The Metropolitan Water Reclamation District of Greater Chicago (MWRD) staff and infrastructure worked overnight Sunday into Monday to manage heavy rainfall totals that exceeded four inches of rain on the North Side in a matter of hours.

Sunday’s rain fell at the rate of a tropical storm, reaching a half-inch in five minutes, according to the National Weather Service (NWS). Rainfall totals on the North Side ranged from 3.3 inches to as high as 4.25 inches in the West Ridge neighborhood, with nearby Evanston registering 3.69 inches, according to the NWS. O’Hare International Airport experienced 1.95 inches while Midway International Airport was at 1.4 inches, representing the wide range of localized rain totals.

MWRD water reclamation plants (WRPs) and the MWRD’s Tunnel and Reservoir Plan (TARP), consisting of 110 miles of tunnels and three reservoirs, managed to keep up with the increased flow. Despite several inches of rain, there is still capacity in the TARP system.

As of 10 a.m., the McCook Reservoir, which holds 3.5 billion gallons, was at 74 percent capacity capturing about 2.5 billion gallons. Together with its two tunnel systems, the system was at a total capacity of 80 percent. The reservoir’s adjoining two tunnel systems are a combined 66 miles and provide an additional 1.6 billion gallons of storage. The McCook Reservoir and Des Plaines and Mainstream tunnel systems serve central Chicago and 36 suburban municipalities that rely on combined sewer systems. View a live stream of the reservoir. 

The MWRD’s Thornton Reservoir, serving the far South Side of Chicago and 13 neighboring suburbs, was 2.6 percent full, capturing more than 200 million gallons of water as of 10 a.m.

TARP, also known as “Deep Tunnel,” is a system of large diameter tunnels and vast reservoirs designed to reduce flooding, improve water quality in Chicago area waterways and protect Lake Michigan from pollution caused by sewer overflows. The water from these reservoirs will be pumped back to the MWRD’s WRPs for treatment when conditions permit.

MWRD staff works around the clock to protect the water environment, mitigating flooding, managing waterway elevations, and keeping operations moving at its seven WRPs.

On days of heavy rain, the MWRD encourages residents to reduce water use before and during a rain event or storm to help prevent combined sewer overflows, which may occur when the MWRD’s intercepting sewers and WRPs reach capacity. Conserving water reduces the chance for local sewer systems to become overwhelmed and lose the capacity to keep stormwater flowing to MWRD plants for treatment.

Storm Update.pdf