Sewage Overflow Prevention


Seattle waterways will benefit from upcoming projects to prevent sewage overflows and polluted stormwater runoff.

Every year, rain washes millions of gallons of untreated sewage and stormwater into the city’s waterways, threatening human and aquatic health and our quality of life. Each year, on average, more than 300 sewage overflows send millions of gallons of raw sewage and stormwater into Seattle’s creeks, lakes, the Ship Canal, the Duwamish River, and Elliott Bay. These combined sewage overflows (CSOs) create significant health and environmental risks.

The Plan to Protect Seattle's Waterways

The final Plan to Protect Seattle’s Waterways has been approved by the Seattle Mayor and City Council. It was submitted to federal and state regulators for final approval on May 29, 2015. Seattle Public Utilities recommended the Integrated Plan alternative, which addresses both sewage overflows and polluted stormwater runoff. Through the Integrated Plan, we will implement stormwater quality projects before some smaller-volume sewage overflow projects. We will also implement sewer system improvements and sewage overflow projects in 11 neighborhoods. The sewage overflow projects that address the largest, most frequent overflows will be built by 2025; smaller-volume projects will be built by 2030.

See a map of projects (pdf) in the Plan to Protect Seattle’s Waterways.

Stormwater projects

  • Expansion of city-wide street sweeping on arterial streets
  • A South Park stormwater quality treatment facility
  • Natural drainage systems partnering in Piper’s, Thornton, and Longfellow Creek drainage areas

More detailed information: