Chicago Sanitary and Ship Canal: The River That Flows Backward

The Chicago Sanitary and Ship Canal is a 45-kilometer (28-mile) artificial waterway in Illinois that connects the south branch of the Chicago River to the Des Plaines River near Lockport. Completed on January 2, 1900, it pulled off one of the most audacious feats in engineering history: it permanently reversed the flow of the Chicago River, sending the city's sewage away from Lake Michigan — its drinking-water source — and toward the Mississippi River instead. That single decision halted deadly epidemics, saved thousands of lives, and reshaped the geography of the American Midwest.

For most of the 19th century, Chicago faced a fatal contradiction. The same Chicago River that carried away the city's waste also flowed straight into Lake Michigan, the lake from which Chicago drew its drinking water. As the population exploded, raw sewage, slaughterhouse runoff, and industrial waste poured into the river and out into the lake, fouling the water supply and triggering repeated outbreaks of cholera, typhoid, and dysentery. The Sanitary and Ship Canal was the radical answer: rather than treat the waste, Chicago would make its river run the other way.

Why Chicago Reversed Its River

By the 1880s, Chicago was one of the fastest-growing cities on Earth, and its public-health situation was dire. The Chicago River was an open sewer that emptied directly into Lake Michigan only a short distance from the city's water intake cribs. Whenever heavy rains overwhelmed the system, the contaminated plume reached the intakes and tainted the tap water of the entire city.

A catastrophic rainstorm in 1885 became the turning point. The downpour flushed an enormous volume of sewage far out into the lake, contaminating the water intake and, by some historical accounts, contributing to waterborne-disease deaths over the years that followed. Whatever the exact toll, civic leaders concluded that incremental fixes — bigger pipes, more distant intakes — would never keep pace with the city's growth. The only durable solution was to sever the link between Chicago's waste and its water by reversing the river itself.

History and Construction

• Construction period: 1892–1900
• Opened: January 2, 1900
• Peak workforce: about 8,500 workers
• Earth and rock moved: roughly 42 million cubic meters

The newly created Sanitary District of Chicago broke ground in 1892. The undertaking was so massive that the techniques and equipment developed for it became known as the "Chicago School" of earthmoving, influencing later projects including the Panama Canal. At its peak the project employed about 8,500 laborers, and crews excavated roughly 42 million cubic meters of earth and rock — more than was removed in the digging of the Panama Canal.

The work demanded cutting a deep, wide channel across the low continental divide that separates the Great Lakes watershed from the Mississippi watershed. On January 2, 1900, the controlling works at the canal were opened and water from Lake Michigan began draining into the new channel. For the first time, the Chicago River flowed away from the lake — and it has flowed that direction ever since.

Geographic Setting

• Length: 45 km (28 miles)
• Connects: Chicago River to the Des Plaines River
• State: Illinois
• Effect: Reversed the flow of the Chicago River

The canal begins at the south branch of the Chicago River in the Bridgeport neighborhood and runs southwest to the Des Plaines River near Lockport, Illinois. From there the diverted water joins the Illinois River and ultimately the Mississippi, carrying Chicago's outflow toward the Gulf of Mexico rather than into the Great Lakes.

The reversal worked because geography offered a narrow but decisive advantage: the elevation difference between Lake Michigan and the Des Plaines River is only a few meters. The Chicago region sits almost exactly on the subcontinental divide, where the land barely tips from one great watershed to the other. By cutting the canal deep enough to create a continuous downhill grade from the lake to the river, engineers gave the water a new path of least resistance — and gravity did the rest.

How the Canal Works

• Flow direction: away from Lake Michigan (reversed)
• Controlling works: locks and sluice gates at several points
• Navigation role: part of the Illinois Waterway linking the Great Lakes to the Mississippi
• Water quality: modern treatment plants now clean wastewater before discharge

The reversed flow is maintained by a system of locks, sluice gates, and controlling works. The Chicago Harbor Lock at the river's mouth on Lake Michigan blocks lake water from re-entering the river system except under controlled conditions, while channel depth and managed water levels keep the current moving steadily westward toward the Mississippi basin.

The canal now serves two purposes at once. It is a working navigation channel — a key segment of the Illinois Waterway that allows barges to travel between the Great Lakes and the Mississippi River system. And it still carries the city's treated effluent away from the lake. Crucially, what flows down the canal today is no longer raw sewage: large wastewater-treatment plants process Chicago's waste before it reaches the channel, a far cry from the open-sewer conditions of 1900.

Economic and Environmental Impact

The public-health payoff was immediate. After the river was reversed, typhoid and cholera rates in Chicago fell sharply, and the city's drinking water became dramatically safer. The canal is widely credited as one of the great public-health engineering achievements of its era.

The benefits, however, came with disputes and consequences. Downstream communities objected to inheriting Chicago's outflow, and in 1900 the state of Missouri, on behalf of St. Louis, sued Chicago, arguing that the city was funneling its sewage down the Mississippi toward St. Louis's own water supply. The legal fight reached the U.S. Supreme Court and became a landmark in American water law.

More than a century later, the canal sits at the heart of a new environmental crisis. Because it physically links the previously separate Mississippi and Great Lakes watersheds, it provides a potential migration route for invasive Asian carp. To stop these fish from reaching Lake Michigan — where they could devastate a multibillion-dollar Great Lakes fishing industry — electric barriers have been installed in the canal. The very connection that solved Chicago's sanitation problem now requires constant defense to protect the lakes' ecology.

Key Facts

• Length: 45 km (28 miles), Illinois
• Completed: January 2, 1900
• Connects: Chicago River to the Des Plaines River, then the Illinois and Mississippi Rivers
• Earth moved: about 42 million cubic meters — more than the Panama Canal
• Signature achievement: permanently reversed the flow of the Chicago River
• Modern use: navigation route plus treated-wastewater carrier
• Current challenge: electric barriers block invasive Asian carp from the Great Lakes

Fun Facts

• Chicago is the only major city in the world to have permanently reversed the flow of its main river.
• The 1900 lawsuit over the canal's discharge went all the way to the U.S. Supreme Court.
• The Chicago River's famous St. Patrick's Day dyeing tradition is made easier by the canal's controlled, reversed flow.

Frequently Asked Questions

Why did Chicago reverse its river?

Chicago reversed the Chicago River to protect its drinking water. The river had been dumping the city's sewage straight into Lake Michigan, right where Chicago drew its tap water, causing repeated cholera and typhoid epidemics. Reversing the flow sent the waste toward the Mississippi River basin instead, away from the lake.

How does the Chicago Sanitary and Ship Canal reverse the river?

Engineers dug a deep channel across the low divide between the Great Lakes and Mississippi watersheds, creating a continuous downhill grade from Lake Michigan to the Des Plaines River. Locks, sluice gates, and controlling works hold the new direction in place, so the river drains southwest toward the Mississippi rather than into the lake.

Where does the Chicago Sanitary and Ship Canal go?

The canal runs about 45 kilometers from the south branch of the Chicago River at Bridgeport southwest to the Des Plaines River near Lockport, Illinois. From there the water flows into the Illinois River and ultimately the Mississippi, eventually reaching the Gulf of Mexico.

Why are there electric barriers in the canal?

The canal connects the Mississippi River system to the Great Lakes, which gives invasive Asian carp a possible path into Lake Michigan. Electric barriers were installed to deter the fish from migrating north, where they could seriously damage the Great Lakes fishing industry and ecosystem.