North Thornton Natural Drainage Systems

Photo of mature plant growth
Example of a natural drainage system a few years after installation

Project Description

Improving creek water quality, reducing drainage issues, and providing other community benefits.

More than 12 million pounds of pollution are carried into our water systems by stormwater runoff every year. Stormwater that falls on hard surfaces in your neighborhood - like roofs, streets, and parking lots - can pick up harmful pollutants before it runs into Thornton Creek, which flows into Lake Washington and eventually into Puget Sound. The pollutants can harm fish, wildlife, and our ecosystems.

SPU is working to minimize pollution at its source. Natural drainage systems (NDS) are shallow depressions built in the roadway shoulder (the public space between the street edge and property line). They are filled with a special soil and deep-rooted plants to temporarily hold and filter stormwater, reducing pollution in Lake Washington and Puget Sound.

The North Thornton Natural Drainage Systems Project aims to:

  • Build natural drainage systems to reduce the amount of pollution in Thornton Creek and Lake Washington and improve stormwater drainage in your neighborhood.
  • Provide other community benefits, which include reduction of drainage issues, traffic calming, more street trees, and added landscaping.


The North Thornton Natural Drainage System project is focused on reducing pollution in the north branch of Thornton Creek. (Project Map

What's Happening Now?

This project is currently in the early design phase. We have identified 22 blocks where a natural drainage system might be built. We are now evaluating these blocks to determine where a natural drainage system could create the most water quality and community benefits.

In Spring 2022, we asked neighbors living and working on the 22 blocks under consideration to share their thoughts through an online survey. We are currently reviewing the survey responses as we evaluate potential locations for natural drainage systems. Thank you for your input!

We’ll be in touch later this year with more information about where we propose to install the natural drainage systems. After announcing the proposed locations, we will work with residents and community to refine the project design, including working to coordinate individual and community needs.

Community Benefits

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

  • Increased landscaping diversity
  • Reduced drainage issues
  • Healthier creek ecosystems
  • Improved pedestrian safety and mobility
  • More street trees

How does SPU decide which blocks to choose?

SPU picks potential project locations by working with other city departments (for example, with planned street or pedestrian improvements), by looking for areas where this project may address a drainage issue, and by collecting information from the community.

The final project location depends on a lot of factors:

  • How much stormwater can be managed at a particular location
  • Location of nearby adverse conditions (steep slopes, high groundwater, or contaminated sites)
  • Potential impacts to existing trees
  • Existing soil conditions
  • Location and condition of existing utilities
  • Community input
  • Width of existing public right-of-way
  • Presence of driveways
  • Existing parking congestion and availability of off-street parking
  • Project construction, operation, and maintenance cost

Community Engagement

We will incorporate community input into our project planning, site selection, design, and final decisions as much as possible. In addition to community input, we will evaluate the sites to identify where it is technically feasible for us to build the natural drainage system.
The project’s first community feedback survey was completed in June 2020. Since then, we’ve been working to understand where natural drainage systems can have the most water quality and community benefit and have identified 22 blocks where a natural drainage system might be built. In spring of 2022, we asked neighbors living and working on the 22 blocks under consideration to share their thoughts through an online survey.
We are planning additional opportunities for SPU to learn more about the community's priorities and how we can work together to improve water quality in Thornton Creek, reduce drainage issues, and make other improvements in your neighborhood. Please sign up for the project email list to receive updates about the project and about opportunities to share your input.



  • Community engagement to inform project planning


  • Technical analysis of project area to identify feasible locations for building NDS


  • Community engagement and updates
  • Identify proposed locations for NDS
  • Project design


  • Complete project design
  • Continue to provide project updates to the community
  • Hire contractor


  • Construction begins
  • Ongoing construction updates to community


  • Construction completed

* Tentative schedule as of September 2022. This schedule is subject to change.