Chicago’s biggest design achievement probably isn’t one of its amazing skyscrapers, but the Chicago River, a waterway disguised as a remnant of the natural landscape. But it isn’t natural, not really. It’s hard to tell when you see the river, but it’s going the wrong way. It should flow into Lake Michigan, but instead fresh water from Lake Michigan flows backwards, into the city. The Chicago River is, in large part, a carefully-designed extension of the city’s sewer system. Reporter Dan Weissmann talked with Richard Cahan (author of “The Lost Panoramas: When Chicago Changed its River and the Land Beyond”) about the amazing lengths the city went to, over the course of several decades, to carry away the sewage that threatened to drown Chicago.