Wet Spring and Combined Sewer Overflows

Have you been surviving the the seemingly neverending spring deluge?

In a sign of our soggy situation, and as the result of a final heavy downpour early this morning, I woke up to this MWRD notification:

5:32 AM (1 hour ago)
to CSOwebmaster


There has been an update to the Metropolitan Water Reclamation District of Greater Chicago’s CSO Events website. A discharge into Lake Michigan has occurred at the Wilmette Gates. This facility is located at Sheridan Road and the North Shore Channel in Wilmette, IL. The lakefront in this area is potentially impacted. Please visit our Please visit our CSO GeoHub for more information or contact 312-751-5133 with questions. Thanks,

Clicking the CSO GeoHub link opens a map that shows all the combined sewer overflows in the MWRD sewer system:

For local residents, when we hear that “discharge into Lake Michigan has occurred at the Wilmette Gates,” we translate that as “they opened the locks.”

But there’s a lot of confusion about what it means to open the locks.

Opening the Locks

The Wilmette Gates, or “locks,” separate Lake Michigan from the North Shore Channel:

During intense rainfall, the eastside combined sewer takes in so much rainfall that it must be offloaded, or it will start backing up into eastside streets and homes.

Here’s how the locks play a part in the eastside Wilmette flood protection system…

When Do They Open the Locks?

Normally, the old eastside combined sewer takes in stormwater, which mixes with sewage, and the mixture flows to a Water Reclamation Plant for cleanup. (It’s inefficient to clean sewage and stormwater together, but it’s an old system, and that’s how it used to be done.)

If the combined sewer gets overloaded:

  1. The overflow goes into the relief sewer, which connects to the TARP system.
  2. The combined sewer also overflows directly into the North Shore Channel at several points (CSOs shown as red dots on the maps, above).
  3. If the channel threatens to overflow, the locks are opened by the MWRD, so the stormwater/sewage mixture can flow into the Wilmette Harbor and lake.

Note: As illustrated in this Wilmette sewer overview, opening the locks is just one part of the eastside flood protection system. Opening the locks has no effect whatsoever on westside flooding.

Notifications About CSOs and Lock Openings

It’s a good idea to stay out of the waterways after a combined sewer overflow (CSO) and avoid swimming in the lake if the Wilmette Gates are opened. Obviously, bacteria counts are high when combined sewers discharge raw sewage directly into a waterway.

To find out when CSOs occur, you can sign up for notifications from MWRD, and get automated alerts via phone or email.

What the Heck?

As I was finishing this post, I received another notification from MWRD:

8:11 AM (23 minutes ago)
to CSOwebmaster


On Wednesday, May 29, 2019, the MWRD inadvertently issued a Lake Michigan reversal notification for the Wilmette Gate. No actual reversal activity occurred. We apologize for the inconvenience.


So in this case, perhaps the lake was spared…? Regardless, it’s clear that there have been many combined sewer overflows in the area, including along the North Shore Channel (which basically functions as an open sewer–ugh!!).

I’ll be following the notifications before taking my dog to the Gillson Dog Beach.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.