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Name: Brenda G.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 4/24/2003

Why do the water levels in the Great Lakes rise at the end of summer?


Most of the lakes actually peak in June or early July, most likely because of thunderstorms. Thunderstorms occur in the Spring and early summer and contribute a large part of the annual precipitation in the upper Midwest. The soil cannot absorb all of this rain and so surface runoff from these storms ends up in the lakes.

Lake Superior is an exception; its level tends to peak later, in September or October because it is at a higher latitude. In higher latitudes it is colder, the storms come later and thus the runoff into the lake occurs later.

A good web site to look at for plots of the lake levels is

It shows a comparison of the levels this year with last year under "Water Level Plots" and the long-term water levels (1900 on) in the Hydrographs.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

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