"Cook County stormwater reservoirs at record capacity after torrential rain," NBC Chicago 5
This weekend’s heavy rainfall in the Chicago area has helped to fuel record amounts of stormwater being stored in Cook County’s reservoirs, nearly 20,000 Olympic-size swimming pools’ worth.
According to the latest estimates from the Metropolitan Water Reclamation District, the McCook Reservoir in suburban Bedford Park has more than 3.6 billion gallons of water in it, meaning it is at full capacity. Officials say this is the sixth time in 2026 the reservoir has hit capacity, compared to the five times it had been full in the previous five years combined.
In addition, the Thornton Composite Reservoir, located near South Holland, is at 94% capacity, with more than 7.4 billion gallons of water being stored. According to estimates, it’s the most water that has ever been in that reservoir since it was opened in 2015.
A dramatic timelapse video released by MWRD shows just how quickly the water rose in the Thornton reservoir during this week’s storms:
In all, more than 13 billion gallons, or nearly 20,000 Olympic-sized swimming pools’ worth, of stormwater is being stored in the area’s Tunnel and Reservoir Plan (TARP) system, according to MWRD.
The TARP system is also known as the “Deep Tunnel” system, capturing and storing stormwater and sewage that would otherwise overflow into area rivers and lakes during rainstorms. The system then transfers that water to treatment facilities, where it is processed and then released back into area rivers and streams.
That water makes its way through approximately 110 miles of tunnels, which are 8-to-33 feet in diameter and located hundreds of feet underground through the Chicago area. Those tunnels carry water into three separate reservoirs, including the Thornton and McCook reservoirs, along with the Majewski Reservoir near suburban Elk Grove Village.
If the system becomes too full of water, untreated sewage can make its way into area rivers and can overflow from sewers, leading to neighborhood and basement flooding. Officials may also be forced to release untreated water into Lake Michigan to alleviate pressure on the TARP system, which would lead to beach closures and potential health hazards.
Officials say the system is still able to contain the water that’s fallen from recent storms, and the NBC 5 Storm Team has good news, as significant rainfall isn’t appearing in the forecast for the foreseeable future, allowing the collected water to be processed and drained out of the reservoir system.
In coming years, MWRD says it is expanding capacity at the McCook Reservoir, which will in turn allow the entire system to contain up to 17.5 billion gallons of water in the event of heavy rainfall.