Thornton Quarry reservoir hits record 94% full after weekend’s heavy rainfall

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THORNTON, Ill. - All that rain from this past weekend had to go somewhere - and that somewhere is the Thornton Quarry.

The Metropolitan Water Reclamation District of Greater Chicago reported Monday the Thornton Composite Reservoir was at 94% full, with 7.3 billion gallons.

That's the fullest the reservoir has ever been since it went online in 2015. The next high-water mark in the reservoir's history was 54.5% full, in 2019.

Brad and Joyce Dykstra shared photos of the Quarry full of water from Monday.

"The MWRD currently has over 13 billion gallons of stormwater in the deep tunnels and reservoirs that has been captured during multiple waves of storms," the MWRD said. "As soon as possible after the storms end, we begin dewatering TARP and cleaning that water at our water reclamation plants to make room for the next storm."

Heavy Rainfall Reported

The National Weather Service in Chicago reported four rounds of thunderstorms this past weekend lead to water-logged soils. The western suburbs saw the most rainfall, with 8.39 inches reported at Sugar Grove and 8.22 at the Aurora Airport. In the South Suburbs, Flossmoor topped the list at 6.37 inches. Lansing received 3.65 inches of rain.

The MWRD reported 3.19 inches of rain on July 3 and 4, almost the average monthly total of rain for this time of year, and 8.15 inches of rain over the last 30 days, which the MWRD said was more than what is normal for June and July combined.

In total, the MWRD said it captured over 13 billion gallons of stormwater that fell across Cook County.

Tunnel and Reservoir Plan

The Thornton Quarry is part of the MWRD's Tunnel and Reservoir Plan (TARP). It's a series of underground tunnels and reservoirs across the Chicago region designed to capture and store stormwater until it can be treated at water reclamation plants.

The system helps reduce flooding and improves water quality, protecting waterways like Lake Michigan from sewer overflows. Overall, the MWRD says the TARP system has 110 miles of tunnels, 8 to 33 feet in diameter and 150 to 300 feet underground.

The Thornton Composite Reservoir is one of three reservoirs, including the Majewski Reservoir near Elk Grove Village and McCook Reservoir in Bedford Park. Thornton's reservoir includes a service area of 91 square miles and 36.7 miles of tunnels from communities including Lansing.https://thelansingjournal.org/2026/07/06/thornton-quarry-reservoir-hits…