September 19, 2021: A CSO advisory is in effect for 48 hours and ends the morning of September 21.
Many of Portland’s older neighborhoods have a combined system. In a combined system, sewage and stormwater are carried in the same pipes to the treatment plant. Before the Big Pipe Project was completed in 2011, these neighborhood pipes used to fill up and overflow to the Willamette River or Columbia Slough when it rained. Now the Big Pipe Project has eliminated 94 percent of overflows to the Willamette and 99 percent to the slough.
Are rare. They occur about four times a rainy season during or after heavy rains.
Usually happen in the winter from October to May.
Contain about 20 percent sewage and 80 percent stormwater.
Carry chemicals and bacteria that could harm people and animals in the water.
See related News & Notices on this page for past CSO events since 2018.