Combined Sewer Overflow Reporting

Understanding Your Sewers and CSOs

Like older cities around the world, most local sewers in the Chicago area were built more than 100 years ago. The sewers were designed to drain sanitary flow and a limited amount of stormwater directly to the river before wastewater treatment existed. Today most of these local sewers are required to carry much more water than when these systems were first put into service. Consequently, the sewers can exceed their flow capacity, cause backups or overflow into the waterways. If the MWRD’s intercepting sewers and water reclamation plants reach capacity during heavy rain, the local sewer continues to drain, or “overflow” to a waterway. That is what we define as a combined sewer overflow, or CSO. To learn more, visit our “Understanding Your Sewer" webpage.

Reducing CSOs

The MWRD’s Tunnel and Reservoir Plan (TARP) system works around the clock to collect water and mitigate CSOs from occurring. The tunnel system captures the “first flush,” a term used by the USEPA to describe the highest pollutant-loaded combined sewer flow. The reservoirs have gone a step further and collected more CSO volume. In fact, the Thornton Composite Reservoir has nearly eliminated CSOs from occurring. Because of this, the region has seen tremendous ecological recovery of the waterways during the implementation of TARP. To learn more about the success of Thornton, visit Water & Wastes Digest.

Mapping CSOs

The map below tracks CSO occurrences in reaches of the Chicago Area Waterway System (CAWS) leveraging Esri GIS software. A dot indicated on the map represents the locations of current CSO occurrences. CSO occurrences that have occurred on a prior date can be viewed by visiting CSO Synopsis Reports. Please ensure your popup blocker is disabled for this site to view the map.

The information presented in the following maps and applications is updated frequently and reflects CSO and diversion activity based on the best available data. Not all CSO outfalls are equipped with monitoring equipment. Therefore, other CSOs may have occurred that are not reflected on the below map.

Regardless of CSO activity, the CAWS like other bodies of water, contains wildlife and harmful bacteria. Swimming, canoeing, or other activities where immersion in water is possible should be avoided and is not recommended, particularly during and immediately following rainfall.