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King County Water Quality Monitoring

King County conducted monthly ambient water quality monitoring at one site in the Yarrow Creek basin at station A499, located on Cochran Springs near the intersection of Points Drive and Lake Washington Blvd, roughly 25 feet downstream from the Lake Washington Blvd culvert. Sampling began in 1975 and continued through 2008 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 station A497 was added on Yarrow Creek 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. Yarrow Creek is placed in Ecology's Noncore Salmon/Trout Aquatic Use Category which sets criterion for the protection of spawning, core rearing, and migration of salmon and trout, and other associated aquatic life.

Land use in 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 watershed 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 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 been making observations at river mile 0.3 (1998, 1999, 2000, 2001) and at river mile 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 under the 2003 rules. Yarrow Creek is on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list for violation of dissolved oxygen and fecal coliform bacteria standards (Category 5). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

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.

Table 1. Routine monitoring summary statistics for this station from 2017 to 2017
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)810.29.011.610.31.0
Temperature (°C)813.38.016.514.13.0
Turbidity (NTU)83.031.565.513.131.26
Conductivity (mSIEMS/cm)8247.8197.0266.0258.522.6
Total Suspended Solids (mg/L)83.471.507.603.091.88
Ortho-Phosphorus (mg/L)80.03200.02520.03850.03330.0045
Total Phosphorus (mg/L)80.05440.04900.06160.05380.0042
Ammonia (mg/L)80.02060.01300.03140.01650.0084
Nitrate (mg/L)80.84630.57700.96400.91050.1400
Total Nitrogen (mg/L)81.07000.86801.19001.10000.1076
Fecal Coliform(CFU/100ML)8110546066150


Stream Sediment

Benthic Invertebrates

Special Studies