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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 U.S. Geological Survey (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 on Soos Creek. Click here for information about Special Studies.


Water Shed Image The Soos Creek watershed is located in South King County in Water Resource Inventory (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 the City of Renton, Washington and east of the City of Kent, Washington. There are 25 tributaries to the Soos Creek totaling over 60 linear miles. The main tributaries to Soos Creek are Covington Creek, Jenkins Creek, Little Soos Creek, Little Soosette Creek, and Soosette Creek. 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, Washington), 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.

Total land use is mostly developed and forest. A majority of the developed land is low intensity and open space while forest is mostly mixed forest. Remaining land use consists of relatively equal amounts of agriculture, scrub, wetlands, and other (barren land, grassland, and open water). See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Agriculture Developed Forest Scrub Wetlands Other
Total 3% 63% 25% 3% 3% 3%

For more information about the Soos Creek watershed, please visit the WRIA 9 webpage 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 report provides more detailed information about habitat conditions in the Soos Creek area.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform (FC) 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. As part of the updated water quality standards, portions of Soos Creek have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from September 15th to July 1st. Soos Creek is not listed on the Washington State Department of Ecology’s (Ecology) 303(d) list for impaired streams. However, several of its tributaries are listed for violation of water quality standards (Category 5) :

  • Covington: water temperature, FC bacteria, and DO
  • Jenkins: water temperature and bioassessment
  • Little Soos: water temperature, DO, FC bacteria, and bioassessment
  • Little Soosette: DO and FC bacteria
  • Soosette: water temperature and FC bacteria

See Table 2 below for routine monitoring summary statistics of water quality data collected to date. If stormwater data is available for this site, it will be shown as Table 3. Historical data reviews can be found in the annual reports produced by METRO/King County DNRP.

To view charts of current water quality data, please visit the Data Download webpage.

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). FC bacteria at A320 exceeded State criterion in storm flows. Total aluminum concentrations exceeded U.S. Environmental Protection Agency (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). 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 DO concentrations decreased, and total phosphorus and nitrate-nitrogen concentrations increased significantly. FC 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. FC bacteria decreased at A320, D320 and G320.

Water Quality Index

A Water Quality Index (WQI) rating system was developed by 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, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 1972 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)35011.19.113.611.11.0
Temperature (°C)4969.81.517.110.03.4
Turbidity (NTU)4543.900.40272.001.6015.63
Conductivity (mSIEMS/cm)317130.963.0163.0131.013.4
Total Suspended Solids (mg/L)49311.870.001092.303.6062.91
Ortho-Phosphorus (mg/L)4950.01490.00400.05300.01420.0055
Total Phosphorus (mg/L)4970.03760.00640.98500.02870.0610
Ammonia (mg/L)4960.01550.00370.13300.01030.0144
Nitrate (mg/L)4960.93440.38601.61000.91700.1990
Total Nitrogen (mg/L)4131.13930.31005.57001.11000.3136
Fecal Coliform(CFU/100ML)4561600850050624

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2024
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 water temperature, precipitation, and stream gages on Soos Creek:

  • Temperature: Soos Creek at mouth (54a) and Soos Creek Meridian tributary (54K)
  • Precipitation: Soos Creek rain gage (54v)
  • Streamflow: Soos Creek at Kent-Black Diamond Road (54j)

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. Acid volatile sulfide/simultaneously extracted metals (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 webpage. To see the benthic macroinvertebrate data for Soos Creek, please visit the Puget Sound Stream Benthos webpage.

Special Studies

Green-Duwamish Water Quality Assessment

In 2001, King County initiated a 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.

Sampling and Analysis Plan and Quality Assurance Project Plan

A temperature and dissolved oxygen total maximum daily load (TMDL) study was conducted on the Soos Creek basin in February of 2009. Ecology suggested that parts of the Soos Creek system did not meet standards for temperature and dissolved oxygen. At the time, the system was subject to two water quality criteria for fish use and it routinely exceeded these criteria. This study populated a water quality model to develop pollutant load reduction quantities to bring the stream into compliance with state water quality standards.