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King County Water Quality Monitoring

King County monitors the ecological health of Soos Creek in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Water quality samples have been collected monthly at four stations in the Soos Creek basin since 1972. Station A320 is located at the USGS gaging station roughly 300 feet upstream of the hatchery near the mouth of the creek. Sampling at A320 began in 1972 and continues today. Station C320 is located in Covington Creek on the bridge on Kent-Black Diamond Rd near Thomas Rd. Sampling began in 1972 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. However, sampling resumed in February 2013. Station D320 is located near the mouth of Jenkins Creek just upstream its confluence with Soos Creek at the bridge on Kent-Black Diamond Rd near 157th Ave SE. Sampling at D320 began in 1972 and continues today. Station G320 is located near the mouth of Little Soos Creek on Covington Way SE, west of the intersection with Hwy 516. Sampling here occurred from 1972 to 2008, then resumed in February 2013. Beginning in 1987, sediment quality samples have been collected from Soos Creek. King County began collecting benthic macroinvertebrate samples in Soos Creek in 2002.

From time to time additional studies have been conducted at various streams in King County. (Click here for information about Special Studies of Soos Creek.)


Water Shed Image The Soos Creek watershed is located in South King County in Water Resource Inventory 9 (WRIA 9) and drains into the Middle Green River at River Mile (RM) 33.7 (Kerwin and Nelson 2000; Herrera 2005). The drainage basin covers an area of approximately 70 square miles and is located southeast of Renton and east of Kent. There are 25 tributaries to the Soos Creek totaling over 60 linear miles. The four main tributaries to Soos Creek are Covington Creek, Jenkins Creek, Little Soos Creek, and Soosette. Lakes that are part of this drainage system have a combined surface acreage of roughly 1,370 acres and include Lake Youngs (a domestic water supply for the City of Seattle,), Shadow Lake, Lake Meridian, Lake Sawyer, Morton, Pipe/Lucerne, and Wilderness lakes. The Soos Creek basin is an extensive system of interacting lakes, wetlands and infiltrating soils that collectively attenuate peak stream flows.

Land use in the Soos Creek basin consists of rural residential, agriculture, and highly urban commercial and residential areas (Herrera 2005). The western area in particular has been subject to heavy urbanization in recent years. Increased impervious surface area has contributed to decreases in summertime low flows and increases in winter storm water flows (King County 1990). Some areas of the Soos Creek basin are expected to have stream flows increase 3.5 times the 1985 levels due to the shift from highly infiltratable soils being converted to urban areas with impervious surfaces (King County 1989). Increased groundwater withdrawal also contributes to the decline in in-stream flows. The City of Kent, the Covington Water District, and King County Water District #111 are the largest consumers of water in the basin.

For more information about the Soos Creek watershed, visit the WRIA 9 Web site for the Middle Green River Subwatershed.


Chinook, coho, sockeye, pink and chum salmon, as well as winter steelhead have been observed spawning in Soos Creek (Kerwin and Nelson, 2000). Resident and anadromous cutthroat have been observed throughout the basin.

The Soos Creek State Fish Hatchery (SFH) has been in continuous operation since 1901. Adult chinook and coho are captured for on and off-season releases. The SFH produces approximately 3.2 million fall chinook sub-yearlings and 6000,000 coho yearlings (Kerwin and Nelson 2000).

The WRIA 9 Habitat Limiting Factors and Reconnaissance Assessment for Salmon Habitat provides more detailed information about habitat conditions in the Soos Creek area.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen, pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform bacteria. Results are compared to State water quality standards. Water quality standards are designed to protect public health and aquatic life. Comparing monitoring results to water quality standards allows an understanding of how safe the creek is for recreational contact as well as for aquatic life. (See link at top of page to view current water data.)

State water quality standards were revised in 2003. Soos Creek is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Primary Contact” for recreational use under the 2003 rules. As part of the updated water quality standards, the lower portions of the creek (stations A320, C320, D320) have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from September 15th through July 1st. Soos Creek is not listed on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list for impaired streams. However, Little Soos is listed for violation of dissolved oxygen, temperature, and fecal coliform bacteria standards (Category 5). Jenkins and Covington Creeks are listed for violation of fecal coliform bacteria and dissolved oxygen standards. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

A water quality data assessment conducted for the Green-Duwamish River in 2003 found that of most parameters were lower at Soos Creek (A320) relative to the Black River, Springbrook, and Mill Creek stations (Herrera 2005). Fecal coliform bacteria at A320 exceeded State criterion in storm flows. Total aluminum concentrations exceeded U.S. EPA chronic criterion during base flow sampling, and acute criterion during storm flow sampling at A320. However, U.S. EPA acknowledges that these criteria may be overprotective. This is because the digestion procedure for analyzing total aluminum includes some aluminum that is not toxic and would not likely be converted to a toxic form under natural conditions (U.S. EPA 1988). U.S. EPA (2002) noted that many high quality water in the U.S. have aluminum concentrations greater than the chronic criterion when either total recoverable or dissolved aluminum are measured.

Long-term Trends

A 28-year (1979 – 2007) trend analysis was conducted with baseflow water quality data collected from all fours sites in the Soos Creek basin. This analysis shows that there have been changes in water quality during that 28 year time period that indicate some decline in water quality. At stations A320, C320, and D320 dissolved oxygen concentrations decreased, and total phosphorus and nitrate-nitrogen concentrations increased significantly. Fecal coliform concentrations increased at C320. Water temperatures increased significantly at D320. Conductivity increased at all four sites. High conductivity can suggest the presence of unidentified dissolved charged substances in the water.p>

Some improvements in water quality in the Soos Creek basin are also indicated in the 28-year trend analysis. Total suspended solids and Ortho-phosphorus decreased at all four sites. Ortho-phosphorus concentrations decreased at all four sites. Total phosphorus and ammonia concentrations decreased at G320. Fecal coliform bacteria decreased at A320, D320 and G320.

Water Quality Index

A Water Quality Index (WQI) rating system was developed by the State Department of Ecology that evaluates several water quality parameters and gives a single rating of “high,” “moderate,” or “low” water quality concern. To see how these ratings compare with other stream sites, visit the Water Quality Index page.

For more information about long-term trends, the Water Quality Index page or to view charts of current water quality conditions in the creek, please visit the links at the top of the page.

Table 1. Routine monitoring summary statistics for this station from 1972 to 2017
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)27311.19.113.611.01.0
Temperature (°C)4199.81.517.110.03.4
Turbidity (NTU)3783.890.40272.001.5016.38
Conductivity (mSIEMS/cm)240130.963.0163.0131.013.1
Total Suspended Solids (mg/L)41712.130.001092.303.5067.34
Ortho-Phosphorus (mg/L)4190.01530.00400.05300.01460.0057
Total Phosphorus (mg/L)4210.03860.00640.98500.02850.0658
Ammonia (mg/L)4200.01780.00370.13300.01200.0155
Nitrate (mg/L)4200.96210.38601.61000.94250.1972
Total Nitrogen (mg/L)3371.17480.31005.57001.13000.3310
Fecal Coliform(CFU/100ML)4141690850050654

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2017
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)5810.98.313.011.01.1
Temperature (°C)799.
Turbidity (NTU)7011.780.66204.004.2026.62
Conductivity (mSIEMS/cm)46124.277.5149.0128.020.4
Total Suspended Solids (mg/L)7825.561.10347.009.9549.59
Ortho-Phosphorus (mg/L)780.02350.00720.21800.02000.0246
Total Phosphorus (mg/L)780.06890.01970.35400.04570.0643
Ammonia (mg/L)780.03190.00540.14900.02600.0261
Nitrate (mg/L)780.90480.53201.49000.90200.2028
Total Nitrogen (mg/L)761.29680.67002.47001.19000.3525
Fecal Coliform(CFU/100ML)78731568002451262


King County maintains two stream flow gauges (54a and 54h) and a precipitation gauge (54v) on Soos Creek.

Stream Sediment

Sediment data were collected from Soos Creek as part of the Streams Monitoring program from 1987 to 2002. Data analysis identified no significant trends for any of the parameters tested.

Basin-wide sampling occurred in 2010, including 11 sites along the Soos Creek reach. Concentrations of arsenic at sites GG320, HH320 and Q320 exceeded the Sediment Cleanup Objective (SCO), meaning adverse effects to sediment-dwelling animals are possible, but uncertain. The SCO for nickel was exceeded at sites 0320, A320 and AA320. The concentrations of di-n-octyl phthalate (a chemical found in many plastics) also exceeded the SCO at site Q320. AVS/SEM analysis suggests that metals are bioavailable at all sites where metals exceeded guidelines.

Benthic Invertebrates

Benthic macroinvertebrates are small animals visible to the naked eye (macro) that lack a backbone (invertebrate) and live in or around the streambed (benthic). This group includes aquatic insects (such as mayflies and dragonflies), crustaceans, clams, snails, and worms. Benthic macroinvertebrates are of interest to scientists and water resource managers because they are an excellent indicator of the biological health of stream ecosystems and are a critical component of the food web in aquatic communities. Scientists quantify the composition and diversity of benthic macroinvertebrate populations in a stream to compare the biologic integrity of different streams. King County has utilized benthic macroinvertebrate sampling to assess biological health of numerous creeks across the county (see map).

For more information about benthic macroinvertebrates and King County’s Benthic Macroinvertebrate Program, please visit the King County Stream Bug Monitoring web page. To see the benthic macroinvertebrate data for Soos Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Green-Duwamish Water Quality Assessment

In 2001 King County initiated a separate comprehensive study of the Green-Duwamish Watershed, called the Green-Duwamish Watershed Water Quality Assessment (GDWQA). Several reports evaluate data collected as part of the GDWQA. Soos Creek is included in this analysis. More information about the GDWQA, as well as the reports, can be found at the Green-Duwamish Watershed Water Quality Assessment Web page.

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

Sampling and Analysis Plan

Appendix 1: 7-DADMAX thermal record graphs

Appendix 2: Historical dissolved oxygen data

Appendix 3: Discharge, water temperature, air temperature graphs.