Chicago is located in northeastern Illinois on the southwestern shores of Lake Michigan. It is the principal city of the Chicago metropolitan area (often called Chicagoland). The city's geography is defined by its relationship with the water. The Chicago River historically flowed into Lake Michigan, but in an incredible feat of modern engineering in 1900, the river's flow was reversed to send water south toward the Mississippi River basin, protecting the city's drinking water in the lake from sanitation runoff.
The city rests on the Chicago Plain, a flat and broad expanse that was once the bottom of ancient Lake Chicago. Because of its location at the intersection of the Great Lakes and the Mississippi River system, it occupies one of the most economically and strategically valuable geographic footprints in North America. Aside from the lakefront, the city's natural environment has been heavily urbanized, though it is punctuated by a sprawling network of parks, including Lincoln Park, Grant Park, and Jackson Park.