'Extreme rainfall has made headline news this month — are we prepared for what’s ahead?,' Evanston Roundtable

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Monday's storm was the latest in a season of wild and erratic weather. Hail and high winds added to the heavy downpours (Evanston received 3 inches of rain, according to some reports) that once again put the region's stormwater management system to the test. Although the headlines weren't as dramatic as they were after four rounds of severe storms swept through the area over the July Fourth weekend - "Deep Tunnel almost full" - by Tuesday morning, the McCook Reservoir was at 85% capacity.

We can expect more extreme weather - including more extreme rainstorms - in the months and years ahead. Is Evanston prepared?

Extreme rain events

On July 5, in a storm update, the Metropolitan Water Reclamation District reported that an average of 8.15 inches of rain had fallen in the communities it serves over the previous 30 days. That was more than the average rainfall for June and July combined.

Most of the rain fell over two days, beginning the evening of July 2. Worst hit were areas to the south and west of Evanston: 8.39 inches of rain fell in Sugar Grove, about 20 miles west of Naperville, according to the National Weather Service.

Even though Evanston was spared the worst of the weekend storms - with rainfall totalling just 3.35 inches, according to the National Weather Service - we weren't immune.

After a wet start to the weekend, rain held off during the July Fourth parade, then started up just as the last marchers neared Green Bay Road. Added to the earlier rounds of rainfall, the storm left the lakefront so wet that the evening concert there was canceled. The fireworks display went on as planned, but was obscured by fog.

As skies cleared, dramatic posts and aerial photographs circulated showing Chicago's stormwater management system nearing capacity. MWRD issued repeated updates and pleas to reduce water use so the system could keep up.

McCook Reservoir on July 6, 2026. Credit: Metropolitan Water Reclamation District

Where does all that stormwater go?

Chicago's Tunnel and Reservoir Plan, or "Deep Tunnel," was designed to collect and store stormwater until it can be treated at water reclamation plants. The goal is to reduce flooding and prevent sewage from overflowing into stormwater systems and entering waterways and Lake Michigan. The system has been under construction since 1975 and now includes three immense reservoirs. McCook is the one where Evanston's overflow ends up.

Even before the July 4 weekend storms had passed, MWRD reported in a July 4 storm update that the McCook Reservoir had reached capacity on July 3 for the sixth time this year. For context, the McCook TARP system filled five times cumulatively between 2021 and 2025.

Under normal conditions, Evanston's sewage and stormwater moves through a system of local combined, relief and storm sewers to a huge MWRD intercepting sewer buried under McCormick Boulevard and from there to the O'Brien Water Reclamation Plant at Howard Street. There it's all treated until it's clean enough to be released into the North Shore Channel.

But when storms drop a lot of rain fast, backup systems are needed to keep basements from flooding and sewage out of waterways. That's where TARP comes in.

TARP drop shaft in Ladd Arboretum near Green Bay Road. Credit: Wendy Pollock

You can sometimes hear water rushing if you stand near one of the TARP dropshafts, like the one in the Ladd Arboretum near Green Bay Road. Dick Lanyon, retired executive director of MWRD, explains: What we're hearing is overflow dropping 200 feet into the Mainstream Tunnel. From there, water flows through a 12-foot diameter tunnel, which gradually increases in size to 33-feet, to the McCook Reservoir, a deep open holding facility excavated in limestone, located in Bedford Park along the Stevenson Expressway. From there, the water is gradually pumped through another five-mile long tunnel to the Stickney Water Reclamation Plant for treatment before it's released into the Chicago Sanitary and Ship Canal - and from there to the Des Plaines River, the Illinois River, and ultimately the Mississippi.

TARP reservoirs. Credit: Metropolitan Water Reclamation District

What happens if a lot of rain falls very fast?

Although the prospect of a 3.5 billion gallon reservoir nearing capacity made headlines, problems can also occur when stormwater falls so fast that it fills up sewers and gets mixed up with sewage. That leads to a CSO, or combined sewer overflow event.

The July storms followed an unusually wet spring and what the National Weather Service Chicago called an "incredibly active stretch of spring weather" that included flash floods in early January, tornados in March, then heavy rainstorms through much of April.

During our soggy spring, one CSO event occurred on April 14 at Evanston's Main Street Outfall, when sewage and stormwater was discharged to the North Shore Channel for about 10 minutes, then another on July 3 lasted about 40 minutes.

Other CSO discharges to the North Shore Channel have occurred on several days in 2026 from outfalls owned by Skokie or MWRD. Although these CSOs occurred beyond our borders, Evanston is still affected because the untreated CSO enters the North Shore Channel and may end up in the lake.

That raises the level of E. coli in the water and beaches are closed as a precautionary measure. NBC5 reported a beach closing in Evanston in July 2023 due to high levels of E.coli.

In response to a FOIA request, the City of Evanston said that they "do not keep records of the specific dates that beaches are closed. However, beaches may be closed for a variety of reasons, including thunderstorms, unhealthy air quality, the opening of Wilmette locks, rainy weather combined with temperatures below 65°F, or reduced staffing levels toward the end of the summer season." The Department of Health and Human Services explains on the city website the methods used to test beach water, which happens seven days a week.

Is more of this kind of rainfall in the future?

This season's storms are part of a trend toward increasingly intense precipitation, with longer dry spells in between. That's because "rising global temperatures have made the atmosphere more waterlogged - providing fuel for wetter and more dangerous storms," according to Sarah Kaplan of the Washington Post . Following the disastrous 2024 floods in western North Carolina, she wrote that "It is a law of physics that, for every degree Celsius increase in temperature, air is able to hold7% more water vapor. This phenomenon increases the moisture available for storms, making individual events wetter than they otherwise would be and increasing the risk of unprecedented rain."

How is Evanston preparing?

Preparing for the stormier weather that's fueled by climate change is a priority of Evanston's Climate Action and Resilience Plan. The Stormwater Master Plan presented to City Council by engineer and project manager Bridget Nash in May 2025 explains the complex model that allows planners to understand the city's hydrology (how much rain infiltrates, how much runs off, where it goes) and hydraulics (the sewer system) and makes recommendations for changes.

Dick Lanyon, former head of MWRD and Evanston resident, describes several of the projects identified in the Stormwater Master Plan in his accompanying report.

These include use of green infrastructure - defined broadly to include trees and open green spaces with deep-rooted plants - as a complement to the grey infrastructure like TARP that's critical during extreme storms. Green infrastructure has multiple co-benefits, including shade that can lower surface temperatures dramatically.

Find out more

MWRD virtual tours are offered monthly. They offer the chance to "go behind the scenes and under water to see how we clean used water; descend 300 feet into the deep tunnel system, and watch our electrofishing crew at work sampling fish on the Chicago River."

You can sign up for Overflow Action Days alerts through MWRD or Friends of the Chicago River.

https://evanstonroundtable.com/2026/07/28/extreme-rainfall-has-made-headline-news-this-month-are-we-prepared-for-whats-ahead/