Thornton Reservoir


Water Resources Development Act 1988, as modified by WRDA 1999 and 2007.


The Chicagoland Underflow Project -Thornton Reservoir, completed by the Metropolitan Water Reclamation District (MWRD) of Greater Chicago in the fall of 2015, provides storage of combined sewer overflow (CSO) for the existing Calumet "Deep Tunnel" System.  The reservoir stores and releases for treatment CSO that would otherwise be discharged directly into area watercourses.  WRDA 1999 modified the project to include the authorized Natural Resources Conservation Service Thorn Creek Reservoir as part of the federal project.  As a result, the original 14,600 acre-feet reservoir was increased by 9,600 acre-feet to a total volume of 24,200 acre-feet (7.9 billion gallons) for the new “composite” reservoir.  Major components include hydraulic structures; tunnels; gates; valves; concrete dam; aquifer protection; and an aeration system.  WRDA 2007, Section 5157 amended WRDA 1996, Section 211, and authorized reimbursement of the sponsor for the Federal share of the design and construction of the authorized reservoir. WRDA 2014, 2016, AND 2018 included provisions for credit in lieu of reimbursement for federal flood risk management projects. 


Average annual benefits of the project are $27,000,000.  Thornton Reservoir benefits the City of Chicago and 13 suburban communities.  It significantly reduces overland and basement flooding by protecting 35,090 structures.  It also improves the water quality of area waterways by providing storage during floods to prevent CSO backflows to area watercourses and ultimately, Lake Michigan.  After the flood peak has passed, the stored CSO waters are released to the Calumet wastewater treatment plant for processing.


Total Project Cost:

  $ 340,397,000

Federal Share and Potential Reimbursement Cost:

  $ 255,298,000

Non-Federal Share:

  $ 85,099,000

Project Manager

Michael Padilla