Ice, snow and nearly two inches of rain over the Chicago area have the Metropolitan Water Reclamation District of Greater Chicago (MWRD) working around the clock to manage this weather event.   

The MWRD’s Tunnel and Reservoir Plan (TARP) is protecting the quality of the region’s water resources and mitigating flooding for 3.71 million people served through combined sewer systems.

TARP consists of three reservoirs and four tunnel systems. As of 10 a.m., TARP is holding nearly 4.3 billion gallons of water. Here is the breakdown:

• McCook Reservoir is approximately 92 percent full, holding 3.2 billion gallons. 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. 
• Thornton Composite Reservoir is approximately 13 percent full, holding 1 billion gallons. The Thornton Composite Reservoir has 7.9 billion gallons of total capacity, including 4.8 billion gallons of combined sewer capacity, serving 91 square miles on the Calumet Tunnel System, covering the South Side of Chicago and 13 south suburban municipalities.
• Majewski Reservoir is approximately 25 percent full, holding 85.6 million gallons. The Majewski Reservoir has 350 million gallons of total capacity, serving 11 square miles on the Upper Des Plaines Tunnel System, covering Arlington Heights, Des Plaines and Mount Prospect. 

As much as 1.76 inches of rain fell in parts of Cook County for the storms that began around 11 a.m. Sunday, before turning into snow early Monday. Before severe storms, the MWRD lowers the levels of the Chicago Area Waterway System at its Lockport Powerhouse to provide added capacity in local waterways. On Sunday morning, the MWRD issued an overflow action alert encouraging homeowners and businesses to conserve water, when possible, to provide more capacity in the sewers.

“Our staff is managing billions of gallons of water at our seven water reclamation plants and through our Tunnel and Reservoir Plan which is operating to protect our water environment and mitigate flooding,” said MWRD President Kari K. Steele. “As we deal with this fast-changing weather this week, we encourage everyone to conserve water, when possible, to create more capacity in our systems.” 

When it rains, stormwater flows through smaller, local pipes owned and maintained by local municipalities. This water then flows to the MWRD’s large 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 and backing up in our basements. TARP has huge capacity, but 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.