Once upon a time, there was a river. As rivers go, it was short and fat, and it didn't move very fast, because it had the misfortune to lie along Lake Michigan, where the slope gradient is less than a tenth of one percent. The total fall of the short, fat, river was less than five feet.
This did not matter, insofar as the river just oozed along, sloshing water back and forth with Lake Michigan in a very unorganized way. Sometimes the river flowed into the lake, and sometimes, when things were dry, the lake flowed into the river. It was one of those "open arrangements" that seem weird to more conventional minds, but it worked.
The people along the lake were nomadic sorts who didn't have very much use for rivers, so they left the short, fat river alone. Unfortunately, these were sent packing by some people who used rivers a lot, and those folks were busier than a hive of bees. They built houses and shops and granaries, and then they built abattoirs to chop up the animals that ate the grain. They noticed the short, fat river, and said, "Instant drainage." They dumped everything into it -- sewage, offal from the slaughterhouses, chemicals from the tanneries and soap makers whose businesses always happen alongside slaughterhouses. Because the people got their drinking water from the lake, through big intake cribs offshore, the short, fat river seemed unimportant.
The short, fat river could not do anything with this mess other than what it already did, which was to slosh back and forth with the lake. Sometimes the foulness from the river reached all the way out to the intake cribs. When that happened, people in the new city got sick. At one point, a sixth of the city's population died of cholera. Besides, it stank. No, it reeked. It produced so much gas from rotting goo that the south branch was called "Bubbly Creek."
Something had to be done, not to clean up the short, fat river, but to carry away all the filth that people wanted to dump into it. Because these people were rather typical of people everywhere, they decided to turn the short, fat river into a long, deep river going the other way. They dug a deep channel, hooked up the short, fat river to the Des Plains river, and watched with joy as Lake Michigan water flowed into the short, fat river and washed the stench and filth toward the Mississippi.
Oh, how the people downstream howled! How they complained about the smell and the disease! But there was nothing they could do, because it's not illegal to divert a river in Illinois, something that good children will be advised to remember. The people in the city rejoiced that the short, fat river was acting like a real river and actually flowing. They forgot that the gradient was so slight, and they forgot that silt and other goo will clog channels. It wasn't long before the short, fat river was just as horrid as before, only now it could foul water in two directions.
Then, one evil night, the city caught fire. The citizens on the north looked at the onrushing flames with dispassion. "The short, fat river will stop the fire before it gets to us," they said. They apparently believed that the short, fat river contained water. What it actually contained was a toxic, flammable stew that caught fire almost immediately. Now it was the northern side of the city's turn to howl, as soon as they rebuilt.
Moral: Don't mess with short, fat rivers.
Addendum: The Chicago river still flows west into the Des Plains, still carries a lot of waste that makes it unfit for animals or people, but the Friends of the Chicago River have gone a long way toward seeing in cleaned up, restricting development along its watercourse, and containing floodwater in huge reservoirs.