Cal-Sag Channel in the context of Metropolitan Water Reclamation District of Greater Chicago

The Calumet River is a system of industrialized rivers and canals in the region between the south side of Chicago, Illinois, and the city of Gary, Indiana. Historically, the Little Calumet River and the Grand Calumet River were one, the former flowing west from Indiana into Illinois, then turning back east to its mouth at Lake Michigan at Marquette Park in Gary. Now the system is part of the Chicago Area Waterway System and through the use of locks flows away from Lake Michigan to the Cal-Sag Channel.

The Chicago Sanitary and Ship Canal, historically known as the Chicago Drainage Canal, is a 28-mile-long (45 km) canal system that connects the Chicago River to the Des Plaines River. It reverses the direction of the Main Stem and the South Branch of the Chicago River, which now flows out of Lake Michigan rather than into it. The related Calumet-Saganashkee Channel does the same for the Calumet River a short distance to the south, joining the Chicago canal about halfway along its route to the Des Plaines. The two provide the only navigation for ships between the Great Lakes Waterway and the Mississippi River system.

The canal was in part built as a sewage treatment scheme. Prior to its opening in 1900, sewage from the city of Chicago was dumped into the Chicago River and flowed into Lake Michigan. The city's drinking water supply was (and remains) located offshore, and there were fears that the sewage could reach the intake and cause serious disease outbreaks. Since the sewer systems were already flowing into the river, the decision was made to reverse the flow of the river, thereby sending all the sewage inland where it could be diluted before emptying it into the Des Plaines.

The Chicago Area Waterway System (CAWS) is a complex of natural and artificial waterways extending through much of the Chicago metropolitan area, covering approximately 87 miles altogether. It straddles the Chicago Portage and is the sole navigable inland link between the Great Lakes and the Mississippi River and makes up the northern end of the Illinois Waterway.

The CAWS includes various branches of the Chicago and Calumet Rivers, as well as other channels such as the North Shore Channel, Cal-Sag Channel, and Chicago Sanitary & Ship Canal. The CAWS ends near the Lockport Navigational Pool, the highest elevated of the eight pools of the Illinois Waterway. There are three major locks within the CAWS, operated by the Army Corps of Engineers: the Chicago Harbor Lock, the Lockport Lock & Dam, and the T.J. O'Brien Lock and Dam.