Shifting land caused by the melting of Canadian glaciers causes Chicago to sink at the rate of about a millimeter a year, a Northwestern University study has found.
The shift also causes water in the Great Lakes to slosh from the upper Great Lakes into their lower reaches.
The study was done by researchers at Northwestern University with collaborators around the U.S. and Canada. It was presented Wednesday at a joint meeting in Montreal of the American and Canadian Geophysical Unions.
Scientists used 10 years of readings from global positioning satellites at more than 200 points across North America to determine the shift.
"All of Canada's going up," said Seth Stein, a Northwestern professor of geological sciences who helped organize the study. "The U.S. is going down."
The reason lies far beneath the Earth's surface, in a mantle of semi-molten rock. More than 20,000 years ago, the weight of glacial ice sheets created depressions in the Earth.
When the ice began to melt around 12,000 years ago, the land returned to its original shape, forcing some areas to sink and others to rise, like a seesaw, scientists said.
Regions south of the U.S.-Canadian border are generally sinking, while regions north of the border are generally rising, the study found.
At the same time, water has been tipped toward Chicago beaches, as well as rivers and marshes all over the northern United States.
"Water is moving from the Canadian side, slowly but surely, to the U.S. side," said Chuck South of Environment Canada, which monitors water levels in the Great Lakes. "Over a century, it's got quite an effect."