The flow of water bodies is typically defined by gravity, directing rivers from higher elevations toward the sea or a larger body of water. This unidirectional movement is fundamental to the definition of a river, making the idea of a river running “backward” seem to defy natural law. While the term usually refers to an unusual flow, the phenomenon is generally either a temporary, natural event or the result of massive human engineering.
Identifying the Famous Reversal
The river most famously known for having its flow permanently reversed is the Chicago River in Illinois. This large-scale, engineered flow reversal forever changed the hydrology of the Great Lakes region. Originally, the river flowed sluggishly east, draining into Lake Michigan, the region’s massive freshwater source. The monumental reversal project was officially completed on January 2, 1900, after years of intense labor and planning by the Sanitary District of Chicago (now the Metropolitan Water Reclamation District of Greater Chicago). Since that time, the main stem and South Branch of the river have flowed west, away from Lake Michigan and toward the Mississippi River watershed.
The Engineering Feat: Changing the Flow
Achieving this permanent reversal required a civil engineering project of monumental scale and complexity. The primary component was the construction of the 28-mile-long Chicago Sanitary and Ship Canal, which began in 1892. This undertaking was designed to connect the Chicago River system with the Des Plaines River, effectively linking the Great Lakes with the Mississippi River basin. Engineers utilized the natural low divide west of Chicago, excavating a channel deeper than both the riverbed and the lake’s surface to allow gravity to pull water in the new direction.
The construction involved the removal of approximately 42,340,000 cubic yards of earth and rock. This effort was one of the largest earth-moving projects ever attempted at the time. The canal was engineered with a gradual slope, allowing water from Lake Michigan to enter the Chicago River and flow through the canal toward the Mississippi watershed.
Control structures, including locks and dams, were integrated into the system to manage the water level and ensure the permanent diversion. The Lockport Lock and Dam, located at the end of the Sanitary and Ship Canal, regulates the drop in elevation from the Chicago area down to the Des Plaines River. By creating this continuous, downhill path, the flow of the Chicago River was forced to discharge away from the lake. This system of canals and control structures was later recognized as a “Civil Engineering Monument of the Millennium.”
Why the River Was Reversed
The historical motivation for this massive public works project was a dire public health crisis in the late 19th century. Rapid population growth and industrial expansion in Chicago overwhelmed the city’s ability to manage its waste. Sewage and industrial effluent were dumped directly into the Chicago River, which naturally carried the pollution into Lake Michigan.
Lake Michigan was simultaneously the city’s sole source of drinking water, creating a catastrophic feedback loop. This contamination led to widespread outbreaks of waterborne diseases, particularly cholera and typhoid fever, which killed thousands of residents. The city attempted solutions, such as building water cribs further offshore, but these efforts proved insufficient as currents and heavy rains carried sewage to the intakes.
Following an extreme weather event in 1885 that severely threatened the water supply, the decision was made to pursue the radical solution of reversing the river. The establishment of the Sanitary District of Chicago in 1889 formalized the goal: to redirect the city’s wastewater away from its drinking source. The reversal was a public health necessity, sending the contaminated flow down the new canal and into the Mississippi River system.
Natural and Tidal Flow Anomalies
While the Chicago River’s backward flow is a permanent, engineered marvel, nature also produces temporary reversals in river flow through different mechanisms. The most common natural flow anomaly is the tidal bore, which occurs in certain estuaries and rivers near the coast. A tidal bore is a phenomenon where the leading edge of the incoming high tide forms a wave that travels up a river, temporarily reversing the water’s current.
This reversal happens in areas that have a large tidal range, where the incoming tide is funneled into a shallow, narrowing channel. The force of the rising tide is so significant that it overcomes the river’s normal downstream flow, pushing the water upstream. Rivers like the Severn in England or the Petitcodiac in Canada experience tidal bores, where the water’s direction of movement abruptly changes for a period.
Unlike the Chicago River’s permanent, gravity-driven reversal, the tidal bore is a temporary oscillation, lasting only until the tide recedes. The reversal is caused by the astronomical forces of the moon and sun, not by civil engineering.