Even though I've lived in the Bay Area for the last half-decade, I still love Chicago – where I was born and raised.
Chicago was growing fast in the late 1800s, from 5,000 to over 1.6 million residents. More and more waste was being dumped into the Chicago River which flowed into Lake Michigan – the source of the city's drinking water. People were becoming sick with waterborne illnesses.
The solution sounded simple but would take an engineering miracle: reverse the flow of the river so that it flowed into the Mississippi River and onto the Gulf of Mexico.
The first step was a series of canal locks to use gravity to move the water away from Lake Michigan.
It also meant a new 28-mile canal that connected the Chicago River and the neighboring Des Plaines River (which connects to the ultimate destination, the Mississippi Rivers). This canal would slope more downward the further it was from the Chicago River, causing an outward flow. This meant that 42 million cubic yards of dirt had to be moved. It required so many new techniques that the innovations became known as the Chicago School of Earth Moving (many of those techniques being useful for the construction of the Panama Canal built a few years later).
After the final dam was broken and the pipeline complete, the Chicago River started to reverse its course. It turned blue and bystanders even observed a large chunk of ice floating in it (from Lake Michigan).
Today, there's still some remnants of Chicago's trouble with waste water. On the South Branch of the Chicago River, there's a section called Bubbly Creek. It literally bubbles with gases that are emitted from the decomposition of blood and guts from the meatpacking businesses that dumped their waste there in the early 1900s (the same meatpacking businesses chronicled in Upton Sinclar's The Jungle).
Other fun facts: Missouri ended up suing Illinois after the Chicago River was reversed. They lost the suit.