CHICAGO — The aftermath of a combined sewer overflow, which flushes sewage and stormwater into the Chicago River, isn’t pretty.
“The water is stained usually like a brown or a sickly gray color and it smells terrible,” said Phil Nicodemus, research director for the nonprofit Urban Rivers. “It’s everything water shouldn’t be.”
The impact is especially evident at Bubbly Creek, the south fork of the South Branch of the Chicago River, where the water is usually stagnant, Nicodemus said.
Pollution seen at Bubbly Creek on Sept. 21, 2026. Credit: Provided by Urban Rivers
This summer Bubbly Creek has experienced eight combined sewer overflows, the most Urban Rivers has seen since the nonprofit started remediating the area in 2022, Nicodemus said. Months of extreme rain events have led to an increase of combined sewer overflows across the city.
This weekend’s heavy rain led to combined sewer overflows at the North Branch Pumping Station and at the Racine Avenue Pumping Station, Patrick Thomas, a spokesperson for the Metropolitan Water Reclamation District of Greater Chicago, said.
Between July 2 and July 5 — an especially rainy stretch of summer — the Metropolitan Water Reclamation District recorded 42 combined sewer overflows throughout the county.
Combined sewer overflows can carry oil, road salt, pesticides, fertilizer, litter and bacteria into the river, making it a harmful environment for humans and wildlife. The annual Chicago River Swim scheduled for Sunday was canceled last-minute in part due to concerns about water quality.
Animals that call the river home can become tangled in the trash that flows into the river and fertilizer runoff can cause algae blooms. When the algae eventually dies, it depletes the river’s oxygen, killing small microorganisms and leading to fish kills.
After a combined sewer overflow, Nicodemus said he will often see fish gathered at the river’s surface where there is more oxygen or even leaping out of the water to breathe, behavior that makes them easier prey for birds.
“Combined sewer overflows can disrupt the entire food chain,” said Margaret Frisbie, the executive director of Friends of the Chicago River.
Rowers row along the South Branch of the Chicago River around Park No. 571 in Bridgeport near Bubbly Creek on Sept. 15, 2025. Credit: Colin Boyle/Block Club Chicago
Investing In Nature
Before the construction of the Chicago area’s Deep Tunnel, a system of underground tunnels and reservoirs that captures and stores stormwater and sewage, combined sewer overflows were a regular occurrence.
In the 1970s, the city averaged an overflow every three days, according to Friends of the Chicago River.
The Deep Tunnel, which was built over the course of more than five decades, has greatly reduced the number of overflows, especially after the Thornton Reservoir was completed in 2015, according to Frisbie.
Data from Friends of the Chicago Rivers shows that in 2025, Cook County saw 24 combined sewer overflow events, down from 434 in 2019.
However, as extreme rain events become more common with climate change, even the Deep Tunnel can’t keep up.
As of Monday night, the Deep Tunnel’s McCook Reservoir was 97 percent full with 5 billion gallons of water. The McCook Reservoir has reached capacity eight times this year. In comparison, it reached capacity only five times between 2021-2025.
During this weekend’s heavy rain, more water entered the city’s combined sewer system than the system could handle, causing the combined sewer overflows, Thomas said.
Thomas said that once the rain subsides, the Metropolitan Water Reclamation District will pump water from the reservoirs to their treatment facilities to create more capacity ahead of the next storm.
The McCook Reservoir is in the midst of an expansion that will add 6.5 billion gallons of capacity. The expansion, however, will likely not be complete until 2032.
“The Deep Tunnel had a huge impact, but at this point in time, it’s not enough,” Frisbie said. “We can’t expect the kind of rain events we used to expect.”
Between June 1 and Aug. 31 of this year, Chicago recorded 15.38 inches of rain, which is 3.32 inches above the average.
Annual precipitation in Illinois has increased by about 5 inches over the past 120 years, according to data from the National Oceanic and Atmospheric Administration. Storms are also getting more severe as the number of 2-inch rain days in the state has increased by 40 percent since the beginning of the 20th century, data shows.
Kids bike in the rain in Edgewater during a thunderstorm warning on July 27, 2026. Credit: Colin Boyle/Block Club Chicago
Instead of relying solely on large engineering feats like the Deep Tunnel to handle extreme rain, it’s time to start investing more in nature, Frisbie said.
Solutions like wetland restoration and replacing lawns with native flowers and grasses, which the Environmental Protection Agency calls green infrastructure, can help the soil capture rainwater where it falls. Native vegetation has deep root systems that hold water onsite, allowing it to move through the soil structure, filtering out pollutants like nitrates and reducing flooding by slowing waters’ return to nearby streams or rivers.
If you’re not “converting the land that’s under our feet to permeable surfaces, you’re missing out on an enormous reservoir that people don’t think about,” Nicodemus said.
Even small-scale projects like backyard rain gardens can make an impact, Frisbie said. Neighbors can also help out by signing up for overflow action day alerts and reducing water usage during extreme weather events.
“The metro region is 5 million people. If every single person chooses to think differently before and after rainstorms, we can actually have an effect,” Frisbie said.
