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The City of Chicago Is Sinking. Here’s Why

Ice sheets that receded 10,000 years ago are responsible for the Windy City dropping at least four inches over the last century

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smithsonian.com

The city of Chicago is sinking, geologically speaking.

Tony Briscoe at The Chicago Tribune reports that the Windy City and all of the towering structures built on its iconic skyline are at least four inches lower than they were a century ago. In the next 100 years, the city will continue sinking at the same rate.

While that might seem like a small drop compared to the height of the city’s skyline, it’s enough to redraw flood plains and interfere with the flow of sewer lines, among other issues. The sinking amounts to just a fraction of an inch per year, but eventually, that adds up. In fact, Chicago is sinking at the same rate as the city of Venice, Italy, which has concerned city planners there for years.

“[O]ver a decade that’s a centimeter. Over 50 years, now, you’re talking several inches,” Daniel Roman, chief geodesist at NOAA, tells Briscoe. “It’s a slow process, but it’s a persistent one.”

So why exactly is the Windy City also the Sinking City? Blame it on the glaciers. The sinking is a belated effect from the massive ice sheets that once covered much of North America, the last of which, a two-mile thick monster, receded 10,000 years ago. The Earth’s crust isn’t quite as rigid as one might infer. Resting on layer of molten mantle, extra weight, like a lake, ocean or megatons of ice, can warp the crust. When that weight is lifted, the crust springs back and reorients itself in a process that can take thousands of years.

Soon after the ice sheets lifted, the geographic area that is now Chicago experienced a rebound. But more recently, the land is sinking as the subterranean mantle flows back to parts of Canada, where the ice sheet was heaviest and the mantle thinned out the most. This has created a line across the northern portion of North America. North of that line, the land is rising. South of that line, the land is sinking.

That line passes through the northern third of Lake Michigan and Lake Erie. meaning the northern lake beds are slowly rising and therefore, getting shallower, while the southern parts are sinking, giving the lakes a tilt that make water levels rise. Because of this, Milwaukee is expected to see lake levels rise 5.5 inches over the next century and Cleveland could experience 4.4 more inches along its lakefront.

Chicago is not alone in having that sinking feeling. The post-glacial rising and falling of the Earth’s crust, called isostatic adjustment, is happening along huge swathes of the Midwest and Eastern United States, especially along what was once the southern edge of those massive, heavy glaciers. That’s due to something called the forebulge. By the latest estimates, Washington, D.C. and the Chesapeake Bay are expected to subside about six inches due to this forebulge collapse.

As glaciers in some parts of the world melt, this is playing out in real time and that’s causing parts of Alaska, Russia, Iceland and Sweden to lift up as well, reports Jenny Chen for Smithsonian.com. It’s also possible, Chen reports, that the shifting caused by the long-gone ice sheets could lead to more volcanic activity and earthquakes in some parts of the world.

For Chicago, there’s not much the city can do to stop the Earth’s crust from doing its thing. But if any city can engineer its way out of trouble it’s Chi-Town, which was built on land just a few feet above the water table. In the 1850s and 1860s, city planners literally jacked it up by about 4 to 14 feet to make a new street level that would allow builders to add stormwater drains and a sewage system. Ironically, that sewage system, which was cutting edge in its day, led to another massive project. Beginning in 1900, the city reversed the flow of the Chicago River so that it would carry the city’s wastewater away from the Lake Michigan shore and its drinking supply, a project that some people now recognize as an environmental boondoggle. And then there’s the Tunnel and Reservoir project, which has been ongoing since 1975 and has created 109 miles of massive sewers beneath the region to deal with billions of gallons of waste and floodwaters.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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