A combined sewer overflow (CSO) occurs when stormwater and untreated, diluted sewage, carried in a single pipe, overload the sewer system and flow into rivers and streams. CSOs can carry a variety of pollutants.

Advisory Programs 

ALCOSAN's Sewer Overflow Advisory Key (SOAK) and CSO Flag Alert Program are complimentary advisory programs to warn the general public of possible river contamination from CSOs. While these programs do not prohibit or discourage river recreational activities, they do caution recreational users to limit contact with river water when boating, fishing, water skiing or engaging in other river recreational activities.

Advisory Programs operate annually from April 1 until October 31 to coincide with ALCOSAN’s NPDES permit summer reporting period. The program will operate between the hours of 8 a.m. and 7 p.m., 7 days a week. If overflows occur during this timeframe, ALCOSAN will raise orange CSO flags at designated points along the waterways and will notify the public accordingly via the ALCOSAN website-based SOAK program, and our subscriber-based text/email notification program. Users will also be notified when overflows have ceased, as well as when the system returns to dry weather operation, 48 hours later. At this point, CSO flags are also lowered to indicate dry weather operation.

Sign up for SOAK Alerts


  • October 26, 2022
    Overflows have ceased; Waterways may still be impaired. In: 10/26/2022 - 08:12 am Out: 10/26/2022 - 11:23 am
  • October 15, 2022
    System is in dry weather operation; No advisories are in effect In: 10/13/2022 - 09:08 am Out: 10/13/2022 - 12:21 pm Dry: 10/15/2022 - 12:21 pm
  • October 13, 2022
    Overflows have ceased; Waterways may still be impaired. In: 10/13/2022 - 09:08 am Out: 10/13/2022 - 12:21 pm
  • October 13, 2022
    Overflows are in effect; Minimize contact with waterways In: 10/13/2022 - 09:08 am
  • October 13, 2022
    Overflows have ceased; Waterways may still be impaired. In: 10/13/2022 - 12:56 am Out: 10/13/2022 - 06:45 am

CSO/SSO Outfall Signage

In addition to the ALCOSAN’s CSO Flag Program and SOAK, the Authority is increasing public awareness of sewer overflows through the installation of signs at each of the Authority’s 259 combined sewer overflow structures and 53 sanitary sewer overflow structures.

Each sign identifies the adjacent ALCOSAN structure and provides notice of the potential for combined or sanitary sewer overflow discharges relative to that structure. Signs will be visible from both the waterway and from publicly accessible land sites and will provide an ALCOSAN hotline number (888-936-9363) for the public to report observed discharges during dry weather.

An Example Outfall Sign
An example of an outfall sign

Sewer Overflow Advisories Frequently Asked Questions

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