The Metropolitan Water Reclamation District of Greater Chicago (MWRD) works around the clock to provide flood protection for Cook County while protecting our greatest asset, the source of our drinking water, Lake Michigan. We monitor the weather, and prior to major storms, we draw down the waterways to provide maximum capacity in the waterways, tunnels and reservoir system.

However, when there is an extreme weather event such as what happened July 2 and 3, we must reverse the flow of the Chicago River when the river level exceeds the elevation of Lake Michigan. During this past weekend’s storm, the elevation of the North Shore Channel topped the elevation of Lake Michigan, and we opened the gates at 2:16 p.m. on July 2. The gates remained open until 9:47 p.m. on July 2. The elevation of the Chicago River downtown exceeded Lake Michigan’s elevation so the locks were opened at 4 p.m. on July 2 and the reversal ended at 2:30 a.m. on July 3. Reversing to the lake only happens in extreme situations. 

Read: Reversals to Lake Michigan (1985 - Present)

In other words, when the Chicago area waterway levels are higher than Lake Michigan, only then can the MWRD open control structures to move as much water as possible out of the system. We cannot open the gates and lock before that time. There is NO MAGIC KEY OR BUTTON to use at will. Opening the gates and lock not only provide overbank flooding protection but they allow for more capacity for stormwater.  As a result, the MWRD can only reverse the waterway to the lake when the river level is ABOVE Lake Michigan levels. If we were to open the lock and gates too early, Lake Michigan would have a tsunami effect, overtaking the river and flooding everything in its path in downtown Chicago and along the waterways, totally decimating the riverwalk and municipalities downstream, on the South side and on the North side. The destruction that would be caused by opening the gates and lock too early is unimaginable.

According to the National Weather Service, “rainfall totals in and in the immediate vicinity of Chicago ranged from roughly 3 to 7 inches, though a few localized areas received over 8 inches of rainfall. The worst of the flooding occurred on the west and southwest sides of Chicago and in the near west and southwest suburbs.”  This includes Cicero and Berwyn. There is no system able to handle that much water in just a few hours. 
View: Significant Flash Flooding in Chicago and Nearby Suburbs

Consistent with the MWRD’s mission of protecting Lake Michigan from pollution, every effort is made to minimize the amount of floodwater discharged to the lake during extreme storms. In a combined sewer system like we have here, sanitary sewage and stormwater drain into the same pipes. Homes, businesses and street drains are connected to the local sewers, which are owned and maintained by municipalities. Local sewers flow by gravity into the MWRD intercepting sewers, which then convey the flow to MWRD water reclamation plants for treatment.

To combat this, we have partnerships with municipalities and other organizations throughout the county to fully or partially fund green infrastructure and other stormwater projects to improve resilience within that community. The 34 Space to Grow® green infrastructure projects that convert asphalt into permeable surfaces at local schools is just one example of a partnership improving communities one neighborhood at a time.

Here are some links that provide additional information and explanations about how the MWRD’s infrastructure and waterway system operate.

Information about the Tunnel and Reservoir Plan (TARP)

How the Chicago Area Waterway System works

How to prepare for stormwater

https://mwrd.org/flood-prevention-101

Booklet: Understanding how sewers work – we also have this as a PowerPoint presentation that we present to communities and organizations

We offer a live virtual tour once a month, there is a recorded version plus we give thousands of people locally and from around the world tours of our facilities, including the TARP pumping stations and reservoir

We would be happy to provide a presentation to your group or a tour of our operations and facilities to answer any and all questions. Please contact forea@mwrd.org for additional information or to schedule a tour.

July 5 storm update.pdf