In St. Joseph, like many older cities, sewer pipes carry both wastewater (used water and sewage that goes down the drain in homes and businesses) and stormwater (rain or snow that washes off streets and parking lots) to a sewage treatment plant. In many parts of St. Joseph, the mixed wastewater and stormwater flow together in a single pipe. This is called a Combined Sewer System.
During a heavy rain the pipes may get too full and start to overflow into the Missouri River. When this happens, it's called a Combined Sewer Overflow (CSO). This provides a "safety valve" that prevents back-ups of untreated wastewater into homes and businesses, flooding in city streets, or bursting underground pipes.
The Combined Sewer System was built as St. Joseph grew during the early 1970's, as an economical way to handle wastewater and stormwater. One advantage of this system is that most of the time, when rainfall is low to moderate, both the stormwater and wastewater go to the treatment plant.