The Chicago River runs counterclockwise.

Chicago saw some of the world's fastest urban growth throughout the second half of the 19th century. It housed 299,000 people in 1870, and 1.7 million at the century's conclusion. But this population increase also had unfavorable side effects, like waterborne illnesses like cholera and typhoid. The main cause of the issue was that the city's sewage ran into the Chicago River, which then empties into Lake Michigan, the city's main water supply. In order to permanently fix the issue, Chicago turned to engineer Ellis S. Chesbrough, who designed the city's sewer system.

​

In the beginning, Chesbrough planned a 2-mile-long tunnel that would run 60 feet below Lake Michigan's surface in order to bring in cleaner water from further offshore. Unfortunately, it only took a significant downpour for this remote water supply to also become contaminated, prompting Chesbrough to look for an alternative. Chicago's water issue would be resolved if the city's named river could simply flow away from Lake Michigan and discharge into the waterways leading to the Mississippi. If the city dug a ditch lower than both the lake and the river through the gap, gravity would carry it from there.

 

The subcontinental divide immediately west of Chicago is what led the river to flow toward the lake.
The difficult operation of turning the Chicago River around was started by workers in 1892. On January 2, 1900, Chicago detonated the final dam after eight years of excavating (and under cover of darkness owing to escalating lawsuits from cities downstream). Chesbrough passed away in 1886, so he was unable to witness this amazing feat of human ingenuity, but his ambitious design rescued the city and ensured its prosperity well into the 20th century.