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 because it is one of the reasons for Chicago's geographic importance: the related Chicago Portage is a link between the Great Lakes and the Mississippi River Basin, and ultimately the Gulf of Mexico.
The river is also noteworthy for its natural and human-engineered history. In 1887, the Illinois General Assembly 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, partly in response to concerns created by an extreme weather event in 1885 that threatened the city's water supply. In 1889, the Illinois General Assembly created the Chicago Sanitary District (now the Metropolitan Water Reclamation District) to replace the Illinois and Michigan Canal with the Chicago Sanitary and Ship Canal, a much larger waterway, because the former had become inadequate to serve the city's increasing sewage and commercial navigation needs. Completed by 1900, the project reversed the flow of the main stem and South Branch of the Chicago River by using a series of canal locks and increasing the flow from Lake Michigan into the river, causing the river to empty into the new canal instead. In 1999, the system was named a "Civil Engineering Monument of the Millennium" by the American Society of Civil Engineers (ASCE).The river is represented on the Municipal Flag of Chicago by two horizontal blue stripes. Its three branches serve as the inspiration for the Municipal Device, a three-branched, Y-shaped symbol that is found on many buildings and other structures throughout Chicago.