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King County Water Quality Monitoring

Beginning in 1975, King County has conducted monthly ambient water quality monitoring in the Yarrow Creek basin at station A499. Station A499 is located on Cochran Springs (a tributary to Yarrow Creek) near the intersection of Points Drive and Lake Washington Blvd and roughly 25 feet downstream from the Lake Washington Blvd culvert. Sampling at A499 was discontinued in 2009 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Sampling was resumed at station A499 in 2014. In 2017, the B499 station on the main stem of Yarrow Creek was added. B499 is located at the bridge on 101st Way Northeast.


The Yarrow Creek basin comprises approximately 640 acres. The creek originates in the Bridle Trails State Park at an elevation of 400 feet and flows 2.95 miles before entering Lake Washington along the northeastern shoreline.

Total land use in the Yarrow Creek basin is predominantly developed and forest. A majority of the developed and forested lands are low intensity and evergreen forest, respectively. Scrub and wetlands combined are less than 1% of total land use; and agriculture is not present. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Scrub Wetlands
Total 71% 28% <1% <1%

The basin includes Bridle Trail State Park, a business complex, condominiums, single family residences, and a portion of State Route 520 (Metro, 1990). Although the water quality in the creek was rated "good overall" in 1989, impacts common to developing watersheds were observed. It was noted that much of the stream had been piped to accommodate development.


The suitability of Yarrow Creek for salmonids was rated "good" in 1989 (Metro, 1990). The benthic invertebrate population was rated "fair" as it was dominated by small mayflies and earthworms. Lack of spawning substrate was stated to be the primary limiting factor for salmonids. In addition, migratory fish passage is blocked on Yarrow Creek at river mile (RM) 0.2 at the culvert under NE Points Drive (Kerwin, 2002).

Little is known about the existing fish population in Yarrow Creek. The creek historically supported coho salmon (Kerwin, 2002). Kokanee were presumed to use Yarrow Creek based on historical records of a Native American village located near its mouth. Volunteers with the Salmon Watcher Program have made observations at RM 0.3 (1998, 1999, 2000, 2001) and at RM 0.5 (1997, 2001). Only one unidentified fish was observed in the creek in 2000 and 2001.

Water Quality

The water quality in Yarrow Creek was characterized as "good' in 1989 (Metro, 1990). The creek was determined to have relatively high temperatures (though none exceeded state criteria), high pH, and high nitrates. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

State water quality standards were revised in 2003. Yarrow Creek is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Extraordinary Contact” for recreational use. As part of the updated water quality standards, portions of Yarrow Creek have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. Yarrow Creek is on the Washington State Department of Ecology’s (Ecology) 303(d) list for violation of dissolved oxygen (DO) and fecal coliform (FC) bacteria standards (Category 5).

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.

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 2017 to 2020
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4110.07.912.39.71.3
Temperature (°C)4112.
Turbidity (NTU)404.171.3220.702.853.73
Conductivity (mSIEMS/cm)41256.2180.0405.0258.041.2
Total Suspended Solids (mg/L)404.370.8026.203.154.46
Ortho-Phosphorus (mg/L)400.02980.01700.04210.03120.0067
Total Phosphorus (mg/L)400.05820.03390.11700.05540.0140
Ammonia (mg/L)400.02540.01160.06240.02420.0104
Nitrate (mg/L)400.70070.42800.96400.69250.1753
Total Nitrogen (mg/L)400.98130.64302.16000.96050.2527
Fecal Coliform(CFU/100ML)40144296064211


King County does not currently operate any gages on Yarrow Creek.

Stream Sediment

Benthic Invertebrates

Special Studies