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Combined Sewer Overflow warning flag (U.S.)

Last modified: 2016-04-04 by rick wyatt
Keywords: united states | cso | combined sewer overflow |
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[Combined Sewer Overflow flag] image by António Martins-Tuválkin, 8 October 2008

See also:

CSO flags

A flag I stumbled upon recently is orange with black initials 'CSO' meaning "Combined Sewer Overflow" and used in the U.S., serving as a warning to the public. Source of this information followed by some relevant quotes:

"In 1994, the CSO Control Policy was announced by the Environmental Protection Agency (EPA). One of the requirements of the policy is to alert the public of the existence and probable impact of CSOs. (...)

Whenever there is a possibility of an overflow, you will see bright orange CSO warning flags flying to alert you to a potential CSO problem. These orange flags will be raised when any of our three rivers may be affected by CSOs."

As explained on above site, the flag is a precautionary warning signal that the sewer system is about to overflow, a situation which occurs during heavy rainfall, when snow melts, etc. The public is then asked not to add to the spate, keep the streets free from litter which might be swept along, etc.

The CSO warning flag is also presented at

"What are CSO flags?
The Allegheny County Health Department regularly posts advisories for the Allegheny, Monongahela, Ohio and Youghiogheny Rivers. On the days that these orange and black flags are flying, residents should be aware that there was sewer overflow into the river due to wet weather."

Jan Mertens, 23 March 2007