The lost West Fork
I never cease to be amazed by facts that I gradually discover about Chicago. The latest: there used to be a West Fork of the Chicago River on the southwest side of the city, running west from the South Branch at Damen Avenue, all the way to the city limits and beyond, finally emptying into the Des Plaines River near the towns of Lyons and Forest View. In the Rand McNally map from 1910 shown above, the West Fork is the dark curvy line above the parallel lines of the Sanitary and Ship Canal ("Drainage Canal") and the Illinois & Michigan Canal. The West Fork was basically made obsolete by the Sanitary Canal (which was fully completed in 1922), and was filled in during the late 1920s. The only obvious traces of the West Fork today are several diagonal property lines (between California Avenue and St. Louis Avenue) which once ran along the river's banks.
The West Fork's current obscurity is somewhat strange since, as Libby Hill notes here in The Chicago River: A Natural and Unnatural History, the fork is the reason Chicago first came to exist, as it was the only waterway link between the Great Lakes and the Mississippi River. Because of the fork, voyagers could travel almost entirely by water, with only a short overland portage required at the fork's western terminus, before continuing on to the Des Plaines, the Illinois River and ultimately the Mississippi.