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CSO Flag Alerts

What are CSO Flag Alerts?
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.

Does this mean that people should stay out of the river?
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.

Do these precautions apply to everyone?
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.

When are advisories issued?
Alerts are issued during the summer months when river recreation season is at its peak. That season lasts from May 1 through October 31 when the likelihood of exposure to river contamination due to sewer overflows and storm water runoff increases.

Where are CSO flags located?
CSO flags are raised at the following eight sites (map) 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)

Why do sewers overflow?
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.

Are alerts cancelled as soon as the rainfall stops?
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.

Are CSO alerts something new?
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.

How often are they likely to occur?
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).

Do these advisories have anything to do with drinking water?
No. Drinking water treatment processes are adjusted to address the changes in river water quality.

How can I find out when an advisory has been issued or cancelled?
In addition to the CSO flag program, ALCOSAN maintains the following methods to inform the public of CSO events:

Sewer Overflow Advisory key: Wet Weather overflows have ceased. Rivers, streams and creeks may still be impaired. For more information:

Customer Care at ALCOSAN, we believe it's our duty and privilege to protect, recover and reuse our most valuable resources for the benefit of our customers.

Consent Decree: To comply with provisions set forth in the Clean Water Act, ALCOSAN must meet a series of requirements for planning, design and construction, operation and permitting. For more information

Watershed Education: Leave small leftovers of paint and solvents to dry rather than pouring them down the drain. Learn more

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3300 Preble Avenue
Pittsburgh, PA 15233
(412) 766-4810