South Thornton Natural Drainage System (NDS) Project

Photo of a street with natural drainage system.

Example of a natural drainage system project a few years after installation

Project description

Enhancing Our Communities with Natural Drainage Systems
Seattle Public Utilities (SPU) is constructing natural drainage systems at 4 sites in the south Thornton Creek basin. Natural drainage systems are built in the roadway shoulder (the space between the street edge and property line) and are filled with deep-rooted plants and spongy soils that temporarily hold and clean polluted stormwater from streets before it reaches Thornton Creek.

Natural drainage systems can offer multiple benefits to local neighborhoods and ecosystems including:

  • Increased landscaping
  • Lower risk of flooding
  • Creation of habitat along streets
  • Healthier creek ecosystems
  • Improved traffic calming
  • More street trees
  • Pedestrian safety improvements

We are partnering with the Seattle Department of Transportation (SDOT) to incorporate walkways into some of the streets that do not currently have formalized sidewalks, which aligns with community requests to install pedestrian improvements as a part of this project.

To provide a comfortable and accessible walking connection to James Baldwin Elementary School, SDOT is planning to construct a new sidewalk on the north side of N 117th St between Meridian Ave N and 1st Ave NE. This project will be constructed in coordination with the natural drainage systems on N 117th St and the redevelopment of James Baldwin Elementary School.

We are also partnering with the City's Office of Arts and Culture to incorporate an art installation in the natural drainage system near Wedgwood Elementary School.

Check out the Project Overview (PDF) and Project FAQ (PDF) for more information.


SPU is constructing natural drainage systems at 4 sites in the south Thornton Creek basin. The sites can be viewed on this Thornton Creek map (PDF).


What's happening now?

Construction on the project began in October 2023 at the 23rd Ave NE and Wedgwood sites and in January 2024 at the N 117th/N 120th St site. Construction at the 41st Pl NE site is scheduled to begin as soon as early to mid-May 2024.  

Please sign up for our project email list to stay updated on construction activities.

Community benefits

Natural drainage systems will reduce pollution in Thornton Creek and Lake Washington and help manage stormwater. This project will provide additional community benefits such as reduced street flooding, improved traffic calming, new street trees, and increased landscaping.

Community engagement

During the project’s design, we engaged the community to gather input, answer questions, and incorporate feedback into the project design.

To prepare residents for construction, members of the project team hosted construction drop-in sessions at each of the sites. If you were unable to attend your neighborhood construction drop-in session and have questions about the project, please contact Luiz Ramirez, Project Manager, at

Please sign up for our project email list to stay informed of project updates and opportunities to engage with the project during construction.

SPU, in coordination with the City of Seattle Office of Arts & Culture, is working to create an art installation for this project.

Proposed artwork to be installed in the NDS near Wedgewood Elementary School.

The artist, Leo Berk, developed an artwork to bring awareness to the ecological benefits of the natural drainage system that SPU is installing in Wedgwood. His design uses the visual metaphor of an oversized kitchen strainer perforated with a pattern that illustrates the function of natural drainage systems. The pattern specifically highlights how these systems reduce the roadway pollutants that enter Thornton Creek and Lake Washington.

Leo Berk is a Seattle artist who has installed numerous public art projects and exhibited in galleries and museums around the Pacific Northwest. His works are fabricated in a wide range of materials with his preferred material being the one he hasn’t used yet.

2017 to 2018

  • Early project planning and site selection
  • Outreach to residents about potential sites

2019 to 2022

  • Design development for selected sites
  • Design phase outreach at key project milestones
  • Finalize design


  • Advertise and award construction contract
  • Construction began in October 2023 at the 23rd Ave NE and Wedgwood sites


  • Construction began in January 2024 at the N 117th St/N 120th St site
  • Construction is scheduled to begin as soon as early to mid-May 2024 at the 41st Pl NE site

Natural drainage systems are living systems, and their appearance will change over time. The grasses, shrubs, and trees installed will grow and change as the garden matures. It may take up to three years for plants and shrubs in the natural drainage system to reach full maturity, and possibly longer for trees. The examples below show what a natural drainage system may look like over time.

Example before construction

Example newly planted

Example growing phase

Example mature phase

Maintenance and Care

SPU is responsible for maintenance of the natural drainage systems, which includes general upkeep of the plantings, ensuring that the system is functioning properly, removing sediment, and replacing or removing plantings and trees in the natural drainage system, if needed. You will not be asked to perform any maintenance related to the natural drainage system.  Maintenance of the planting strip that is not part of SPU’s natural drainage system remains the responsibility of the adjacent property owner.

Natural drainage systems slow stormwater and filter out pollutants before they can harm our creeks and other waterways. Keeping pollutants out of our water is important for fish, aquatic life, and people.
Building a natural drainage system in the Thornton Creek basin is part of the Plan to Protect Seattle's Waterways. SPU used to call these "roadside rain gardens" but now calls them "natural drainage systems." 
Why Thornton Creek? Thornton Creek is a salmon-bearing urban creek with degraded water quality. Stormwater picks up pollutants from streets—including oil, heavy metals, and fertilizers—and flows into the creek through ditches or pipes. This project will remove pollutants from stormwater before it reaches Thornton Creek.

Construction of the South Thornton Natural Drainage System project was financed by the Clean Water State Revolving Fund (CWSRF). The CWSRF program is administered by the Washington State Department of Ecology with joint funding from the U.S. Environmental Protection Agency. The South Thornton project will provide water quality treatment for street runoff that drains to Thornton Creek by retrofitting the roadsides with natural drainage systems and addressing localized flooding issues to the extent feasible. CWSRF programs operate around the country to provide states and communities the resources necessary to maintain and improve the infrastructure that protects our valuable water resources nationwide.

This project was paid for in part by the King County Flood Control District.

Logo of the WA State Department of Ecology

Logo of King County Flood Control District.

All documents in PDF format.

Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.