Chicago River, reversed

120 years ago engineers permanently reversed the flow of Chicago River.

Canal lock
National Archives and Records Administration / Public domain

Yesterday I wrote about the reversal of the Amazon – the flow of the river permanently flipping direction. The other notable example of this is not natural but artificial: the 1900 reversal of the river flowing through central Chicago.

In the 19th century, Chicago River was a problem for the growing city. The river was heavily polluted from industrial and human waste, and it carried that waste through the city and right into Lake Michigan. Lake Michigan was Chicago’s supply of clean water… so you can see the problem. Any reasonable civic leader would look at methods of reducing that pollution, but the solution in this case was an engineering project of unprecedented scope.

First, some geography. Chicago River has two branches – the North and South Branches – that join together in a (formerly) muddy plain to form the Main Stem. The Main Stem was the bit that drained into the lake. First, engineers needed to connect the South Branch to the nearby Des Plaines River, which leads south and eventually becomes part of the Mississippi. The Chicago Sanitary and Ship Canal, 45km long, was that connection.

(Side note: I’m glossing over a bunch of other canal projects that contributed to this reversal – the Chicago Sanitary and Ship Canal was the culmination of many massive earthworks to connect the Great Lakes and the Mississippi basin.)

The direction of the South Branch and the Main Stem were then reversed by setting up a series of locks along the route and adjusting the relative heights so that the water went in the opposite direction… and hey presto, Chicago River now permanently flows away from the lake.

It was not an easy path. At the time, the canal was the largest earthworks project in North America. A strike by the workers over insufficient pay ended when the state’s Nation Guard were called out and five people died. Legal questions over the amount of water coming out of Lake Michigan ended up in the US Supreme Court. But the river remains reversed to this day.

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