The Metropolitan Water Reclamation District — a county agency, not the city — has built TARP since 1975: 110 miles of tunnel up to 350 feet down, feeding reservoirs that now hold over 11 billion gallons. MWRD publishes reservoir fill levels on a public live map. Overflow days fell from about 100 a year to 50; McCook Stage 2 has slipped to 2032.
MWRD reports combined sewer overflows fell from an average of 100 days a year to 50 once the tunnels came online, and that overflows in the Thornton Composite Reservoir's service area have been nearly eliminated since 2020. TARP has captured over 1 trillion gallons of combined sewage since 1981, about 130 billion of it by McCook Stage 1 alone. All figures are the district's own; no independent evaluation was found.
🎓 Lesson
Capacity is not protection. Fifty years and more than $3 billion halved overflow days, but in July 2026 McCook sat between 96% and 100% full while 311 logged about 400 flooded basements — the century-old street sewers, not the tunnels, are the bottleneck.
Evidence🏛️ Official
Scale🗺️ Regional
Costover $3bn across 30 years of construction
FundingMWRD, with federal cost-sharing through the US Army Corps of Engineers
Sources
↗ MWRD — Tunnel and Reservoir Plan (TARP) (authority, operator)Operator page. Carries the scale and the outcome figures sentence-level: 'four tunnel systems totaling 110 miles of tunnels, 8 to 33 feet in diameter and 150 to 300 feet underground'; 'The system currently has a capacity of more than 11 billion gallons for combined sewage and stormwater'; 'since the first portions of the system were put in operation in 1981, TARP has captured over 1 trillion gallons of combined sewage'; 'CSOs have been reduced from an average of 100 days per year to 50'; Thornton eliminating CSOs in its service area since 2020. Reservoir dates: Majewski 1998, Thornton 2015, McCook Stage 1 2017.
↗ MWRD — TARP Reservoir Levels (authority, the data layer)This is the measurement layer the entry rests on: a public interactive map of live reservoir fill levels, with a full-screen view and a live camera on McCook. The page states no update interval or units, so the entry claims only that fill levels are published live, not any accuracy or frequency.
↗ MWRD — issues update on the Tunnel and Reservoir Plan, 15 August 2025 (authority)Currency source, and the correction that matters: 'The current estimate for McCook Stage 2 going online is now Dec. 31, 2032.' Consent decree entered 6 January 2014 originally required mining complete by 31 December 2028 and operation by 31 December 2029; the delay is attributed to a slowdown in Chicago-area stone demand. Also: McCook Stage 1 'has already captured approximately 130 billion gallons'; Stage 2 brings McCook to 10 billion gallons; the system serves 3.1 million people across Chicago and 36 suburban municipalities.
↗ WBEZ Chicago — Deep Tunnel nearly full after days of heavy rain, 6 July 2026 (press)Counterweight and 2026 currency, sentence-level: 'The McCook reservoir in southwest suburban Bedford Park fluctuated between 96% and 100% full' after 'more than 8 inches of rain over the past 30 days'; 'The city's 311 system received almost 400 complaints about water in basements and nearly 600 reports about flooded streets between Thursday and Monday afternoon.' Also carries the independent view: 'Experts say that Deep Tunnel isn't enough to prevent flooding.'
↗ Ven Te Chow Hydrosystems Laboratory, University of Illinois — Tunnel and Reservoir Plan (academic, boundary source)Boundary source, and the reason this entry claims no automation: 'The MWRDGC now desires to develop a new, updated and enhanced computer model for each TARP system, to allow real-time control of the TARP systems.' Real-time control is a stated goal of a modelling research programme, not a documented operating system. Earlier models 'were based on design, rather than as-built specifications for the system and are now outdated.'
↗ Wikipedia — Tunnel and Reservoir Plan (reference)Reference work, used only for dates and cost where MWRD does not state them: 'adopted in 1972, commenced in 1975, and completed and operational by 2006'; '109.4 miles of drainage tunnels ranging from 9 to 33 feet in diameter, up to 350 feet underground'; 'Across 30 years of construction, over $3 billion has been spent on the project.' Its reservoir capacities (Majewski 350 million, Thornton 7.9 billion, McCook Stage 1 3.5 billion gallons) and 2032 Stage 2 date agree with MWRD.
↗ Smart Cities Dive — with extreme rainfall overwhelming Chicago's sewers, the city tries a new approach, July 2026 (trade)The city's local answer to TARP's limits, and the source of the agency split: the $12 million Wing Storage pilot is 'a partnership between the Chicago Department of Water Management and the Metropolitan Water Reclamation District of Greater Chicago' — two vaults under West Side streets holding nearly 1.7 million gallons for more than 2,900 buildings, announced 8 July 2026. No sensors, monitoring or control layer is described; water is released back into the sewer as capacity frees up.
📎 Cite this project
1001 Smart Cities (2026). “TARP: Chicago's Deep Tunnel & Reservoir System” — Chicago, United States. The Smart City Atlas. https://1001smartcities.org/projects/chicago-tarp-deep-tunnel/ (last verified 2026-07-26). Data: CC BY 4.0.
The underlying data is free to reuse with attribution — see the open data page.
