In 1900, Chicago made its river flow backward to keep sewage out of Lake Michigan, redirecting the water toward the Mississippi Basin instead

Imagine your city drawing its drinking water and sewage from the same river. That was Chicago’s problem in the late 1800s. The Chicago River flushed the city’s waste into Lake Michigan, the city’s sole source of drinking water. According to the Metropolitan Water Reclamation District of Greater Chicago (MWRD), a fast-growing population and fear of disease pushed officials to act. Their fix was bold. They decided to reverse the flow of an entire river.

A city poisoning its own water
By the late 1800s, Chicago was one of the fastest-growing cities in the world. The more people there were, the more waste there was, and that waste had no place to go but the river. According to WTTW Chicago's account of the reversal, the city was battling typhoid, cholera, and other waterborne illnesses. A bigger sewer system wasn’t sufficient. Engineers had to send the waste far away from the lake, for good.

How do you turn a river around?
Rivers don't turn around on command. But Chicago is near a low ridge. That ridge separates the Great Lakes basin from the Mississippi River basin. Engineers had a simple idea. Dig a deep ditch towards the Des Plaines River, and gravity would take the water the other way.


That channel became the Chicago Sanitary and Ship Canal. According to MWRD's own history of the project, the excavation took a lot of people and equipment, working in tough conditions. Chicago Sun-Times op-ed by MWRD president Kari K. Steele, chief engineer Isham Randolph later applied what he learned here to help build the Panama Canal.

According to a Chicago Sun-Times op-ed by MWRD president Kari K. Steele, workers officially finished the reversal on January 2, 1900. That day the last barrier was broken, and almost overnight the current of the river changed. It no longer drained into Lake Michigan. Instead it went along the new canal to the Des Plaines River, then the Illinois River, and finally the Mississippi.

The neighbors were not happy
Chicago fixed its problem, but created another one downstream. St. Louis filed suit over the project on January 17, 1900, backed by the state of Missouri, according to WTTW Chicago’s history of the reversal. That was just two weeks after the river changed course. Leaders there worried a new flow could threaten their water supply.

The case went to the U.S. Supreme Court and took years to settle. The Court agreed with Illinois and the Sanitary District of Chicago in 1906, and the river continued to flow in reverse. It was one of the first big water battles between states in the United States.

A surprise problem, over a century later
But here’s the twist. It still matters today if you live near the Great Lakes. The canal that saved Chicago's drinking water also tied together two river systems that nature had kept apart for thousands of years: the Great Lakes basin and the Mississippi basin.

That link works both ways. Fish are now able to swim between what used to be separate basins. According to a University of Illinois study covered by ScienceDaily, invasive Asian carp are moving north through this waterway, heading for Lake Michigan and the rest of the Great Lakes. Researchers pinpointed 18 risky spots along the canal where invasive species could enter. Since then, electric barriers have been installed in the water to help keep the carp out.

So the fix that once protected Chicago's drinking water is now part of a new problem. Engineers are fighting a very different, modern threat.

Why this old story still feels new
This is easy to dismiss as odd old trivia. But it’s really a story about trade-offs. Chicago solved a public health emergency with one bold, expensive project. That project saved lives, but it also altered the water map of North America in ways no one could have anticipated at the time.

American cities face their own hard choices today. Pipes are aging. Floods are getting worse. New pollutants, like microplastics and PFAS, often called “forever chemicals,” are also showing up in water supplies. As MWRD itself reflects on the project, these modern problems call for the same bold thinking that once turned a river around.