Tunnel and Reservoir Plan



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.

The Deep Tunnel

The District adopted the Tunnel and Reservoir Plan (TARP) in 1972 as the Chicago area’s plan to cost-effectively comply with Federal and State water quality standards in the 375 square miles 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.

Construction of the Phase I tunnel systems commenced 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 109.4 miles of deep, large diameter, rock tunnels providing 2.3 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 20.55 BG.


TARP Service Areas/Communities Served
Complete TARP Status Report
TARP Fact Sheet

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 yielded over $350 million in flood damage reduction benefits to the three communities it serves.

McCook Reservoir

The McCook Reservoir is currently under construction and, when completed, the reservoir will have a total capacity of 10 BG. McCook Reservoir Stage 1 will be completed in 2017 and provide 3.5 billion gallons of storage. Stage 2 will be completed in 2029 and provide 6.5 billion gallons of storage. The McCook Reservoir will provide more than $143 million per year in flood damage reduction benefits to 3,100,000 people in 37 communities.

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 33.7 BG of flood water during 52 fill events.

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 provides 7.9 BG of CSO storage. In operation since the Fall of 2015, it provides benefits to 556,000 people in 14 communities. In its first year of operation, it captured more than 4.5 BG of polluted water.

TARP 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.