Question:
If it is true that there is essentially no tide on the great lakes, why do the water levels fluctuate quite a bit during the day? I would estimate that in Door County, Wisconsin, it could be as much as 2-4 inches several times during the day.


Replies:
There is some disagreement about this topic, but many researchers describe the Great Lakes as having small but measurable tides. The tides described are in the range of a few centimeters, which is similar to the magnitude you have described in Door County, WI. That is a very small tide indeed, compared to the true tides experienced on marine coasts, and such small tides may often be invisible or masked by other phenomena such as wind. Here is a good reference:

http://www.great-lakes.net/teach/chat/answers/100100_tides.html

Christopher Perkins

David,

There could be several reasons for the fluctuations in water level, of which the most important may be wind. Wind drives water ahead of it, resulting in the water level being highest on the shore towards which the wind is blowing. If the wind speed is particularly high, the water level on one side of Lake Michigan could be several inches greater than on the other side.

Furthermore, if the wind direction shifts during the day, the water level will also change in response.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory

Many factors affect the water level in a lake. For the complex answer see:

http://www.cwr.uwa.edu.au/~adiyanti/NonDim.html

I have not done the calculation but my intuition says thermal expansion (heating) during the day would be insufficient to account for a 2-4 inch rise "several times during the day"-- is a key observation. What could vary in Lake Michigan on that time scale? My hunch is that it is wind pushing the water north, south, east, west. There are probably prevailing winds at any give season. The analogy is blowing across the surface of a cup of hot tea.

Vince Calder


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