Water Quality Improvement Project
Soos Creek Watershed Multi-Parameter

Portions of Soos Creek have unhealthy conditions that cause them to fail to meet state water quality standards. We are developing water quality improvement projects, also known as Total Maximum Daily Load (TMDL) projects in the Soos Creek watershed to address water temperature, dissolved oxygen, and the health of the organisms living in the water, including fish, insects, algae, plants and others. Our study of this is called a bioassessment.

Along with our partners, we will develop a water quality improvement report that will address water quality standards in the watershed. We will determine what needs to be done and who will carry out the recommendations so that the water will meet state water quality standards.

The Soos Creek watershed includes portions of King County and the cities of Auburn, Black Diamond, Covington, Kent, Maple Valley, and Renton. Major streams draining into Big Soos Creek include Soosette, Little Soosette, Jenkins, Covington, and Little Soos creeks.

Water quality issues

On the most recent water quality assessment (2014), stream segments in the Soos Creek watershed do not meet water quality standards for temperature, dissolved oxygen, and bioassessment.

Land use in the study area varies from urban and residential, commercial, some industrial, commercial forestry, and small scale agricultural land uses. In parts of the Soos watershed these land uses caused loss of intact riparian zones and riparian trees, disconnected hydrological conditions, high water temperatures, low dissolved oxygen, impacted benthic invertebrates, and fish. Benthic invertebrates are organisms that live in or on the bottom sediments of rivers, streams, and lakes, and are measured as bioassessment, which is a score of the overall health of a stream.

Several salmon species use these streams as important migration corridors to get to spawning and rearing areas. These species include:

Soos Creek photo.  Courtesy of Joan Nolan, WA State Department of Ecology.
  • Puget Sound Chinook
  • bull trout
  • coho
  • chum
  • pink
  • sockeye
  • kokanee
  • steelhead/rainbow
  • cutthroat trout

What we have done

In 2006 we initiated a TMDL water quality improvement project for temperature and dissolved oxygen in the Soos Creek watershed. In 2007 we collected data for the TMDL study, along with King County and others. In 2011-2012 we partnered with the Environmental Protection Agency (EPA), EPA contractor Tetra Tech, King County, and Muckleshoot Indian Tribe to initiate a similar effort for the watershed, along with a new Soos Creek TMDL initiative for bioassessment.

In 2016 we partnered with EPA to do a benthic invertebrate stressor identification study to help identify what pollutants and habitat concerns are leading to depressed bioassessment scores.

Status of the projects

With help from EPA, King County, and the Muckleshoot Indian Tribe, we are presently compiling data and modeling to address temperature, dissolved oxygen, and bioassessment issues in the Big Soos Creek and its tributaries. We will look at where we need additional tree shade to keep streams from over-warming, particularly during the late summer. We are looking at how low flows affect the streams' ability to absorb solar radiation and what that means in terms of water withdraws and recharge. We will look at how nutrient delivery and in-stream vegetation can use up the available dissolved oxygen in the streams. We will look at how high flows, sediment delivery, watershed hydrology, and riparian habitat affect stream bugs.

As the TMDL study progresses we will work to identify the pollution problems and specify how much pollution needs to be reduced to achieve clean water. Throughout the TMDL study we will work closely with the local community to prepare an implementation plan that details the specific actions needed to improve water quality in the basin. The plan will describe management roles, activities, and schedules for partners.

Local partners involved in this effort include:

  • Muckleshoot Indian Tribe
  • King County
  • King County Conservation District
  • Cities of Auburn, Black Diamond, Covington, Kent, Maple Valley, and Renton
  • Implementation groups, such as Middle Green River Coalition and Midsound Fisheries Enhancement Group
  • Washington Department of Transportation
  • Water purveyors such as Seattle Public Utilities, Covington Water, and King County Water District #111
  • Watershed residents
  • Local businesses

We will support the existing good work done by local partners and the public in new and ongoing riparian restoration projects. Streamside projects may be initiated through grants or using current and proposed city and county critical area ordinances and shoreline master programs, which encourage preservation and restoration of riparian vegetation.

King County Department of Natural Resources, the Soos basin municipalities, and local watershed groups have ongoing stream restoration projects. The TMDL improvement report plan, which is under development, will provide a basis for additional work. Water management can be improved by water use efficiency, managing water withdrawals, practicing conservation, and creating opportunities for water recharge through low impact development.


Soos Creek TMDL Projects Timeline

Note: Dates are subject to change. Please check this webpage for updates.

March 7, 2017 B-IBI Stressor Identification and Technical Direction Meeting
May 2019 Bioassessment/Sediment Technical Memo
June 2019 Advisory Group Meeting #1
May 2020 TMDL Technical Report  (combines bioassessment, temperature, and dissolved oxygen and includes draft TMDL allocations)
August 2020 Preliminary Draft TMDL completed
September-December 2020 Advisory Group Meetings # 2-5
February 2021 Draft TMDL Report – External to Public
March 2021 Soos Creek  Public Meeting
April 2021 Respond to Public Comments
May 2021 Final Soos Creek TMDLs submitted

What we all can do

Citizens can help reduce stream temperatures and provide better habitat for stream dwellers. Some strategies include:

  • Reducing lawn irrigation, pesticides, and fertilizer.
  • Using ecological products such as phosphate-free detergents.
  • Ensuring a properly functioning on-site sewage system.
  • Providing livestock exclusion and proper pet waste disposal.
  • Planting trees, especially along stream buffers.
  • Keeping as much native vegetation and undisturbed land as practicable.
  • Installing rain gardens.
  • Practicing water conservation.
  • Washing cars on lawns or using a commercial car wash.
  • Practicing “only rain down the drain” for stormwater conveyances.
  • Practicing “don’t drip and drive.”

Why this matters

Bioassessment indicates how healthy streams are. Intact stream channels, riparian vegetation, and intact watershed hydrology all act to help create conditions that support the kind of benthic invertebrates that fish eat. Healthy streams means aesthetically pleasing streams that people can use for irrigation, recreation, and fishing.

Oxygen dissolved in healthy water is vital for fish and aquatic life to “breathe” to survive. It is more difficult to transfer oxygen from water to blood than it is to transfer oxygen from air to blood. Therefore, it is critical that an adequate amount of oxygen is maintained in the water for this transfer to take place efficiently and sustain aquatic life. Oxygen is also necessary to help decompose organic matter in the water and bottom sediments as well as for other biological and chemical processes.

Water temperature influences what types of organisms can live in a water body. Cooler water can hold more dissolved oxygen that fish and other aquatic life need to breathe. Warmer water holds less dissolved oxygen. Many fish need cold, clean water to survive.

Technical information

Unless otherwise specified, the following documents are Ecology publications.

Modeling Quality Assurance Project Plan for Soos Creek Watershed Temperature and Dissolved Oxygen TMDL Technical Analysis

Squalicum Creek and Soos Creek: Bioassessment Monitoring and Analysis to Support Total Maximum Daily Load (TMDL) Development

King County documents for Soos Creek project

Soos Creek Watershed Temperature and Dissolved Oxygen Total Maximum Daily Load Study Sampling and Analysis Plan

Related information

Unless otherwise specified, the following documents are Ecology publications.

March 7, 2017 meeting presentations and Ecology Response to Comments - Soos Stressor Identification and Bioassessment Meeting:

Soos Creek watershed streams are too warm, have too little oxygen, and aquatic habitat is degraded

Focus on Soos Creek Watershed

Green-Duwamish River Basin

WRIA 9: Duwamish-Green Watershed Information (Water web site)


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Last updated September 2017


WRIA: #9 (Duwamish-Green)
County: King

Water-body Names:
Big Soos Creek
Little Soos Creek
Covington Creek
Jenkins Creek
Soosette Creek
Little Soosette Creek

Dissolved Oxygen
Fecal Coliform

# of TMDLs: ---

Bioassessment, Dissolved Oxygen, and Temperature project - TMDL under development
Fecal coliform project - TMDL under development

Contact Info:
Joan Nolan
Phone: 425-649-4425
Email: Joan.Nolan@ecy.wa.gov

Northwest Region
Department of Ecology
3190 160th Ave. SE
Bellevue, WA 98008-5452