Working for a safe, affordable, vibrant, innovative, and interconnected city.
Learn More Home Page This Department

Sewage Overflow Prevention

Protecting Seattle’s Waterways


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 draft Long-term Control Plan, draft Integrated Plan, and draft Environmental Impact Statement (DEIS) are available. Read the
Plans and Impact Statement.

Seattle Public Utilities is evaluating two alternatives in the Plan:

Integrated Plan stormwater projects

  • Expansion of street sweeping on arterial streets (citywide)
  • New stormwater quality facility (South Park)
  • Natural Drainage Systems Partnering (Piper’s, Thornton, and Longfellow Creek drainage areas)

Read the Community Guide to the Integrated Plan (pdf)

Soil testing in Ballard, Fremont and Wallingford

As described in the Plan to Protect Seattle’s Waterways, over the next ten years SPU will need to construct underground storage for approximately 9.4 million gallons of sewage and polluted stormwater to prevent overflows into the Lake Washington Ship Canal.

From late August through early fall 2014, Seattle Public Utilities (SPU) will test soils near the ship canal to investigate potential storage locations.

For more information about this work, contact: Dan Enrico, at 206-684-7413 or

More detailed information: