The Tunnel and Reservoir Plan (TARP), also known as "The Deep Tunnel," is the Metropolitan Water Reclamation District’s (MWRD) large scale engineering project designed to reduce flooding and combined sewer overflows (CSOs) within the MWRD’s service area including the Chicago-Calumet River watershed.

TARP’s primary purpose is to reduce CSOs, flooding, and backflows into Lake Michigan by using a combination of tunnels and reservoirs to store combined stormwater and sewage after heavy rainfalls until it can be treated, rather than discharging the polluted mess into local waterways. 

The system currently has a capacity of more than 11 billion gallons for combined sewage and stormwater. Once complete, TARP will have a total capacity of 17.5 billion gallons. 

TARP Timeline

• 1972 – TARP approved as the regional plan to address flooding and water pollution

• 1975 – Construction of TARP begins

• 1981 – Upper Des Plaines Tunnel System (near O’Hare) begins operation

• 1984 – Original scheduled completion date for TARP

• 1998 – Gloria Alitto Majewski Reservoir (smallest TARP reservoir) completed

• 2006 – Entire TARP tunnel system becomes operational

• 2015 – Thornton Reservoir completed, delivering major immediate benefits

• 2017 – McCook Reservoir Stage 1 (3.5 billion gallons capacity) begins operation

• In progress – McCook Reservoir Stage 2 (final phase of TARP reservoir system)