Staff and infrastructure for the Metropolitan Water Reclamation District of Greater Chicago (MWRD) are working around the clock Friday to manage a rare January storm that hit the Chicago region last night.

The National Weather Service reported that approximately 1.92 inches of rain fell over the Chicago area yesterday, with many gauges across the region reporting over 2.5 inches in communities southwest of the city, rain falling heavily in a matter of hours. 

The MWRD’s Tunnel and Reservoir Plan (TARP), comprised of 110 miles of deep tunnel and three reservoirs, is working to mitigate flooding and protect area waterways from pollution across a 360-square-mile area serviced by combined sewers. MWRD staff are keeping operations flowing at our seven water reclamation plants.

The McCook Reservoir and Thornton Composite Reservoir are currently holding approximately 3.4 BG of water. Here is the breakdown:

McCook Reservoir is currently 86% full = 3 BG
The McCook Reservoir has 3.5 billion gallons of capacity, serving central Chicago and 36 municipalities covering 252 square miles on the Des Plaines and Mainstream Tunnel Systems. Water contained at McCook Reservoir is conveyed to the MWRD’s Stickney Water Reclamation Plant to be treated, cleaned, and returned to the environment.

Thornton Composite Reservoir is currently 5% full = 400 million gallons. 
The Thornton Composite Reservoir has 7.9 BG of capacity, including 4.8 BG of combined sewer capacity, serving 91 square miles on the Calumet Tunnel System, covering the South Side of Chicago and 13 south suburban municipalities.  Water contained at the Thornton Composite Reservoir is conveyed to the MWRD’s Calumet Water Reclamation Plant to be treated, cleaned, and returned to the environment.

During rainfall, stormwater flows through sewer systems owned and maintained by local municipalities. This water then flows to the MWRD’s larger intercepting sewers that convey wastewater from homes and stormwater from local sewers to the MWRD’s water reclamation plants for treatment. In heavy rainfall, the water flows past the MWRD intercepting sewer and falls into a TARP tunnel to help keep this water from overflowing into our waterways or backing up in our basements. Water must pass through local sewers first before it even reaches the TARP system.

Learn more about TARP and other MWRD programs 

How you can help prevent flooding

Learn about the types of sewer systems in Cook County

Receive direct Overflow Action alerts via text

Sign up for combined sewer overflow alerts

Visit these links for more information about the MWRD, Stormwater Management and TARP.

By reducing water during these storms, area residents and businesses can help reduce flooding and protect water quality.