Some cities, such as Fort Wayne, collect both rainwater runoff and sanitary wastewater in the same set of sewer pipes. These are called "combined sewers." Sometimes when it rains, combined sewers do not have enough capacity to carry all the rainwater and wastewater (sewage) to the Water Pollution Control Plant and/or the Plant does not have the capacity to accept all of the combined flow. In this situation, the combined wastewater then overflows from the collection system into the nearest body of water -- in Fort Wayne's case, into one of the three rivers -- creating a combined sewer overflow (CSO).

Combined Sewers serve many communities especially in the northeast and Great Lakes region of the United States. Fort Wayne has a typical combined sewer system. In Fort Wayne, combined sewers are found most in the older part of the City. Fort Wayne has 44 combined sewer overflow outfalls locations going to the three rivers and their tributaries.