As Chicago sinks, some Great Lakes rise, others fall

CHICAGO – The sightlines at Wrigley Field, the panorama from Navy Pier, the vantage points at the Adler Planetarium observatory — all structures built more than 100 years ago — are at least 4 inches lower now.

In the northern United States and Canada, areas that once were depressed under the tremendous weight of a massive ice sheet are springing back up, while others are sinking. The Chicago area and parts of southern Lake Michigan, where glaciers disappeared 10,000 years ago, are sinking about 4 to 8 inches each century.

One or 2 millimeters a year might not seem like a lot, but "over a decade that's a centimeter. Over 50 years, now, you're talking several inches," said Daniel Roman, chief geodesist with the National Oceanic and Atmospheric Administration. "It's a slow process, but it's a persistent one."

While Chicago's dipping is gradual, this dynamic could eventually redefine flood plains and work against household sewer pipes that slope downward to the sewer main.

The greatest effect of this imperceptible phenomenon likely won't be inland, however. The contour separating the part of the continent that is rising from that which is falling bisects the Great Lakes.

In Lake Michigan, that line passes from Death's Door in Wisconsin (the strait linking Lake Michigan to Green Bay) to Grand Traverse Bay in Michigan, signaling that the far northern end is rising while the rest of the lake is lowering. Over time, that has created a tilting effect, generally translating into higher lake levels for the southern end of the lakes and lower watermarks for the northern shorelines.

The slow-motion seesaw effect is also happening in other cities on the southern end of the lakes: Cleveland could see a 4-inch swell in Lake Erie's levels and Milwaukee is projected to see a rise of 5½ more inches. Places such as Canada's Hudson Bay, which was covered by glaciers up to 9,800 feet thick, are rising up to 3 feet per century.

When the hefty ice sheet melted and the pressure was lifted, the mantle began to flow back to areas where it had thinned out.

Chicago, on the fringe of the last glacial intrusion, rebounded for a time after the glaciers disappeared. But now the city is slowly submerging because more of the mantle is returning to Canada, where the thickest part of the ice sheet bore down on the ­landscape.

"We can still see the land moving now even though the ice came off Chicago 10,000 years ago," said Seth Stein, professor of geological sciences at Northwestern University.

This shift will also affect the movement of water between the lakes. In Lake Superior, the eastern end that lets out into lakes Michigan and Huron is rising, while the far western edge is slumping. Researchers suggest that could slow Superior's outflow while causing lake levels to rise 10 inches per century near Duluth.

"If you're tilting one direction, the water flows might change direction or water might accumulate in a way different than you expected in the past," the NOAA's Roman said. "That's important for on-land and near-shore environments. You might get more water, but not where you want it."