Holiday weekend storms overwhelmed Chicago's stormwater system, dumping untreated water into area rivers
The rain over the July Fourth holiday weekend overwhelmed a massive system meant to handle Chicago's stormwater, causing untreated water to be dumped into area rivers.
The Fourth of July weekend in Chicago has a tradition of bad weather, and this weekend was no different, with multiple rounds of flooding storms.
"It's been a very busy month, and an even busier weekend," said Ed Staudacher of the Metropolitan Water Reclamation District. "We have over 13 billion gallons of flow that we captured and prevented from going out to the waterways."
That's equivalent to 20,000 Olympic-sized swimming pools.
The MWRD captured the rain using a network of enormous underground tunnels to hold combined sewage and stormwater until it can be treated. But this weekend's heavy rains proved to be too much. Pounding rainfall filled local sewers, filled the deep tunnels and even filled the reservoirs.
"This is the most we've seen in [the Thornton reservoir] ever. It came online in 2015," Staudacher said.
The McCook Reservoir also filled to the brim. It is large enough to hold 22 Soldier Field stadiums full of water.
"We've filled the McCook Reservoir six times this year alone, where between 2021 and 2025 we filled it five times over that span," Staudacher said.
When the entire system fills, combined sewage and stormwater rush into area rivers. On July 3rd, 4th and 5th, 18 discharges into Chicago waterways were reported on each of those days.
"That sewage and stormwater has decomposing matter in it that consumes all the oxygen," said Dr. Austin Happel at the Shedd Aquarium. He tracks the impact these discharges have on fish. "And that causes stress on any sort of animal that needs to breathe underwater."
He and his team track the movements of more than 100 different fish.
"On previous events, we see that they tend to avoid the areas where those sewage discharges occur, and that's because they can't breathe in them," Happel said. "[The last time this happened] there was a good number of fish that did not survive the event."
Happel said most fish are able to move into other areas until the water clears, often going to the downtown riverwalk where freshwater is pulled into the river from Lake Michigan. And while contaminants make water near the discharges unsafe for swimming and kayaking, the flow rate means the river is refreshed and clean in about one week.
MWRD said they won't know how much combined sewage was discharged for a few days. They encourage residents to sign up for Overflow Action Day alerts, with tips to help reduce your wastewater output during heavy rain.
