For years, Chicago's growing population got rid of waste through the river, which flowed into Lake Michigan, the city's source of water. Especially during heavy rains, when the watershed would overflow, the filthy water would contaminate drinking water, killing many people (at times, "more than five percent of the population" in a year).
Congress authorized plans to reverse the flow in 1822, and construction began in 1836, the year before Illinois went bankrupt due to economic depression.
In mid 1885, heavy rains resulted in 12% of the Chicago population dying from waterborne illness.
The reversal was completed at 1:00pm on Jan 1, 1990, and remains today, using a series of canals and locks.