ultiple rounds of heavy rain fell between July 2 and July 4, leading to flash flooding in streets and rising river levels across the greater Chicago region, with some municipalities reporting 8 to 10 inches of rain over the 48-hour period.

According to the Metropolitan Water Reclamation District of Greater Chicago, its two largest Deep Tunnel reservoirs are nearly at capacity.

The McCook reservoir, serving central Chicago and 36 municipalities, was approximately 98% full and holding 3.4 billion gallons of water as of Monday morning. It actually hit capacity on July 3, which is the sixth time that’s happened this year alone. McCook has filled five times between 2021 and 2025.

The Thornton reservoir, serving the South Side of Chicago and 13 south suburban municipalities, was 94% full and holding 7.3 billion gallons. That’s the fullest it’s ever been since coming online in 2015, MWRD said. Thornton’s previous high-water mark was approximately 54.5% in 2019.

If the Thornton reservoir reaches capacity, the MWRD warned that communities in the south suburbs could face an increased risk from overbank flooding.

Between its combined reservoirs and tunnels, MWRD has captured and is holding 13 billion gallons of water from the recent wave of storms. This water is eventually sent to reclamation plants for treatment, clearing room in the tunnel and reservoir system for the next storm.

The first of four rounds of storms struck the evening of July 2, followed by two rounds on July 3 that also saw damaging winds. Some of the heaviest rains fell on July 4, at a point when soils were already nearly completely saturated, leading to a “significant flash flood event across the western Chicago suburbs,” the National Weather Service said.

Official gauges at Chicago’s airports recorded 3.53 inches and 3.05 inches at O’Hare and Midway, respectively. Further south in Cook County, Flossmoor recorded 6.37 inches. To the west, the Aurora airport notched 8.2 inches of rain.

Drier and cooler weather is expected through mid-week, with the chance for more rain returning on Thursday, according to the weather service.