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Quote/Term of the Week
Reservoir / noun
An artificial lake where water is stored, often created to supply drinking water, irrigate crops and prevent flooding.
Ex: The Chicagoland area features multiple major reservoirs which contribute to a larger system called The Deep Tunnel, meant to prevent stores of stormwater and sewage from overflowing into waterways.
When it rains, it pours
The impact of Chicago’s near-record rainfall this summer goes beyond flooded basements. Chicago experienced a total of 15.38 inches of rain, making it the wettest meteorological summer since 2018.
After a slew of storms in July, Chicago’s Office of Emergency Management and Communications received 569 reports to 311 of water on the street and 540 reports of water in basements.
After those storms, the Metropolitan Water Reclamation District’s Deep Tunnel reservoirs were holding a combined 6 billion gallons of stormwater and wastewater.
According to MWRD President Kari Steele, the Thornton reservoir, which serves the South Side of Chicago and 13 south suburban municipalities and holds 7.4 billion gallons of water, reached 94% capacity – the fullest it’s ever been.
The McCook reservoir, which serves central Chicago and 36 municipalities and holds 3.6 billion gallons of water, reached its capacity seven times. For context, McCook has hit capacity just five other times between 2021 and 2025.
PAUSE: Why does this matter?
Both the Thornton and McCook reservoirs are critical parts of MWRD’s Tunnel and Reservoir Plan, known as TARP or “The Deep Tunnel.” It is a system of deep, large-diameter tunnels and massive reservoirs to help reduce flooding, improve Chicago-area water quality and prevent sewage overflows.
When it rains, this underground tunnel system is there to catch and evenly distribute all the stormwater. If there’s heavy rain and that tunnel system nearly reaches capacity, sewers around the city could overflow into Chicagoland’s surrounding rivers and Lake Michigan, the largest public drinking water supply in the state, serving nearly 6.6 million people.
PLAY: What’s being done about this?
Trying to manage these systems when they’re at capacity is, “like trying to drain a swimming pool with a straw,” Steele said. Instead of using that metaphorical straw, Steele said the best solutions happen when various departments and organizations join forces to help mitigate the risks of flooding.
A major long-term solution includes a project to expand the McCook reservoir by 6.5 billion gallons, bringing its capacity to 10 billion gallons. That part of The Deep Tunnel expansion has been in works for nearly three decades (the first part of the project was completed in 2017). But the work has been delayed because it requires mining and removing materials from an existing stone quarry. It’s not expected to be complete until late 2032.
Until then, more short-term solutions include prioritizing and integrating more green infrastructure across the city, especially in more flood-prone areas. Green infrastructure such as rain barrels, native landscaping, rain gardens and permeable (water-absorbing) pavement help absorb rainwater so it doesn’t flow into our sewers.
City Council Committee on Environmental Protection and Energy by Annabel Rocha and Colette Willard
What you can do:
Learn more about green infrastructure:
Contact your alderperson: Want more green infrastructure in your community? Click here to find your alderperson.
Attend a MWRD meeting: The next MWRD Board of Commissioners meeting is 10:30 a.m. Thursday, Sept. 17 at Barbara J. McGowan Main Office Building (100 E. Erie St.). Public comments can be made in-person.
A version of this story was first published in the September 9, 2026 issue of the Newswire, an email newsletter that is your weekly guide to Chicago government, civic action and what we can do to make our city great. You can sign up for the weekly newsletter here.
Have thoughts on what you'd like to see in this feature? Email Civic Editor Dawn Rhodes at dawn@citybureau.org.