A July 2 storm dropped more than 8 inches of rain on some parts of the nation’s third-largest city and its suburbs, flooding tens of thousands of basements and prompting a federal disaster declaration. Once the tunnels and main reservoir were full, sewage and storm water poured into the Chicago River, forcing officials to reverse its flow and send the tainted mix into Lake Michigan, the region’s main source of drinking water.  

The $3.8 billion Tunnel and Reservoir Plan, also known as the Deep Tunnel—comprising over 100 miles of tunnels and three reservoirs covering different parts of the region—has kept a trillion gallons of sewage-tainted water out of local waterways since 1981, according to officials at the Metropolitan Water Reclamation District of Greater Chicago. 

A huge addition to the McCook Reservoir serving Chicago and certain suburbs is expected to open in 2029, adding 6.5 billion gallons of capacity to the current reservoir’s 3.5 billion gallons. That will further reduce the frequency of spills into the Chicago River, but officials aren’t certain even that will be enough to bottle up the biggest storms. 

“It’s sort of a Catch-22,” said Kevin Fitzpatrick, assistant director of engineering for the reclamation district, just before heading 300 feet below ground to show a visitor the giant pumps that send the water after storms to a huge sewage treatment plant. “We’re in a lot better spot because we have the reservoirs, but the storms that we’re getting now are a little bit more than maybe they designed this thing for in the 1970s. It’s so unpredictable with these weather patterns.”

Chicago isn’t alone in wrestling with storm runoff and sewage spills amid more intense storms kicked up by the warming climate. Infrastructure built for the storms of the past might not be enough to withstand the events of the future as climate change alters rainfall patterns, raising difficult questions about how cities will be able to adjust massive projects to handle more water without incurring prohibitive costs.

“Almost everything that’s been built in the past couple of decades is just undersized for the type of rain events that we get today because of climate change,” said Bob Dean, chief strategy and program officer at the Center for Neighborhood Technology, a Chicago-based nonprofit that recently published a study showing that storm and wastewater systems in Chicago, Milwaukee, Detroit, South Bend, Ind., and Buffalo, N.Y., were built to address now-outdated storm data.

Even projects still on the drawing board, such as the so-called Ike Dike in Galveston Bay, a $31 billion coastal barrier to protect the Texas Gulf Coast from hurricanes, might not be up to the task of countering the biggest storms. 

“A lot of times we’re designing these things for yesterday’s storms, not tomorrow’s storms,” said Rob Moore, senior policy analyst with the Natural Resources Defense Council, a New York-based environmental group. “Beyond that, it’s the fallacy that the single engineered solution is going to protect us. Whether it’s a deep tunnel project or a storm surge barrier or levee, the single mode of protection is also a single mode of failure.”

Philadelphia and other cities are leading the way in using what is known as green infrastructure to divert water from the storm-drainage system through things such as rain gardens, permeable alleys and other projects, he said. Reclamation district officials for the Chicago area are also pouring money into projects such as converting hard-surface school playgrounds into artificial-turf soccer fields that can store rainwater and release it slowly into the storm-water system, officials said.

In Chicago, the deep tunnel project does little to alleviate one major source of flooding: areas where older local pipes aren’t big enough to send water out of the neighborhoods during big rains. 

That is one reason why there were so many flooded basements with the July 2 storm, officials said. “We were getting flooding complaints before the reservoir was full,” said Ed Staudacher, assistant director of maintenance and operations with the reclamation district. “It’s not even getting to us quick enough before they’re flooding.”

Lavelle Parker, 44, who drives a street sweeper for the city’s streets and sanitation department, said the 2½ feet of tainted water that flooded his basement apartment in the Austin neighborhood on the West Side sent a couch across the living room and ruined most of his possessions.  

“I’ve been living in Chicago all my life and I’ve never seen a flood like that,” said Parker, who has been staying with his girlfriend since the flood. 

Austin and the suburbs of Cicero and Berwyn had rainfall that would be expected to happen only once every 500 years, while other areas received the equivalent of 50- to 100-year rainfalls, all within about six hours, said Kevin Doom, a meteorologist with the National Weather Service. “The system as a whole was absolutely crawling across the area. There was just copious amounts of water vapor to work with,” he said. 

To get ready for the coming rains, workers in the glassed-in control room off the lobby of the reclamation district’s Chicago headquarters opened gates southwest of the city to lower river levels and make room for storm water, Staudacher said. 

As the deluge gained steam, runoff and sewage began flowing through the massive deep tunnel—most of it 33 feet in diameter and hundreds of feet below city streets—toward the McCook Reservoir southwest of the city. Soon workers called in reinforcements, including Staudacher, as the reservoir filled at the fastest pace he says he can remember.

When the reservoir finally topped off, the dirty water started flowing straight into the river, which rose 6 feet, threatening to spill its banks. That is when reclamation district officials and the U.S. Army Corps of Engineers opened locks and gates, allowing water to flow into Lake Michigan from the Chicago River. It was just the second time Chicago River water had been released into the lake since the first phase of the McCook Reservoir opened in late 2017, compared with four times in the previous six years.

“It’s stressful in this room during these storms,” he said. “There’s always curveballs thrown at you.”

Researchers say greater use of green infrastructure and analysis of massive data sets through tools like artificial intelligence and machine learning could help the district cope with bigger storms ahead. “You have to adapt to what Mother Nature throws at you, and right now Mother Nature is as upset as it can possibly be,” said Marcelo Garcia, a professor of civil and environmental engineering at the University of Illinois Urbana-Champaign.