The Chicago River is a system of rivers and canals with a combined length of 156 miles (251 km) that runs through the city of Chicago, including its center (the Chicago Loop). Though not especially long, the river is notable for being a reason why Chicago became an important location, with the related Chicago Portage being a link between the Great Lakes and the Mississippi Valley waterways and eventually the Gulf of Mexico.

The River is also noteworthy for its natural and man-made history. In 1887, the Illinois General Assembly, partly in response to concerns arising out of an extreme weather event in 1885 that threatened the city's water supply, decided to reverse the flow of the Chicago River through civil engineering by taking water from Lake Michigan and discharging it into the Mississippi River watershed. In 1889, the Illinois General Assembly created the Chicago Sanitary District (now The Metropolitan Water Reclamation District) to replace the Illinois-Michigan Canal, which had become inadequate to carry the city's increasing sewage and commercial navigation needs, with the Chicago Sanitary and Ship Canal, a much larger waterway. The District completed this man-made hydrologic connection between the Great Lakes and Mississippi watershed in 1900 by reversing the flow of the Main Stem and South Branch of the river using a series of canal locks, and increasing the river's flow from Lake Michigan, causing it to empty into the new Canal. In 1999, this system was named a 'Civil Engineering Monument of the Millennium' by the American Society of Civil Engineers (ASCE). The river is also noted for the local custom of dyeing it green on St. Patrick's Day.

The river is memorialized, in part, by two horizontal blue stripes on the Municipal Flag of Chicago. The river also serves as inspiration for one of Chicago's ubiquitous symbols: a three-branched, Y-shaped symbol (called the municipal device) is found on many buildings and other structures throughout Chicago; it represents the three branches of the Chicago River.

en.wikipedia.org/wiki/Chicago_river