Cook County contains about 400 CSOs, or combined sewer outflows, points that can release sewage water when storms cause sewers to fill to capacity faster than water treatment plants are able to process the water.
Construction began on the city's $3.5 billion Tunnel and Reservoir Plan (TARP) in the late 1960's. The project has decreased the number of CSO events from 100 per year to 35. The Metropolitan Water Reclamation District hopes when TARP is completed in 2029, there will be fewer than five occurrences each year.
Sewer overflow events force Chicago River Canoe and Kayak to close or redirect their customers about five times a year, sometimes for two to three days at a time. The other 30 CSOs each year usually occur during storms, when the company is already closed.
CSOs prevent Chicagoans from taking full advantage of all the river has to offer.