The Chicago River is a 156-mile system of waterways that winds directly through the heart of one of America’s largest cities. This unique urban river is comprised of three main sections: the North Branch, the South Branch, and the central Main Stem that flows through the downtown Loop. Historically, the river’s location provided a geographical link between the Great Lakes and the vast Mississippi River basin, fueling the city’s early development. While water quality has dramatically transformed from its industrial past, the river remains susceptible to pollution events.
Historical Context of Pollution and Flow Reversal
The river once flowed eastward, emptying into Lake Michigan, which served as the city’s sole source of drinking water. As Chicago’s population exploded in the mid-19th century, the river became heavily polluted with raw sewage and industrial waste. This contamination led to severe public health crises, including outbreaks of typhoid and cholera.
To safeguard its water supply, the city embarked on one of the largest civil engineering projects of its time: reversing the river’s flow. The Chicago Sanitary and Ship Canal, completed in 1900, forced the river to flow inland, away from Lake Michigan. This action protected the drinking water source, but it turned the Chicago River into an open sewage channel for decades, creating a legacy of severe pollution.
Current Water Quality Metrics and Recreational Safety
Today, the condition of the river is gauged primarily by two measurable indicators: levels of fecal bacteria and the amount of dissolved oxygen (DO). Fecal bacteria, often measured as E. coli, is monitored because it signals the potential presence of pathogens that pose a risk to human health during water contact. The Illinois standard for acceptable recreational water quality is a concentration below 200 Colony Forming Units (CFUs) per 100 milliliters.
During dry weather, the river’s water quality often meets this standard, but conditions can fluctuate dramatically due to weather events. After heavy rainfall, bacteria levels can spike significantly, sometimes exceeding 50,000 CFUs per 100 milliliters, making contact unsafe.
Dissolved oxygen levels are the primary metric for the river’s ecological health, as aquatic life requires sufficient oxygen to breathe. Historically low DO levels have been a major problem, but improvements have allowed the number of fish species to increase from only 10 in the 1970s to nearly 70 species today. Despite these improvements, major pollution events can still cause temporary periods of low oxygen, stressing aquatic life.
While the river is cleaner than in the past, direct full-body contact, like swimming, is generally advised against, except for sanctioned events with pre-tested water. Recreational activities like kayaking, canoeing, and fishing are popular and permitted. Users are strongly cautioned to avoid water contact immediately after major rain events, and a new notification system is being developed to alert the public to these high-risk periods.
Primary Sources of Contamination
The main reason for sudden drops in water quality is the city’s aging Combined Sewer System (CSS). In older parts of Chicago, this system collects both sanitary sewage and stormwater runoff in the same pipes. This design works well during dry periods when all the flow is directed to water reclamation plants for treatment.
During periods of heavy rain, the combined volume of sewage and stormwater quickly overwhelms the system’s capacity. The excess mixture is then discharged directly into the Chicago River through hundreds of outfalls, known as Combined Sewer Overflows (CSOs). As little as 0.3 inches of rain can trigger a CSO event, sending untreated wastewater into the waterway.
These overflows introduce high concentrations of fecal bacteria and raw pollutants, severely impacting the river’s ecosystem and human safety. Stormwater also contributes non-point source pollution, carrying oil, litter, and other contaminants from streets into the river. The resulting surge of nutrients consumes dissolved oxygen as it decomposes, further stressing aquatic life.
Major Infrastructure Projects and Long-Term Restoration
The long-term strategy for mitigating contamination is centered on the massive Tunnel and Reservoir Plan (TARP), often called the Deep Tunnel project. TARP is an extensive civil engineering initiative designed to capture and store combined sewage and stormwater that would otherwise be released through CSOs. This system consists of over 100 miles of deep, large-diameter tunnels, excavated 150 to 300 feet below the city.
The tunnels divert the overflow into vast reservoirs, where the water is held until treatment plants regain capacity to process it. The system is designed to provide a total storage capacity of approximately 17.5 billion gallons upon completion. Stage 1 was completed in 2006, and the plan has already reduced the average number of days with sewer overflows from about 100 per year to roughly 50.
The final major components of TARP are the large surface reservoirs, including the McCook Reservoir, scheduled for completion by 2029. When fully operational, the Deep Tunnel system is expected to nearly eliminate CSOs across its service area, transforming the river’s overall water quality. These infrastructure projects underscore a strong commitment to the river’s environmental recovery.