The System Built To Prevent Massive Floods In Chicago Near Capacity After ‘Stunning’ Rainfall
CHICAGO — A soggy start to summer with near-record rainfall has left Chicago’s Deep Tunnel stormwater retention system almost full.
The Deep Tunnel, which was constructed over the course of more than five decades, is a system of underground tunnels and reservoirs that captures and stores stormwater and sewage that could otherwise overflow into waterways.
After an especially rainy first five weeks of meteorological summer, the Deep Tunnel is nearly full.
Flooding in Beverly. Credit: Crystal Paul/Block Club Chicago
The Thornton Reservoir, which was completed in 2015 and is the largest combined sewer reservoir in the world, was 94 percent full as of Monday morning, according to the Metropolitan Water Reclamation District. With 7.4 billion gallons of water, this is the fullest the reservoir has ever been. The reservoirs’ previous record was set in 2019, when it was 54.5 percent full.
The Thornton reservoir serves the South Side and 13 south suburban municipalities. The Metropolitan Water Reclamation District warned that if the Thornton Reservoir reaches capacity, the south suburbs could experience river flooding.
The McCook Reservoir, which serves central Chicago and 36 municipalities, was full as of Monday morning, with 3.6 million gallons of water.
This is the sixth time the McCook Reservoir has reached capacity just this year. In comparison, the reservoir only reached capacity five times between 2021 and 2025, according to the Metropolitan Water Reclamation District.
A shot of the McCook Reservoir on Monday, July 6, (left) compared with the reservoir’s water levels on July 2 (right). Credit: Screenshot via Metropolitan Water Reclamation District
The McCook Reservoir is in the midst of an expansion that will add 6.5 billion gallons of capacity. The expansion, however, will likely not be complete until 2032. You can find a livestream of the McCook Reservoir here.
Chicago’s official rainfall totals are determined at O’Hare Airport, which has measured 8.56 inches since June 1. That total is well above average for this point in the summer, according to WGN-TV meteorologist Bill Snyder.
But areas including Midway Airport, and much of the South and Southwest Sides of the city, have seen much more.
“Midway is at 11.43 inches,” Snyder said. “That ranks as the second wettest on record on the South Side since records began in 1928.”
The only rainier start to summer was in 1993, an infamous summer that saw vast swaths of the Midwest experience heavy rains and intense flooding. The Mississippi and Missouri rivers experienced months of flooding, creating one of the most costly natural disasters in the country’s history.
Independence Day weekend saw much of the Chicago area receive more rain in 48 hours than is typical for the entire month of July, according to Snyder.
“Aurora Municpal Airport got nearly 8.5 inches, Naperville nearly 8 inches,” Snyder said. “Chicago’s average July rainfall for the entire month is 3.7 inches.
“It’s absolutely stunning,” he said.
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The weekend rainfall follows storms that battered the city in mid-June, causing tens of thousands of residents to lose electricity and thousands of reports of downed trees.
Chicagoans also had to deal with flooded basements after heavy rainfall this spring and some suburban residents even had to surround their homes with sandbags when the Des Plaines River flooded in April.
Annual precipitation in Illinois has increased by about 5 inches over the past 120 years, according to data from the National Oceanic and Atmospheric Administration. Storms are also getting more severe as the number of 2-inch rain days in the state has increased by 40 percent since the beginning of the 20th century, data shows.
The start of this week should be drier, according to the National Weather Service. Monday, Tuesday and Wednesday have no rain in the forecast and moderate temperatures.
Thursday sees storms return and Friday could produce some rain as well.
