North Thornton Natural Drainage Systems

Photo of mature plant growth
Mature plant growth

Project Description

Reducing flooding, improving creek water quality, and providing other community benefits.

More than 12 million pounds of pollution are carried into our water systems by stormwater runoff every year. Water that falls on the roofs, streets, and parking lots in north Lake City neighborhoods makes its way into the north branch of Thornton Creek, which flows into Lake Washington.

SPU is working to minimize pollution at its source. Natural drainage systems (NDS) slow down 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 help manage stormwater in your neighborhood.
  • Deliver other community benefits, which include reduction of street flooding, traffic calming, new trees, and added green space.


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 planning stages as we study locations to construct natural drainage systems in north Lake City neighborhoods.

Community Benefits

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

  • Increased landscaping
  • Lower risk of flooding
  • Creation of habitat along our streets
  • Healthier creek ecosystems
  • Traffic calming
  • More street trees

What Options Are We Looking At?

We plan to build natural drainage systems in several locations within the project area. NDS are built in the space between the street and private property. Currently, we are studying a range of location options.

These natural drainage systems filter out pollutants at the source:

  • After rainwater washes from the impervious (hard) surfaces in your neighborhood, it is captured in the natural drainage systems
  • A layer of plants and soils filter the pollutants
  • The filtered stormwater may seep into the ground or to a downstream drainage system
  • The system is designed for all water to flow out within 24 hours

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 be doing evaluations to identify where it is technically feasible for us to build the NDS.
The initial project community feedback survey was complete in June 2020.
We are planning additional opportunities for SPU to learn more about the community's priorities and how we can work together to help improve drainage, reduce flooding, 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
  • Finalize selection of locations to build 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 December 2021. This schedule is subject to change.

Project Documents