New forecast services for Mainstream Deep Tunnel at McCook Reservoir, IL

Effective May 26, 2026, forecast services will begin for the Mainstream Deep Tunnel at McCook Reservoir near McCook, Illinois. In addition, Action Stage will be changed to a level approximately equivalent to the reservoir being 75% full. These changes are the result of almost 6 years of discussion, modeling, and meetings with multiple federal, state, and local agencies.

These changes were announced via a Service Change Notice issued April 24, 2026.

Mainstream Deep Tunnel at McCook Reservoir

Action Stage
Change from -40 feet to -100 feet

Minor Flood Stage
Set at -30 feet

Human reviewed forecasts for this location will be issued when the water level has reached, or is expected to reach, Action Stage. No flood watches or warnings will be issued for this location. Observations and forecasts will be used to monitor the potential for combined sewer overflow events and Chicago River flooding.

Due to the design of the reservoir, flood stage is not expected to ever be reached.

Forecast services for this reservoir were developed in collaboration with the North Central River Forecast Center, representatives from the City of Chicago, Cook County Emergency Management and Regional Security, Metropolitan Water Reclamation District of Greater Chicago, and the US Army Corps of Engineers Chicago District.

For more information about Metropolitan Water Reclamation District of Greater Chicago's Tunnel and Reservoir Plan, click here.