Soos Creek watershed fine sediment TMDL

The Soos Creek watershed is located in South King County, in the Seattle metropolitan area. The watershed includes several urbanized, fast-growing communities, including Covington, portions of Kent, Auburn, Maple Valley, Black Diamond, and Renton, and unincorporated King County.

Portions of Soos Creek fail to meet state water quality standards and the beneficial uses for aquatic organisms are compromised. To address water quality impairments, we are developing a water quality improvement plan, also known as Total Maximum Daily Load (TMDL). The water quality improvement plan will identify the primary causes of these impairments and provide recommendations to improve the habitat to support healthy aquatic organisms.

Based on the data available in the water quality assessment, stream segments in the Soos Creek watershed do not meet water quality standards for temperature, dissolved oxygen, bacteria, and fine sediment. This TMDL will focus on the excess of fine sediment, as a result of human activities, that impairs benthic macroinvertebrates. We are developing another TMDL to address the remaining impairments.

Water quality impairments

Benthic invertebrates are organisms that live within the gravel (sediment) on the bottom of rivers, streams, and lakes. Benthic invertebrates can be studied using methods developed by EPA, to create a measure of stream health called the benthic index of biological integrity (B-IBI). This index gives the waterbody a score, that is based on which benthic invertebrates are found in the sediment. Benthic invertebrates are considered the “canaries in the coal mine” of water quality because some of these "water bugs" are highly sensitive to pollution, including high volumes of fine sediment. Benthic invertebrates need healthy waters to thrive, meaning that if they aren’t doing well, other aquatic organisms like salmon, that rely on the same habitat, are jeopardized too. In other words, the B-IBI score not only represents the condition of the “water bugs” but also of the overall health of a stream.

Land use in the Soos Creek watershed varies from urban and residential, commercial, some industrial, to small scale agricultural land uses. Changes in how we use the land, such as increasing impervious surfaces, can impact benthic invertebrates’ ability to thrive in several ways:

  • Increases in impervious surfaces impact the rate and volume of stormwater being discharged to streams due to lack of retention and infiltration.
  • Stormwater runoff that is not adequately managed can change the patterns of stream flows by increasing peak flows during storms to levels that are much higher than those that occurred before development.
  • Stormwater runoff also becomes a conduit for fine sediment from upland areas into streams.
  • Loss of streamside vegetation and disconnection from floodplains increase streambank erosion.
  • Fine sediment from erosion and from stormwater runoff settles in the small spaces between the gravel at the bottom of the streams, smothering or displacing sensitive aquatic life.

The diagram below helps demonstrate the relationship between development and the health of benthic organisms.


Illustrates the cycle described in the bulleted list above.

Read a text explanation of the Soos Creek Fine Sediment TMDL Conceptual Model

While some erosion is part of the streams’ natural processes, too much fine sediment is an indicator of significant disruptions to the flow pathways in the watershed. The TMDL will describe these flow disruptions and provide recommendations to help stakeholders prioritize work.

What has been done so far?

In 2016, we partnered with EPA to do a benthic invertebrate stressor identification study to help identify what pollutants and habitat impairments are contributing to low B-IBI scores in Soos Creek. The study found that three main stressors are the primary culprits to impaired benthic invertebrates:

  • Fine sediment
  • High peak stream flows during storm events
  • Habitat degradation

We recently developed a watershed model to help us understand how urban development in the watershed can change the movement of water (hydrology) and impact the pathways that move fine sediment. 

Status of the project

We are currently working on estimating by how much each pollution source will need to be reduced to achieve clean water. We are also in the process of developing a draft implementation plan that we can bring to stakeholders for discussion later in 2023. We will work closely with our local stakeholders and implementation partners to prepare an implementation plan that will describe actions and programmatic recommendations needed to improve water quality in the basin. The map below shows the local jurisdictions in the Soos watershed that will be involved in the implementation of the TMDL.

The watershed includes all of Covington and parts of Kent, Auburn, Black Diamond, Maple Valley, Renton, and unincorporated King county

Soos Creek watershed jurisdictions

We expect to start engaging stakeholders to discuss and develop an implementation plan in 2023 and to finalize the TMDL plan for EPA review by early 2024.

Why this matters

Benthic macroinvertebrates live a year or more and spend a good portion of their life cycle as larvae. During this life stage, the macroinvertebrates don’t move much and are entirely dependent on their local water quality. Some taxa are more sensitive to specific pollutants than others, so their presence, abundance, or total absence can signal a pollution problem and make them a good indicator that instream habitat conditions are out of balance.