A new study published in Knowledge and Management of Aquatic Ecosystems reveals how a massive combined sewage overflow (CSO) event in July 2023 significantly altered the ecology of the Chicago River. Researchers from Shedd Aquarium documented major shifts in zooplankton populations, water quality, and fish behaviour following the discharge of untreated wastewater.
Chicago’s combined sewer system collects both wastewater and stormwater. During heavy rain, the system overflows, releasing untreated water into the river to prevent flooding. In July 2023, more than 12.3 billion litres of untreated wastewater and stormwater entered the river, with Bubbly Creek receiving 9.1 billion litres.
The study highlights the vulnerability of aquatic ecosystems to sudden pollution events, particularly in cities with aging or combined sewer infrastructure
“It’s alarming any time a human-caused disturbance alters the natural ecosystem,” said Dr. Austin Happel, research biologist at Shedd Aquarium and lead author of the study. “This study reveals another piece of the puzzle in understanding how CSOs affect the Chicago River, which provides a home to hundreds of different animal and plant species.”
Water samples collected throughout the summer showed that one group of zooplankton, Chydoridae, nearly disappeared from Bubbly Creek after the overflow. Meanwhile, Cyclopoida and Moinidae populations spiked. Low oxygen and high nitrogen levels persisted in the creek for weeks, driving fish to flee the area, as confirmed by Shedd’s acoustic telemetry data.
“In the midst of an urban environment, the Chicago River is a biodiversity hotspot teeming with life, and it’s everyone’s job to protect this critical habitat,” said Dr. Happel. “As climate change increases the frequency and severity of extreme weather, this research on CSOs helps us understand and prepare for the threats facing our freshwater ecosystem.”
The study highlights the vulnerability of aquatic ecosystems to sudden pollution events, particularly in cities with aging or combined sewer infrastructure. Shedd Aquarium continues to work with local and regional partners to rewild parts of the South Branch of the Chicago River. Ongoing projects include the installation of floating wetlands to provide habitat for fish and birds, environmental education initiatives, and public engagement campaigns encouraging recreation and stewardship of the river.
Residents are also encouraged to help reduce overflow risk during storms by signing up for Overflow Action Alerts from the Metropolitan Water Reclamation District of Greater Chicago and conserving water during heavy rainfall. Simple actions like shortening showers, delaying laundry, and disconnecting downspouts can make a meaningful difference.
