The Chicago Sanitary and Ship Canal is the only shipping link between the Great Lakes (specifically Lake Michigan by the Chicago River) with the Mississippi River system, by way of the Illinois and Des Plaines rivers. The canal also carries Chicago's treated sewage into the Des Plaines River. Before completion of the canal in 1900, the sewage of Chicago was dumped into Lake Michigan, the city's drinking water supply. The canal is part of the Chicago Wastewater System , which has been named a Civil Engineering Monument of the Millennium by the American Society of Civil Engineers (ASCE). The Chicago Sanitary and Ship Canal is 30 miles (48 kilometers) long, 202 feet (62 meters) wide, and 24 feet (7.3 meters) deep.

Early Chicago sewage systems discharged directly into Lake Michigan or into the Chicago River, which itself flowed into the lake. The city’s water supply also came from the lake, through water intake cribs located offshore. Inevitably, sewage infiltrated the water supply, leading to typhoid fever, cholera and dysentery. A cholera epidemic in 1854 killed more than 5 percent of the city’s population. Deaths from typhoid fever averaged 65 per 100,000 population a year between 1860 and 1900. The water cribs were moved farther out into the lake, but this effort did not stem the epidemics.

By 1887, it was decided to reverse the flow of the Chicago River through civil engineering. Engineer Rudolph Hering noted that a ridge about 12 miles from the lake shore divided the Mississippi River drainage system from the Great Lakes drainage system. A plan soon emerged to cut through that ridge and carry waste water away from the lake, through the Des Plaines and Illinois rivers, to the Mississippi River and the Gulf of Mexico. In 1889, the Illinois General Assembly created the Metropolitan Sanitary District of Greater Chicago (now the Metropolitan Water Reclamation District of Greater Chicago) to carry out the plan.

The canal, linking the south branch of the Chicago River to the Des Plaines River at Lockport, was completed in 1900. The rate of flow is controlled by sluice gates at Chicago Harbor and at the O'Brien Lock in the Calumet River , and also by pumps at Wilmette Harbor. Two more canals were later built to add to the system: The North Shore channel in 1910, and the Calumet Sag Channel in 1922.

Construction of the Ship and Sanitary Canal was the largest earth-moving operation that had been undertaken in North America up to that time. It was also notable for training a generation of engineers, many of whom later worked on the Panama Canal.

Water diversions from the Great Lakes system are now regulated by an international treaty with Canada and by governors of the Great Lakes states.