(February 9, 2017) – Environmental Services is reporting a short combined sewage overflow (CSO) that was detected at the SW Carolina Street outfall  to the Willamette River last night. The overflow occurred at 11:47 p.m. and ended at midnight. 

Environmental Services suspect that the cause was a temporary blockage from debris that cleared itself and this incident remains under investigation.

Because of increased bacteria in the water, the public should avoid contact with the Willamette River in the area north of Willamette Park for 48 hours after the CSO event ended. Find out more about CSOs, what they are and why they occur at: https://www.portlandoregon.gov/bes/article/565063

This is the fourth CSO this 2016-2017 winter season. Previous overflows this season occurred on Super Bowl Sunday, February 5 due to unusually heavy rains that dropped about 2.5 inches within 24 hours; on January 18  due to a combination of snowmelt and heavy rain; and Thanksgiving Day due to heavy rain.

Before the city completed the CSO control program, combined sewers overflowed an average of 50 times a year. Today, the combined system overflows to the Willamette River an average of four times per winter and once every three summers.

Portland’s combined sewer system carries sewage and stormwater runoff in the same pipes. In December 2011, Portland completed a 20-year program of sewer improvements, including constructing big pipes on both sides of the Willamette River and along the Columbia Slough. The improvements eliminate 99% of CSOs from the slough and 94% from the river.

During heavy storms, the big pipes store large quantities of stormwater and sewage while pumping it to the Columbia Boulevard Wastewater Treatment Plant. During very heavy storms, some combined sewage can overflow. A combined sewer overflow is about 80% stormwater and 20% sanitary sewage.

 The Bureau of Environmental Services works with Portland residents and businesses to protect water quality, public health, and the environment through wastewater collection and treatment, sewer construction and maintenance, stormwater management, and stream and watershed restoration. www.portlandoregon.gov/bes @besportland