When it rains, pollution from our streets ends up in Seattle’s four major receiving waters: Lake Washington, the Ship Canal and Lake Union, the Duwamish River, and Puget Sound. Three urban basins, or watersheds, stand between these receiving waters and runoff from stormwater drainage and combined sewer overflows (CSOs).

Urban creek watersheds are not only home to fish and wildlife—they help filter stormwater before it enters lakes, the Duwamish River and Puget Sound. Because of the important role that these watersheds play to ensure our city’s waterbodies avoid pollution from runoff, it’s crucial that we improve the quality and reduce of the flow of water that travels to them. SPU and local groups are working to slow stormwater from rushing into the creek and restore important native plants for its protection. King County’s Clean Water Healthy Habitat Initiative is investing in healthier watersheds and protecting habitat.

These plants are a part of natural drainage systems (NDS) that help protect the creek watersheds. NDS projects, built by SPU and King County, consist of shallow depressions in the public right-of-way, or “planter strips,” filled with deep-rooted plants and spongy soils that temporarily hold and clean polluted stormwater from streets and sidewalks. These features capture and clean pollutants before they can reach the creek.

Natural drainage systems offer multiple community benefits, including:

• Greener, more attractive neighborhoods
• Lower risk of flooding
• Healthier creek ecosystems
• Calmer traffic patterns
• More street trees

Preserving and improving the health of our urban watershed is essential for providing healthy and livable communities.

Our city’s combined sewer system collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Under normal conditions, it transports all of the wastewater it collects to a sewage treatment plant for treatment, then discharges to a water body. Sometimes the volume of wastewater exceeds the capacity of the combined sewer system or treatment plant, such as during heavy rain or snowmelt. When this happens, untreated stormwater and wastewater overflow to nearby streams, rivers, and other water bodies.

Combined sewer overflows (CSOs) contain untreated or partially treated human and industrial waste, toxic materials, and debris, as well as rain. Along with stormwater runoff, they are the top water pollution concern for our region.

As part of its CSO Reduction Plan, the city and King County intend to add storage capacity, build Green Stormwater Infrastructure, optimize our existing infrastructure, and develop a long-term control plan.

Protecting Our Waters is a program developed by King County to prevent pollution from excess stormwater in the sewer system on rainy days.

Local drainage and wastewater utilities develop and fund these capital improvement projects to proactively address challenges like combined sewer overflows, creek protection, drainage system capacity issues, and other high priority issues around stormwater management. These projects most typically retrofit the public right-of-way to manage polluted stormwater runoff from the road system. Projects are designed to improve water quality, prevent sewer overflows or back-ups, and prevent damage to creeks. These projects are also designed for neighborhood co-benefits such as traffic calming, tree canopy recovery, improved streetscape aesthetics, and climate resilience.

Two projects capture efficiencies in design and construction: the Sidewalk Development Program and SPU’s Natural Drainage Partnership program to treat polluted stormwater within creek basins. These projects foster improved coordination between City departments and their individual projects.