The Great Loop

Reversing the flow

During the last ice age, the area that became Chicago was covered by Lake Chicago, which drained south into the Mississippi Valley. As the ice and water retreated, a short 12-to-14-foot (3.7 to 4.3 m) ridge was exposed about a mile inland, which generally separated the Great Lakes’ watershed from the Mississippi Valley, except in times of heavy precipitation or when winter ice flows prevented drainage. By the time Europeans arrived, the Chicago River flowed sluggishly into Lake Michigan from Chicago’s flat plain. As Chicago grew, this allowed sewage and other pollution into the clean-water source for the city, contributing to several public health problems, like typhoid fever. Starting in 1848, much of the Chicago River’s flow was also diverted across the Chicago Portage into the Illinois and Michigan Canal. In 1871, the old canal was deepened in an attempt to completely reverse the river’s flow but the reversal of the river only lasted one season.

Finally, in 1900, the Sanitary District of Chicago, then headed by William Boldenweck, completely reversed the flow of the Main Stem and South Branch of the river using a series of canal locks, increasing the river’s flow from Lake Michigan and causing it to empty into the newly completed Chicago Sanitary and Ship Canal. In 1999, this system was named a “Civil Engineering Monument of the Millennium” by the American Society of Civil Engineers (ASCE). 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 that 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.

In 2005, 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 seasonally 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 3,200 cubic feet per second (91 m³/s) of water from the Great Lakes system; about half of this, 1 billion US gallons per day (44 m³/s), 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.