The region's main flood defense is running out of room, and more rain is on the way.
The Metropolitan Water Reclamation District issued an Overflow Action Alert for Monday, Aug. 10, asking Park Ridge and Niles residents to delay showers, flush less and hold off on laundry as the McCook TARP Reservoir sat at 95% capacity with the system holding nearly five billion gallons of stormwater.
That alone would be routine.
What isn't routine: the McCook reservoir has already hit full capacity six times in 2026, according to MWRD officials. Between 2021 and 2025, it reached capacity five times total.
The district confirmed the sixth capacity event occurred July 3 in a July 7 Facebook post, noting the reservoir "filled 5 times cumulatively between 2021 and 2025."
Both Park Ridge and Niles are among the 37 communities the McCook reservoir is designed to protect from sewer backups and basement flooding.
What happens when it fills
When the TARP system reaches capacity during continued heavy rain, untreated sewage can overflow into area rivers and back up through neighborhood sewers into basements and streets. Officials could also be forced to release untreated water into Lake Michigan, triggering beach closures.
Even when the reservoir has room, local street flooding and basement backups can still occur if neighborhood sewers lack the capacity to move water into MWRD's intercepting tunnels.
More storms forecast through mid-week
The National Weather Service forecast severe weather for the Chicago area Monday, with the primary threat window running from 3 p.m. Aug. 10 through 5 a.m. Tuesday. Hazards include damaging winds, hail, flash flooding and possible tornadoes. Additional rounds of storms are expected through mid-week, particularly Tuesday and Tuesday night, according to Patch.
MWRD officials expressed concern that the current weather pattern could allow heavy rains to repeatedly hit the same areas, leading to extreme precipitation totals.
Expansion delayed to 2032
The McCook reservoir's first stage, completed in 2017, holds 3.5 billion gallons. A second stage under excavation will add 6.5 billion gallons for a total of 10 billion, but MWRD pushed the completion estimate to Dec. 31, 2032, back from a previous target of 2029. The district attributed the delay to a slowdown in demand for quarried stone in the Chicago area, according to an MWRD update on the project.
"We encourage everyone to conserve water when possible to create more capacity in our systems," MWRD President Kari K. Steele said in the district's Aug. 10 alert. Steele issued a nearly identical statement July 31 during the previous overflow alert, the district's 10th of the year.
What residents should do
MWRD asks residents to take these steps during and after heavy rain:
- Delay showers and baths
- Flush toilets less frequently
- Wait to run dishwashers or washing machines
Residents can sign up for future Overflow Action Alerts through MWRD's website. The MWRD Board of Commissioners meets next on Thursday, Aug. 13, at 10:30 a.m.
