Soos Creek's journey from decline to recovery

The Soos Creek watershed in south King County is home to streams that once supported a thriving community of salmon, trout, stream insects, and other native aquatic life. Today, these waters are struggling - though they are showing some signs of stabilization.

Too much fine sediment - tiny soil particles - has built up in the streambeds. This sediment chokes the spaces between gravel where young salmon develop and stream insects live, disrupting the entire food chain.
Scientists call this decline "urban stream syndrome." It's common in urban areas that have seen rapid development.

In the Soos Creek watershed, prior development resulted in uncontrolled stormwater runoff that carries fine sediment into the streams. High peak flows from impervious surfaces like roads and rooftops - when not controlled - make the problem worse by eroding streambanks and flushing even more sediment downstream.

A plan for recovery

To put Soos Creek on a path to clean water, we developed a draft water quality improvement plan - called a Total Maximum Daily Load, or TMDL. This plan sets science-based limits on how much fine sediment the streams can handle, and it lays out actions needed to control fine sediment delivery and erosive flows.

The draft Soos Creek Fine Sediment TMDL focuses on two main solutions:

• Better stormwater management - Installing stormwater controls where they don't exist will help slow and filter runoff before it reaches the streams.
• Habitat restoration and protection - Restoring streamside vegetation (riparian buffers) and stabilizing streambanks will help prevent erosion and improve habitat for fish and other wildlife.

The plan includes both wasteload allocations for municipal, industrial, and construction stormwater permit holders, and nonpoint source controls for pollution coming from areas like farms, rural properties, and residential lands not served by municipal stormwater systems.

Municipal stormwater permittees in the watershed include Auburn, Black Diamond, Covington, Kent, Maple Valley, Renton, and King County.

Ecology staff will hold an online informational meeting on the draft Soos Creek Fine Sediment TMDL on Oct. 7 at 1:30 p.m.

Register now for the Zoom meeting.

We will present information about the draft plan and answer attendee questions.

Managing stormwater

Stormwater runoff is a leading pollution threat to water in urban and suburban areas of Washington. In the state's most populated counties and cities, stormwater is managed by local governments under Municipal Stormwater Permits, and by property owners and operators under stormwater permits for specific operations, such as construction and industrial activities.

Because Soos Creek has been and continues to be a rapidly urbanizing watershed, the largest reductions of fine sediment are expected to come from managing urban stormwater.

Municipal stormwater programs are funded through local stormwater utility fees. Washington is unique in the amount of state funding passed through to local governments in the form of stormwater grants and loans to support permit implementation. Ecology's funding is used by municipal stormwater permittees to build new stormwater facilities, such as bioretention facilities or engineered wetlands, to treat and manage urban runoff across the state.

Cities, counties, and ports can also be awarded water quality funding to help employ stormwater staff, inspect development projects, inform business owners and community members about local stormwater issues, and conduct stormwater system maintenance.

Signs of resilience

Monitoring of benthic macroinvertebrates - tiny "stream bugs" that indicate water quality conditions - shows that there are parts of the Soos Creek watershed where habitat is healthy. These areas - often less developed - remind us that Soos Creek can recover when pressures are reduced.

By working together, we can protect healthy areas and bring degraded reaches back to fine sediment levels that support healthy aquatic life. Monitoring benthic macroinvertebrates will continue to be a barometer for the progress we are making toward these restoration goals.

A long but necessary journey

Restoring Soos Creek will take years of effort and investment with our partners in the watershed. It will also need public support for projects that slow stormwater runoff, reduce erosion, and restore habitat.

"Restoring a watershed does not happen quickly. Investments over time in stormwater infrastructure, including public retrofits and private partnerships, as well as habitat restoration, will improve the health of Soos Creek and its tributaries by reducing fine sediment pollution and minimizing erosive flows,"

- Rachel McCrea, Water Quality Section Manager for Ecology's Northwest Region

In addition to supporting stormwater projects, Ecology's competitive Water Quality Combined Funding Program can also fund qualifying habitat restoration projects.

The draft Soos Creek TMDL is more than a technical document; it's a roadmap for restoring a living system that supports clean water, fish, wildlife, and people. We invite everyone to be a part of this process by providing feedback on the draft plan, attending our public meeting, and by actively participating in restoring Soos Creek.