John Murray, Executive Director of the Metropolitan Water Reclamation District of Greater Chicago, who oversees MWRD's operations, says McCook Reservoir is nearing its capacity of about 3.5 billion gallons. It's currently at about 95% full and Thornton Reservoir, part of the region’s Deep Tunnel flood-control system is at about 60% full, "what this means is that there's a higher chance of combined sewer overflows to area waterways."
If conditions arise where the elevation of waterways in the Chicago area raise higher than Lake Michigan, there could be overflows into the lake.
Murray said "last time that there was an overflow in Lake Michigan was in July of 2023 when we had just an incredible amount of rainfall in a very short period of time. It was about eight inches in just a couple hours. However, before a rainstorm hits, we've lowered the area of the Chicago Area Waterway System, and we dropped the elevations down. So that helps provide additional capacity."
When MWRD knows a storm is coming, an overflow action alert may be issued to encourage people to use less water by, for example, taking shorter showers or delaying doing doing laundry to help conserve space in the sewer system for rain water.
