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 Sewer Overflow Advisory line (412-734-6249), 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 16, 2021
    Overflows have ceased; Waterways may still be impaired. In: 10/16/2021 - 07:44 am Out: 10/16/2021 - 12:07 pm
  • October 16, 2021
    Overflows are in effect; Minimize contact with waterways In: 10/16/2021 - 07:44 am
  • October 12, 2021
    System is in dry weather operation; No advisories are in effect In: 10/08/2021 - 9:42 pm Out: 10/09/2021 - 12:31 am Dry: 10/12/2021 - 12:31 am
  • October 09, 2021
    Overflows have ceased; Waterways may still be impaired. In: 10/08/2021 - 09:42 pm Out: 10/09/2021 - 12:31 am
  • September 23, 2021
    Overflows have ceased; Waterways may still be impaired. In: 09/22/2021 - 05:09 pm Out: 09/23/2021 - 12:07 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

Orange CSO Flags are raised at designated points along our waterways to warn the general public of possible river contamination and caution them to limit contact with river water when boating, fishing, water skiing or engaging in other river recreational activities.
No. An alert does not prohibit nor discourage river recreational activities. It is intended to inform the general public when river water may be contaminated so they can take precautions to minimize water contact during their outings.
Yes, but these precautions are particularly aimed at people who have open cuts or sores, and those with weakened immune systems. These people are most vulnerable to infection from exposure to contaminated river water.
Alerts are issued during the months when river recreation season is at its peak. That season lasts from April 1 through October 31 when the likelihood of exposure to river contamination due to sewer overflows and storm water runoff increases.

CSO flags are raised at eight sites when the system is in overflow:

Silky’s Crows Nest & Marina (Allegheny)
Fox Chapel Yacht Club (Allegheny)
Peggy’s Harbor Marina (Ohio)
Washington’s Landing Marina (Allegheny)
RiverQuest (Ohio)
Three Rivers Rowing Club (Allegheny)
Southside Boat Ramp (Monongahela)
Braddock Boat Ramp (Monongahela)

Sewers overflow during a significant rainfall because the combined sewer systems fill up beyond their capacity to carry both sewage and storm water, thereby discharging the excess flow into rivers and streams. Since streams also are affected, people should limit stream contact as well.
No. Runoffs and overflows subside gradually and river water quality usually does not return to normal until for up to 48 hours after a sewer overflow event ends.  After 48 hours without additional overflow, CSO flags are lowered and the system returns to normal, dry weather operation.
No. CSO alerts were instituted in 1995 to comply with federal regulations requiring public notice when overflows and runoffs increase the likelihood of river contamination.
The frequency and duration may vary depending on the amount of rainfall. For example, in the "dry" summer of 1999, eleven advisories were issued lasting a total of 33 days (an average of three days each). In 1998, ten advisories were issued lasting a total of 50 days (an average of five days each).
No. Drinking water treatment processes are adjusted to address the changes in river water quality.
You can visit this webpage, check one of the eight CSO Flag locations, call the ALCOSAN Sewer Overflow Advisory line at 412-734-6249, or sign up for text/e-mail notifications. (Text/email advisories are active between 8:00 a.m. and 7:00 p.m. daily. Status changes outside of those hours are reported at 8:00 a.m. the following morning.)  

Quick Links

Clean Water

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Stormwater protection with native plants and gravel
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