The Chicago River is a river that runs 156 miles and flows through Chicago, including the downtown. Though not especially long, the river is notable for the 19th century civil engineering feats that directed its flow south, away from Lake Michigan, into which it previously emptied, and towards the Mississippi River basin. This was done for reasons of sanitation. The river is also noted for the local custom of dyeing it green to commemorate St. Patrick's Day.

Geography Originally, the river flowed into Lake Michigan. Its course jogged southward from the present river to avoid a baymouth bar, entering the lake at about the level of present day Madison Street. Today, the Main Stem of the Chicago River flows due west from Lake Michigan, past the Wrigley Building and the Merchandise Mart to Kinzie Street, where it meets the North Branch of the river. The North Branch is formed by the West Fork, the East Fork (also known as the Skokie River) and the Middle Fork, which join into the North Branch at Morton Grove, Illinois. From downtown, the river flows south along the South Branch, and into the Illinois and Michigan Canal and Chicago Sanitary and Ship Canal. From there, the water flows into the Des Plaines River and eventually reaches the Gulf of Mexico.

Jean Baptiste Pointe du Sable, the Founder of Chicago, was the first non-Native American to establish a permanent residence near the Chicago River. He built his farm on the northern bank at the mouth of the river in the 1780s. In 1808, Fort Dearborn was constructed on the opposite bank on the site of the present-day Michigan Avenue Bridge.

At one time, and as late as 1830, the north branch of today's Chicago River was known locally as Guarie’s (or Gary's) River. Guarie is a phonetic spelling of the name of an early settler/trader by the name of Guillory, who lived along the Chicago river sometime around 1778.

Early improvements In the 1830s and 1840s, considerable effort was made to cut a channel through the sandbar to improve shipping. In 1900, the river's flow was reversed in order to keep Lake Michigan clean. In 1928, the South Branch of the Chicago River between Polk and 18th Street was straightened and moved 1⁄4 miles west to make room for a railroad terminal.

Originally, the river flowed into Lake Michigan. As Chicago grew, this allowed sewage and other pollution into the clean-water source for the city. This contributed to several public health problems, including some problems with typhoid fever. Starting in the 1850s, much of the flow was diverted across the Chicago Portage into the Illinois and Michigan Canal. In 1900, the Sanitary District of Chicago, then headed by Rudolph Hering, completely reversed the flow of the river using a series of canal locks, and caused the river to flow into the newly completed Chicago Sanitary and Ship Canal. Before this time, the Chicago River was known by many local residents of Chicago as "the stinking river" because of the massive amounts of sewage and pollution which poured into the river from Chicago's booming industrial economy. Through the 1980s, the river was quite dirty and often filled with garbage; however, during the 1990s, it underwent extensive cleaning as part of an effort at beautification by Chicago Mayor Richard M. Daley.

Recently, researchers at the University of Illinois at Urbana-Champaign created a three-dimensional, hydrodynamic simulation of the Chicago River, which suggested that density currents are the cause of an observed bi-directional wintertime flow in the river. At the surface, the river flows east to west, away from Lake Michigan, as expected. But deep below, near the riverbed, water travels west to east, toward the lake.

All outflows from the Great Lakes Basin are regulated by the joint U.S.-Canadian Great Lakes Commission, and the outflow through the Chicago River is set under a U.S. Supreme Court decision (1967, modified 1980 and 1997). The city of Chicago is allowed to remove 3200 cubic feet per second water from the Great Lakes system; about half of this, 1 billion US gallons a day, is sent down the Chicago River, while the rest is used for drinking water. In late 2005, the Chicago-based Alliance for the Great Lakes proposed re-separating the Great Lakes and Mississippi River basins to address such ecological concerns as the spread of invasive species.

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