In case you hadn't noticed, the Chicago River flows backwards. It's been doing this for over a hundred years.
Like any normal river, the Chicago River used to flow into a larger body of water--namely, Lake Michigan. This became a problem in the middle of the 19th Century. As Chicago grew into a major city, the raw sewage of civilization was dumped into the river and flushed through to the lake. And the lake was where Chicagoans got their drinking water.
Besides being gross, this was dangerous. All those germs in the drinking water produced outbreaks of cholera or typhoid or other diseases.
You'll often hear the story of the Great Chicago Plague. It's said that cholera wiped out 70,000 people in a single year, about 20% of the city's population. Don't believe it. Someone cooked up the tale to make a point.
In any case, the solution to the pollution was simple. Just reverse the flow of the river so that it didn't empty into the lake.
Build a barrier at the east end of the Chicago River to block it off from the lake. At the same time, connect the west end to the Des Plaines River.