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12 Square Miles - Seattle's Largest Urban Watershed

Thornton Creek
Watershed

Bioswale design, landscaping, and stormwater management for Northeast Seattle. Every bioswale we install upstream keeps sewage out of the creek downstream.

Serving the Thornton Creek Watershed since 2026

Free Assessment

11.6 mi2

Watershed area

Thornton Creek Alliance

7,400

Acres draining to one creek

King County

2

Major forks (North + South)

SPU Watershed Data

10+

Neighborhoods we serve

swale.bio

The Problem Under Your Yard

Thornton Creek is one of the most polluted urban streams in Washington State. And it starts in your neighborhood.

Combined Sewer Overflows

When it rains hard in Northeast Seattle, the combined sewer system can't handle the volume. The result: raw sewage overflows directly into Thornton Creek and out to Lake Washington at Matthews Beach - where families swim.

Every impervious surface in the watershed - every roof, driveway, and compacted lawn - accelerates runoff into a system that was never designed for this much water. The city's solution is massive infrastructure projects that cost hundreds of millions.

The homeowner's solution is simpler: absorb the water before it hits the pipe. That's what a bioswale does.

Combined Sewer Overflow
Matthews Beach
The North Fork of Thornton Creek enters Lake Washington at Matthews Beach Park. During heavy rain events, combined sewer overflows discharge untreated stormwater and sewage near one of Seattle's most popular swimming beaches.
Every bioswale installed upstream in the Thornton Creek watershed reduces the volume of runoff that reaches the combined sewer system - directly reducing overflow events at the beach.

Your Yard Is Part of the Solution

A single residential bioswale can absorb up to 90% of the stormwater runoff from your property.

Absorb Stormwater

Engineered soil layers and native plantings capture rainwater on your property before it enters the storm drain. Up to 90% on-site absorption.

Filter Pollutants

Seven-layer bioretention system removes 99% of sediment, heavy metals, and petroleum products before water reaches Thornton Creek.

Increase Property Value

Professional landscaping that pays for itself. Reduce your stormwater utility bill while adding curb appeal that stands out on the block.

Follow the Creek

Thornton Creek flows 7+ miles from its headwaters to Lake Washington. Here's the journey - and where your bioswale fits in.

Headwaters

Jackson Park & Haller Lake

The North Fork begins near Jackson Park Golf Course in Shoreline. Water from Haller Lake and surrounding neighborhoods feeds into the creek's upper reaches. This is where the watershed starts.

North Fork

Pinehurst & Victory Heights

The creek flows south through Pinehurst, picking up residential runoff from one of the densest parts of the watershed. Victory Heights contributes from the east. Every yard matters here.

South Fork Origin

Licton Springs & Northgate

The South Fork originates near Licton Springs and North Seattle College. It flows through Northgate, gathering volume as it moves east toward the confluence.

Mid-Watershed

Maple Leaf & Lake City

The heart of the watershed. Maple Leaf sits between the two forks. Lake City is where density meets the creek - the commercial district adds significant impervious surface to the system.

Confluence

Meadowbrook & Cedar Park

The North and South Forks converge at Meadowbrook Pond - a constructed wetland that serves as the creek's last major filtration point before the final stretch to the lake.

The Mouth

Matthews Beach

Thornton Creek enters Lake Washington at Matthews Beach Park. Everything upstream - every yard, every roof, every bioswale or lack thereof - shows up here. This is where the watershed's health is measured.

Your Neighborhood, Your Watershed

We serve every neighborhood in the Thornton Creek Watershed. Find yours.

Let's make the Matthews Beach CSO obsolete.

Every bioswale installed in the Thornton Creek Watershed is one more property absorbing its own stormwater instead of sending it to an overwhelmed sewer system.

Serving the Thornton Creek Watershed since 2026

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