In the 1970s the MWRD began work on the Tunnel and Reservoir Plan (TARP), an innovative and ambitious plan to reduce flooding and pollution in the Chicagoland area from combined sewer overflows (CSOs) that previously discharged a polluted mix of stormwater and sewage into our waterways approximatley 100 days per year at over 400 locations. TARP now captures and stores the CSOs in deep tunnels and enormous reservoirs until the storm is over and the water can be treated at the MWRD's water reclamation plants.

The Tunnel and Reservoir Plan (TARP) is being carried out in two phases. 

The first phase involved the construction of 109 miles of deep, large-diameter rock tunnels, which was completed in 2006 and is currently in operation. These tunnels capture the first flush of highly polluted CSOs. 

The second phase involves constructing flood and pollution control reservoirs for the Chicagoland combined sewer area. The Majewski Reservoir became operational in 1998, followed by the Thornton Reservoir in 2015, and Stage 1 of the McCook Reservoir in 2017. Stage 2 of McCook is being mined and scheduled to become operational in 2029, completing the TARP project.

Once TARP is completed, the total storage capacity will reach 17.5 billion gallons, with 86.8% of its capacity coming from reservoirs and 13.2% from deep tunnels. We are more than halfway there; the TARP Status Report presents construction progress, cost, State/Federal grant, and revolving loan funding information.

The amount of CSOs captured is directly related to the amount of rainfall received. From 2017 through 2022, the entire MWRD Tunnel and Reservoir Plan (TARP) system captured 255 billion gallons of CSOs. Most of the volume was captured by the Mainstream/Des Plaines TARP System. All of the captured CSO receives full treatment at one of our water reclamation plants before being discharged back to the environment.

TARP comprises three distinct tunnel and reservoir systems:

• The Calumet TARP System serves the south side of Chicago and south suburbs and was completed in 2015 with the addition of the Thornton Composite Reservoir.

• The Mainstream and Des Plaines TARP System is the largest, serving most of Chicago and several suburbs to the north, west, and southwest. It includes the partially completed McCook Reservoir. The completion of Stage 1 of the McCook Reservoir in 2017 greatly reduced the amount of CSOs in this service area. While this system already captures the most CSO due to its large service area, Stage 2 of the McCook Reservoir is still under construction, so it has not reached its full potential in terms of percentage of CSO captured.

• The Upper Des Plaines TARP System is located in the northwest suburbs and is the smallest of the three. It has captured nearly all of the CSOs within its service area since becoming fully operational with the completion of the Majewski Reservoir in 1998.

Through a variety of green and gray infrastructure projects and flood-prone property acquisitions, MWRD’s Stormwater Management Program addresses both regional and local flooding problems throughout Cook County. MWRD has made significant investments in developing 229 capital stormwater projects since it assumed the authority for stormwater management in 2004. These projects, which range in size and scope, provide flood protection for thousands of homes, businesses, and critical infrastructure. See a map of our Stormwater Infrastructure Projects.

Our Green Infrastructure Partnership Opportunity Program (GIPP) encourages and supports implementing green infrastructure (GI), which includes a range of engineered installations that store, infiltrate, and/or evaporate stormwater, thereby mimicking the natural water cycle. Since 2014, MWRD has partnered with dozens of local partner agencies to fund GI projects such as rain gardens, bioswales/bioretention areas, permeable pavement systems, and rain water harvesting systems which help minimize the potential for basement backups and combined sewer overflows. MWRD solicits applications from various governmental agencies within its jurisdiction for GI projects on an annual basis. Visit our Stormwater Management page for more information about the GIPP and other MWRD Stormwater Partnership Opportunities.