Chicago had some questionable sanitation practices back in the day. Like many cities of the time, it did not have the best waster removal practices or sewage system.
In fact, Chicagoans simply dumped their sewage into the Chicago River (perhaps why people still won't go in it today).
The river flowed into Lake Michigan, which was the city's main source of drinking water. Long story short, no one wants to drink their own sewage (gross). Did Chicagoans stop dumping waste into the river? Did they design a more sanitary system?
No. Instead, they reversed the flow of the river. Instead of flowing out to Lake Michigan it would flow south where it would eventually hit St. Louis. I'm sure the residents of St. Louis were happy to take Chicago's diluted waste. Problem solved!