Soos Creek watershed fine sediment TMDL

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 is used to give 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 water 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 path for fine sediment from upland areas into streams.
• Loss of streamside plants 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.

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.

In Soos Creek, a stressor identification study made the connection between low B-IBI scores and fine sediment loadings. The TMDL describes these flow disruptions and provides recommendations to help local entities prioritize work.

Our final plan

We submitted the Soos Creek Fine Sediment TMDL to EPA for approval on Dec. 30, 2025. This cleanup plan addresses elevated fine sediment in the Soos Creek watershed. The final TMDL includes watershed model results and allocations, defines each partner's role, and sets pollution reduction targets to meet water quality standards.

We held a comment period from Sept. 17, 2025 through Oct. 20, 2025. Public comments and our response to comments are included in an appendix to the plan. You can view all comments we received on our comments webpage.

Once implemented, this plan should reduce fine sediments in the Soos Creek watershed. This will create the proper conditions for the restoration of the overall health of the watershed, so it can support all native aquatic life. 

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.