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Metropolitan Water Reclamation District of Greater Chicago

The MWRD’s Tunnel and Reservoir Plan, also known as TARP or “The Deep Tunnel,” is a system of deep, large diameter tunnels and vast reservoirs. TARP is designed to reduce flooding, improve water quality in Chicago area waterways and protect Lake Michigan from pollution caused by sewer overflows. 

An aerial view of the McCook Reservoir with a boat passing by on the Chicago Sanitary and Ship Canal
McCook Reservoir is located between the Chicago Sanitary and Ship Canal (left) and the Des Plaines River (right).


Stormwater storage 

TARP captures and stores combined stormwater and sewage that would otherwise overflow from sewers into waterways in rainy weather. This stored water is pumped from TARP to MWRD water reclamation plants to be cleaned before being released to waterways. 

Workers walk through a large tunnel
The Des Plaines Inflow Tunnel connects the 26 mile long Des Plaines TARP Tunnel to McCook Reservoir.


Huge scale

TARP includes four tunnel systems totaling 110 miles of tunnels, 8 to 33 feet in diameter and 150 to 300 feet underground. The four TARP tunnel systems capture and carry combined sewage and stormwater to the Majewski Reservoir near Elk Grove Village, Thornton Composite Reservoir near South Holland and McCook Reservoir in Bedford Park. 

Workers wearing high visibilty vests stand in a tunnel in front of a gate structure
The Des Plaines Inflow Tunnel gate structure can be used to isolate the tunnel from McCook Reservoir.


The system currently has a capacity of more than 11 billion gallons for combined sewage and stormwater. McCook Reservoir is being completed in two stages. Stage 1 is in operation now and when Stage 2 is complete, TARP will have a total capacity of 17.5 billion gallons. That is more than 4,666 gallons for each person in its service area! 

A limestone wall being excavated with explosives
Explosives are used to excavate the large TARP reservoirs. 


Since the first portions of the system were put in operation in 1981, TARP has captured over 1 trillion gallons of combined sewage.

What if TARP fills up?

If TARP is full and heavy rains continue, sewers can still overflow to waterways.

Red and white water intake structures in the foreground and the Chicago skyline in the distance
Protecting Lake Michigan, the region's water supply, from pollution is one of the goals of TARP.


Goals of TARP

TARP’s main goals are:

  • to protect Lake Michigan—the region’s drinking water supply—from raw sewage pollution.
  • to improve water quality of area rivers and streams.
  • to provide an outlet for floodwaters to reduce street and basement sewage backup flooding.
A large reservoir filling with combined sewage and rain water in January 2018
Water entered McCook Reservoir for the first time in 2018 from the 33-foot diameter Mainstream TARP Tunnel. 


The challenge: combined sewer overflows

Most of Chicago has a combined sewer system, which means that sewage from homes, offices, and businesses flows into the same pipes as rainwater. These combined sewers were built before wastewater treatment technology existed, so they were designed to empty the sewage straight into the river. 

In the early 20th century, the Metropolitan Water Reclamation District of Greater Chicago (MWRD) built big intercepting sewers to redirect the sewage to new wastewater treatment plants. Our treatment plants clean sewage and release clean water to the river. This system works great in dry weather, but when it rains heavily, the intercepting sewers and treatment plants can get overloaded and sewage follows the original path of the sewers and overflows into the river. These combined sewer overflows pollute the water and also increase the danger of flooding.

Underground construction of 33 foot diameter tunnels with workers and equipment


Planning TARP

As development spread through the Chicago area in the early 20th century, paved surfaces sent more and more amounts of stormwater runoff into the combined sewer system. By the 1960s, Chicago area sewers were overflowing to the river more than 100 days a year and flooding had become an ongoing problem. 

Flood Control Coordinating Committee

In 1967, officials of the MWRD, the state of Illinois, Cook County and the city of Chicago formed the Flood Control Coordinating Committee to find a solution to the region’s flooding and water pollution problems caused by combined sewer overflows. 

Alternative plans

The committee reviewed over 50 alternatives and selected TARP as the most cost-effective approach to providing the most benefits with the least negative effects. The most obvious solution, replacing combined sewers with separate storm and sanitary pipes, was found to be too expensive, disruptive to communities, and unable to provide flood relief. The MWRD officially adopted TARP as the area’s plan to meet federal and state water quality standards in 1972.


TARP tunnel construction began in 1975. Construction was planned so that completed portions of the system could be put into operation as work continued elsewhere. 

Tunnel boring machines

The scale and depth of the project was unlike anything previously undertaken and required innovative approaches to tunneling. Newly-developed tunnel boring machines were used instead of traditional blasting to reduce vibrations, speed progress, and lower costs for long sections of tunnels. 

A large tunnel boring machine next to a van and a person, which look small by comparison
Huge tunnel boring machines were used to excavate the long TARP tunnels.
The rotating/grinding front of a tunnel boring machine breaking through a wall underground with a small crowd of people watching


Building reservoirs

Partnering with commercial quarries has allowed the huge TARP reservoirs to be completed economically and efficiently. The dolomite limestone excavated from the reservoirs is sold by the quarries and used for construction and road building.

Protecting groundwater

To protect groundwater from leakage and protect TARP tunnels from water seeping in, cracks in the limestone are sealed with grout and the tunnels are lined with concrete. The large reservoirs are surrounded by “grout curtains” that seal their perimeters to keep water from escaping and groundwater from entering. A naturally occurring watertight layer of shale beneath the reservoir floors keeps water from seeping out the bottom. Groundwater monitoring wells around the reservoirs and along the tunnels confirm that the system is working as designed. 

Current status

TARP was planned so portions of the system could be put in operation as they were completed. The Upper Des Plaines Tunnel System, located near O’Hare Airport, began operation in 1981 and the entire tunnel system was operational in 2006. The smallest of the TARP reservoirs, Gloria Alitto Majewski Reservoir, was completed in 1998. Thornton Reservoir was completed in 2015 and yielded nearly instant benefits. McCook Reservoir is being completed in two stages. The first stage, with a capacity of 3.5 billion gallons, started operations in 2017. 




As portions of the system have been completed, TARP has been extremely successful in preventing flooding and pollution caused by combined sewer overflows. Since the tunnels were completed in 2006, CSOs have been reduced from an average of 100 days per year to 50. Thornton Reservoir has completely eliminated CSOs within its service area since 2020. 

Water quality

As water quality has improved, our waterways have become home to increasingly healthy and diverse fish populations and popular destinations for recreation. Other cities around the world have taken note of TARP’s success and are now undertaking similar deep tunnel projects.

More information about TARP

Visit the McCook Reservoir Live Stream to see current conditions at the reservoir. 

See current TARP reservoir levels

For more detailed information about funding and construction status see our 2023 TARP Status Report