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 the same name, including its center (the Chicago Loop). Though not especially long, the river is notable for being the reason why Chicago became an important location, as the link between the Great Lakes and the Mississippi Valley waterways. In the 19th century through civil engineering, the flow of the river was reversed to head toward the Mississippi River basin, away from Lake Michigan, into which it previously emptied. This was done for reasons of sanitation. 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.

Chicago (ⁱ/ʃɪˈkɑːɡoʊ/ or /ʃɪˈkɔːɡoʊ/) is the largest city in the US state of Illinois and the third most populous city in the United States after New York City and Los Angeles. The city has around 2.7 million residents. Its metropolitan area, sometimes called "Chicagoland", is the third largest in the United States, with an estimated 9.8 million people. Chicago is the county seat of Cook County, though a small portion also extends into DuPage County.

Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed. Today, Chicago is listed as an alpha+ global city by the Globalization and World Cities Research Network, and ranks seventh in the world on the 2012 Global Cities Index. The city retains its status as an international hub for finance, industry, telecommunications and infrastructure, with O'Hare International Airport being the second busiest airport in the world in terms of traffic movements. In 2008^[update], the city hosted 45.6 million domestic and overseas visitors. Among metropolitan areas, Chicago has the 4th largest Gross Domestic Product (GDP) in the world, ranking just behind Tokyo, New York City, and Los Angeles. Chicago is one of the most important Worldwide Centers of Commerce and trade.