UPDATE 4 P.M.  - The advisory issued this morning is canceled. Environmental Services' inspection determined that no overflow occurred to the Willamette River today. Environmental Services' monitoring system earlier indicated a five-minute overflow but field inspectors determined that all stormwater and sewage was contained. Enjoy your river.

ORIGINAL MESSAGE (September 14, 2018 9:04 a.m.) Isolated rainburst triggers 5-minute combined sewer overflow (CSO) occurs to Willamette River near Cathedral Park; avoid contact downstream for 48 hours

 An isolated rain burst last night near the east end of the St. Johns Bridge led to a short combined sewer overflow (CSO) from an outfall near Cathedral Park.

The rain burst dumped four-tenths of an inch within 15 minutes.  The overflow lasted about five minutes and led to an estimated 2,500 gallons of combined stormwater and sewage to overflow to the river. 

The majority of the discharge is stormwater, mixed with about 20 percent sewage. 

CSOs are rare. But when they occur the public is advised to avoid contact with the river water for 48 hours due to increased bacteria. In this case, the area affected is at Cathedral Park and downstream.

Since completing the Big Pipe project in 2011, a 20-year $1.4 billion program to reduce overflows, the number of CSOs have been reduced by 94 percent to the Willamette River and 99 percent to the Columbia Slough.  Before the project, CSOs occurred to the Willamette River from multiple outfalls an average of 50 times a year, with some instances lasting days.

For more information about CSO events, what they are and why they occur, visit https://www.portlandoregon.gov/bes/article/565061.