Tunnel and Reservoir Plan

Unmatched in size throughout the world, the Tunnel and Reservoir Plan (TARP) works to protect and improve area waterways and reduce flooding throughout Cook County. TARP is one of the country’s largest public works projects for pollution and flood control.

TARP, also known as “Deep Tunnel,” is a system of deep, large diameter tunnels and vast reservoirs designed to reduce flooding, improve water quality in Chicago area waterways and protect Lake Michigan from pollution caused by sewer overflows. 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 water reclamation plants (WRPs) to be cleaned before being released to waterways.

TARP includes four tunnel systems totaling 109 miles of tunnels, 8 to 33 feet in diameter and 150 to 300 feet underground. The four TARP tunnel systems capture and convey combined sewage and stormwater to the Majewski Reservoir, Thornton Composite Reservoir and McCook Reservoir (Stage I). The system will have a combined sewage capacity of 17.5 billion gallons when Stage 2 of McCook Reservoir is completed. That is more than 4,666 gallons for each person in its service area. Construction on the McCook Reservoir Stage 2 is scheduled for completion in 2029.

  • One of the largest civil engineering projects on earth, TARP has been extremely effective and widely emulated since the initial tunnels went online in 1981.

Protecting our Region’s Health

The MWRD adopted TARP in 1972 as the Chicago area’s plan to cost-effectively comply with Federal and State water quality standards in the 375 square mile combined sewer area consisting of Chicago and 51 suburbs. TARP’s main goals are to protect Lake Michigan—the region’s drinking water supply—from raw sewage pollution; improve water quality of area rivers and streams; and provide an outlet for floodwaters to reduce street and basement sewage backup flooding.

Phase I of TARP, intended primarily for pollution control, is made up of four distinct tunnel systems: Mainstream, Des Plaines, Calumet and Upper Des Plaines. The separate tunnel systems and their service areas are shown on Figure 1.  After a storm event, pumping stations dewater the tunnel systems as Water Reclamation Plant (WRP) capacity becomes available, making the tunnel and reservoir capacity available for the next storm event. All captured combined sewer flow pumped to the WRP receives full secondary treatment prior to being discharged to the waterway pursuant to the National Pollutant Discharge Elimination System permits.

Combined Sewer Overflows

Despite the reversal of the Chicago River, and even the construction of the largest wastewater treatment plant in the world, contaminants continued to accumulate in the rivers, canals and Lake Michigan. The persistence of the problem was due mainly to the fact that Chicago and many of the older suburbs are served by combined sewers, in which both sanitary and storm flow are conveyed through the same pipes.

As the area developed and more land was paved, the amount of rain water entering the sewer system dramatically increased. During rain events, the sewer system and treatment plants could not accommodate the additional flow, and combined sewage would overflow to the local waterways over 100 days per year. Within the combined sewer areas there were over 450 outfalls that released polluted combined sewer overflows (CSOs) into the waterways. During particularly large storms, the rivers were forced to reverse to their natural direction, releasing raw sewage into the lake. Beach closings were frequent along the Lake Michigan shoreline and the area waterways were polluted and devoid of aquatic life. In addition, combined sewage would back up into basements of homes and businesses.

Construction of the Phase I tunnel systems began in 1975. The tunnel systems were put into service as portions were completed, starting in 1985. By 2006, all of Phase I was completed and in operation. The total system consists of more than 110 miles of deep, large diameter, rock tunnels providing 2.4 billion gallons (BG) of volume to capture CSOs that previously discharged at hundreds of outfall locations.

Phase II of TARP consists of reservoirs intended primarily for flood control, but it will also considerably enhance pollution control benefits being provided under Phase I. The U.S. Army Corps of Engineers' (USACE) Chicagoland Underflow Plan (CUP), Final Phase I General Design Memorandum (GDM) of 1986 defined the Federal interest in TARP Phase II based on the Federal National Economic Development Plan criteria. The three reservoirs proposed under TARP Phase II/CUP are: the Gloria Alitto Majewski, McCook and Thornton reservoirs. When all three reservoirs are completed, the reservoirs will increase the TARP system storage volume to 17.5 BG.

Click below for more information about TARP, CSO and our service areas.

Providing relief to area residents

  • Majewski Reservoir: The 350-million-gallon Majewski Reservoir was completed by the USACE in 1998 at a cost of $45 million. Since its completion, the Majewski Reservoir has captured over 7.8 BG of combined sewage over 80 events and prevented flood damages in the three communities it serves (as of April 2022). 
  • McCook Reservoir: McCook Reservoir Stage 1 was completed in 2017 and provides 3.5 BG of storage. Stage 2 will be completed in 2029 and provide 6.5 BG of storage. The McCook Reservoir will provide more than $143 million per year in flood damage reduction benefits to 3.1 million people in 37 communities, along with a total capacity of 10 BG. Since becoming operational, Stage 1 of the McCook Reservoir has captured over 88 BG of combined sewage (as of April 2022). 
  • Thornton Composite Reservoir: The Thornton Composite Reservoir was constructed in two stages. The first stage, a temporary 3.1 BG Natural Resources Conservation Service (NRCS) flood control reservoir called the Thornton Transitional Reservoir, was completed in March 2003 in the West Lobe of the Thornton Quarry. This reservoir provides Thorn Creek overbank flood relief for nine communities and has captured over 58 BG of flood water during 83 fill events (as of April 2022). The second stage is a permanent combined NRCS/CUP reservoir, called the Thornton Composite Reservoir, constructed in the North Lobe of the Thornton Quarry. The Thornton Composite Reservoir currently provides 7.9 BG of storage, including 4.8 BG of CSO storage. In operation since the fall of 2015, the reservoir provides benefits to 556,000 people in 14 communities. Since becoming operational, the Thornton Composite Reservoir has prevented over 50 BG of combined sewage from entering the waterways during 115 events (as of April 2022). By the end of 2022, the temporary 3.1 BG Thornton Transitional Reservoir will be returned to the quarry and all of the flow will be held in the Thornton Composite Reservoir.

Award-winning Success

The success of the TARP is evident by the dramatic improvements in the water quality of the Chicago River, the Calumet River and other waterways. Game fish have returned, marinas and riverside restaurants abound, river recreation and tourism are booming, and waterfront real estate values have skyrocketed as Chicago area residents see the river system as a major asset rather than an embarrassment.

TARP has received many awards, including the American Society of Civil Engineers (ASCE) award for most outstanding Civil Engineering Project of 1986. In 2016, the Thornton Composite Reservoir was recognized with several achievements, including the American Public Works Association (APWA) National Conference Project of the Year, the APWA Chicago Metro Chapter Project of the Year, the ASCE Illinois Section Project of the Year, the Illinois Department of Natural Resources Mined Land Reclamation Award, and the Illinois Association for Floodplain and Stormwater Management Flood Reduction Project Award. TARP has been named by the U. S. Environmental Protection Agency as one of the nation's top Clean Water Act success stories and is serving as a model urban water management tool worldwide.

Frequently Asked Questions

What is the purpose of the Tunnel and Reservoir Plan?

The MWRD’s Tunnel and Reservoir Plan (TARP) or “Deep Tunnel” system is designed to reduce flooding and pollution caused by combined sewer overflows. TARP tunnels act like very large intercepting sewers, capturing the excess flow from the combined sewers before it can reach the waterway. Excess flow is conveyed through the tunnels to the large storage reservoirs. The “tunnels” portion of TARP was completed in 2006 and is fully operational. The “reservoir” portion of TARP is currently mainly complete, with one expansion of the McCook Reservoir still under construction. The stormwater captured by TARP is eventually pumped to the MWRD water reclamation plants for treatment.

What if TARP is full?

Although it is one of the largest civil engineering projects ever undertaken, TARP capacity is not unlimited and can fill during intense storms. When TARP is full, the local sewers overflow by gravity into the waterways, as they did before TARP was built.

The tunnels currently are used for storage and can hold 2.4 billion gallons. When totally complete, the reservoirs will add 15.15 billion gallons of capacity, but it will still be possible for the system to fill during extreme storms.

What causes sewer backups?

Backups are usually caused by insufficient flow capacity at some point in a sewer system. The system cannot drain as quickly as it is being filled.

One way to understand the concept of flow capacity and the difference between normal sewer flow and a sewer backup is to imagine a kitchen sink being filled with water faster than it can drain.

How can I sign up for CSO alerts?

You can sign up to receive notification regarding CSO event or diversion to Lake Michigan by completing a registration form.