Combined sewer overflows (CSOs) occur when wastewater and stormwater overwhelm system capacity and are released into nearby waterbodies. The Environmental Protection Agency has a great, brief explanation of CSOs.

The Big Pipe project completed in 2011 is designed to reduce the volume of combined sewer overflows to the Willamette River by 94 percent and to the Columbia Slough by 99 percent.  The Big Pipe project is not intended to prevent all CSO discharges, but to control them to a high standard that substantially improves water quality and minimizes health risks from exposure.

The National Pollutant Discharge Elimination System (NPDES) permit that allows the City of Portland to operate the wastewater system has separate standards for the Willamette River and the Columbia Slough.  The standards are based on

1. the environmental sensitivity of the water body; and,
2. the risk of human contact with water if a CSO occurs.

The threshold for CSOs is significantly higher during the peak recreation season between May 1st and October 31st.

The Big Pipe system’s performance is measured by the number of overflows. Before Big Pipes, an average of 50 Willamette River overflows occurred each year, sometimes lasting for days. Now, on average, the City of Portland will experience three to four CSO events during the wet season every year and one during the dry season every three years. Not only are there fewer overflows, the volume and duration of overflows are also much lower due to the Big Pipe project improvements. Our system is working as intended and has greatly improved the water quality.

Overflows are tracked and reported as they occur.

At the Big Pipe Project webpage, you can read more about the need for the project, the different parts of the project, and more.