Author: Sangyoon Lee

Program of Study: Chemistry PhD in the Physical Sciences Division

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The direction of the Chicago river has been reversed to prevent the pollution of Lake Michigan from the city’s sewage. The gigantic engineering project was a success, but at what cost? Here we focus on the reversal’s various environmental effects on the river’s ‘new downstream’.




(transcript was provided by the student and is unedited)

Welcome to the ELI’s Finding Chicago Global Perspectives Podcast Series for AEPP 2022. I’m your host, Sangyoon Lee. And I’m currently enrolled in the University of Chicago’s Physical Sciences Division. Today, we will be exploring the topic of the environmental effects of the reversal of the Chicago river. One of the biggest tourist attractions in downtown Chicago is the Chicago river cruise. Naturally, this is the first thing I did when I went to the downtown a few days after the arrival in Chicago. During the event, a river cruise guide explained about all the architectures and histories of downtown Chicago. And the one piece of information about the Chicago river that totally blew my mind was that the direction of the Chicago river has actually been reversed.

The boldness of the attempt and the sheer scale of the challenge in the engineering point of view is usually what surprises people when first encounter this piece of information. However, today we are going to turn our heads from the massive scale of the project itself, to the consequences that happened after the project.

A ‘river’ is not just an independently flowing big body of water. There is a whole ecosystem imbedded underwater, which is very sensitive to the environmental changes. Also, the river is connected to the other bodies of water. Its source, and its destination. With many factors at steak, the consequences of the reversing of the Chicago river was never ‘less complicated’ compared to the engineering problems of the reversal itself.

Before diving into the consequences, let me introduce ‘how’ and ‘why’ the reversal of the river’s flow was attempted in the first place.

After the big fire in 1871, the downtown Chicago was soon rebuilt, and the industry of Chicago grew rapidly. According to the Encyclopedia of Chicago, in 1880, the population of Chicago reached 500,000, and In 1910, it reached to almost 2.2 million and was continuing to grow. At this point, the Chicago river’s role was to drain out all the waste and pollution from the industry and from more than a million people. The destination of this drainage was Lake Michigan. A big problem here was that Chicago was using Lake Michigan as a source of drinking water. This was indeed a serious problem, because the sewage will eventually pollute the lake and it will most likely lead to a massive spread of waterborne illness.

So, the civic leaders of the Chicago at the time made a bold decision. The reversal of the Chicago river.

This giant engineering project can be called successful in many ways. The flow of the water was reversed, and the massive amount of sewage from Chicago was no longer flowing into lake Michigan.

However, this does not mean that the polluted water just disappears. Instead of flowing into Lake Michigan, the sewage was drained to Illinois river, to St. Louis, into the Mississippi river, and of course, to the Gulf of Mexico.

Naturally, draining the sewage to St. Louis resulted in the first pollution case tried in the U.S. Supreme Court. According to the US Supreme Court record provided by the Legal Information Institute of the Cornell Law School, the State of Missouri brought a lawsuit against the State of Illinois and the Sanitary District of Chicago to restrain the discharge of the sewage of Chicago through an artificial channel that empties into the Illinois river which empties to the Mississippi river at about 43 miles above the city of St. Louis. Unfortunately for the state of Missouri, there were no visible evidence of the additional pollution of the Illinois river available by the technology at the time. One possible evidence they had was that the typhoid fever has increased considerably around the time of the reversal. However, the scientific research around this problem, that is, the maximum traveling distance of the typhoid bacteria was not agreed between different research teams. Although it has been shown during the research that the sewage water released from Chicago river did indeed contain a higher number of bacteria, The case was denied because the State of Missouri have failed to give a solid proof that the pollution of Illinois river was increased because of the reversal of the Chicago river.

Despite the denial of the case, there exists clear negative effects on the downstream of the new flow of the Chicago river.

The reversal had a tremendous effect on the fish species in the Illinois river. As the Chicago river is connected to the Illinois river by the artificial canal, the Chicago’s sewage pollution slowly went downstream into the Illinois river. It is a known fact that polluted sewage, sewage treatment plants, and agricultural waste contains a lot of organic waste that serves as a nutrition source for algae. The uncontrolled thriving and increase of the algae coupled with the solid sediments from sewage will cause the sunlight transmittance to decrease. This leads to the ceasing of aquatic plants. And as the total amount of algae is dramatically increased, the amount of the ceasing algae also increases. The aerobic bacteria in the river uses up oxygen while digesting the remnants of the decaying algae. With the high oxygen consumption rate and low oxygen production rate due to the decrease of aquatic plants, the underwater oxygen concentration drops. Most fish cannot live in this ‘hypoxic’ zone, and it leads to a decrease in the number of the observed species.

 This effect does not stay within the Chicago river and Illinois river. The Illinois river flows into Mississippi river and the river flows to the Gulf of Mexico. In the Gulf of Mexico, there is one of the world’s largest hypoxic zone. According to the National Ocean Service(NOAA), the area of the hypoxic zone of the Gulf of Mexico is over 6000 square miles as measured in 2021.

Although one can easily say that there are many areas and cities that are contributing to the formation of this ‘dead zone’ in the Gulf of Mexico, it is said that the Chicago river is the largest contributor of the hypoxia according to Mark Templeton, director of the Abrams Environmental Law Clinic.

The Chicago river connecting to the Mississippi river has another significant meaning. The two of the world’s largest freshwater ecosystems were connected by the new canal. Unfortunately, this change allowed a new entrance for the invasive species like Asian carp to expand its territory from Mississippi river, up into the great lakes.

Currently, Chicago is trying to make up for the negative effects of the river’s reversal. Adding purification steps of the sewage before draining, planning to significantly reduce the amount of phosphorus output by 2030, and applying measures to prevent further intrusion of the invasive species. However, ecologists say that the negative impacts of the reversal are still far from being completely reversed.

Now, questions arise. Is it possible to eliminate the negative environmental impacts on the river’s ecosystem while the city should always produce a large amount of waste? Is it really an inevitable destiny of the rivers close to the big cities to be used for sewage draining? Or will the technological development allow us to get around the problem and conserve the aquatic ecosystem untouched by the pollutions? With these questions for you to think about, I’m closing this podcast here. Thank you for listening.



  • The Encyclopedia of Chicago –
  • The Supreme Court Record from Legal Information Institute of Cornel Law School –
  • Dead Zone in the Gulf 2021, NOAA –
  • Remark of Mark Templeton, Energy Policy Institute at the University of Chicago News –