Combined Sewer
Overflow(CSO) Information

Combined Sewer Overflow (CSO) Information

The following is the City of Alton’s Combined Sewer Overflow (CSO) information. 

The sewer collection system in Alton is made up of combined and sanitary sewers.  A combined sewer conveys both sanitary sewage and storm water while a sanitary sewer is not designed to collect and convey storm water.  The older, southwest portion of the City is served by combined sewers and the east and north portions are served by sanitary sewers.

When it rains or when the water level in the Mississippi River is elevated, some sewers in the combined sewer areas of Alton can overflow sending untreated rainwater and sewage into the river or into the Wood River Levee District impoundment area, and then to the river. These are called combined sewer overflows (CSOs)

The City of Alton has four CSO points which are:

1. #002 – Shields Valley

2. #003 – Central Avenue

3. #004 – Piasa Street

4. #007 – Turner Tract

These CSO’s act as relief valves when too much rainwater beyond the pipe’s capacity enters the sewer system.  Without them, our community could experience basement backups, extensive street flooding, or manhole covers popping off in the lower areas of the system.

When the overflows occur, there is a possibility of contaminants or floatable debris from the combined sewer entering the discharge areas.  Special caution is required when the water level at the discharge areas are high.  There is a risk of being swept away into the current and a risk of illness associated with the personal contact with these waters.  Please avoid all contact with waters in or near a combined sewer outlet area.  After a rainstorm, you should avoid contact with waters in the combined sewer area for at least 72 hours.

To view a map of Alton’s CSOs, please click here.


Disclaimer:  Information on this page is subject to change.  Please check back for updates.