Cook County Stormwater Reservoirs at Record Capacity After Torrential Rain, With More Than 13 Billion Gallons Stored
The McCook Reservoir is full for the sixth time this year and the Thornton reservoir holds more water than at any point since it opened in 2015, the Metropolitan Water Reclamation District says — and officials say the Deep Tunnel system is still containing it all.
This weekend's heavy rainfall has pushed Cook County's stormwater system to levels it rarely sees. According to the Metropolitan Water Reclamation District, the McCook Reservoir in suburban Bedford Park holds more than 3.6 billion gallons of water and is at full capacity. Officials say it is the sixth time in 2026 that the reservoir has hit capacity — compared with five times in the previous five years combined.
The Thornton Composite Reservoir near South Holland is at 94 percent capacity with more than 7.4 billion gallons stored, the most water that has ever been in that reservoir since it opened in 2015, according to the district's estimates. In all, more than 13 billion gallons — nearly 20,000 Olympic-size swimming pools' worth — is being stored across the Tunnel and Reservoir Plan system, known as the Deep Tunnel, which captures stormwater and sewage that would otherwise overflow into area rivers and lakes and holds it until it can be treated and released.
Officials say the system is still able to contain the water that has fallen, and with no significant rainfall appearing in the near forecast, the collected water can be processed and drained down. The margin matters: if the system becomes too full, untreated sewage can overflow into rivers and basements, and officials can be forced to release untreated water into Lake Michigan, which would bring beach closures and health hazards. The district says a coming expansion at McCook will let the whole system contain up to 17.5 billion gallons.
