So what is a CSO? A CSO, or combined sewer overflow, is the discharge of wastewater and stormwater from the combined sewer system directly into the river. In Portland, CSOs used to be a frequent occurrence, especially after heavy rainfall. In December 2011, the City completed its "Big Pipe" project which it says has reduced CSOs to the Willamette River by 94%.
CSOs are no big deal for paddlers, despite the fact that you may hear about them from time to time in the news. CSO advisories only suggest that people avoid activities in the river during which water could be swallowed (we're not drinking this water, are we?) and that we wash after coming in contact with the river (something we all should do all the time anyway).
CSOs are different from sanitary sewer overflows, which happen much more infrequently, and to which we will send notifications to team contacts advising whether practices should be canceled if one occurs. If you want to get these updates directly from the City, you can sign up at the City's website for River Alert notifications here (sign in and select "Sewer Overflow Notification").