I found the Lake Michigan Federation looking for more information on the link between untreated sewage, heavy rains and Chicago beach closings. This group (which I joined online here, you should too), has all the information.
Check this fact sheet out.
MWRD opens the Wilmette Locks to release sewage to Lake Michigan when the Channel can no longer handle wastewater overflows.
This must stop. There has got to be a better way than releasing sewage into the Lake. What is this, the 17th century? And we have to close the beaches because of bacteria/sewage in the water? COME ON!