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King County Water Quality Monitoring

Beginning in 1975, King County has conducted monthly ambient water quality monitoring in the Cochran Springs 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.

Watershed

Water Shed Image The Cochran Springs 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. Cochran Springs is placed in the Washington Department of Ecology's (Ecology)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 watersheds were observed. It was noted that much of the stream had been piped to accommodate development.

Fisheries

The suitability of Cochran Springs 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 Cochran Springs at River Mile (RM) 0.2 at the culvert under NE Points Drive (Kerwin, 2002).

Little is known about the existing fish population in Cochran Springs. The creek historically supported coho salmon (Kerwin, 2002). Kokanee were presumed to use Cochran Springs based on historical records of a Native American village located near its mouth. Volunteers with the Salmon Watcher Program have been making 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 Cochran Springs 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 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.

State water quality standards were revised in 2003. Cochran Springs 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 Cochran Springs have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. Cochran Springs 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 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 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 1975 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)27110.97.915.610.90.7
Temperature (°C)39810.72.516.010.91.9
Turbidity (NTU)3983.800.2058.802.455.94
pH3247.936.458.907.990.23
Conductivity (mSIEMS/cm)239228.382.0256.7236.028.4
Total Suspended Solids (mg/L)39810.371.40292.005.9021.76
Ortho-Phosphorus (mg/L)3980.04710.00700.11600.04600.0100
Total Phosphorus (mg/L)3980.06890.03580.35400.06310.0273
Ammonia (mg/L)3980.01770.00390.15400.01200.0187
Nitrate (mg/L)3972.11990.69604.54002.18000.3265
Total Nitrogen (mg/L)3272.31371.08003.30002.34000.2585
Fecal Coliform(CFU/100ML)359247025000321478

Table 2. Storm water monitoring summary statistics for this station from 1975 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)3710.79.213.210.80.9
Temperature (°C)5210.56.515.010.62.0
Turbidity (NTU)5410.881.0068.005.6513.61
pH497.686.408.407.710.38
Conductivity (mSIEMS/cm)25168.362.0243.0172.056.7
Total Suspended Solids (mg/L)5445.543.00312.0021.3060.60
Ortho-Phosphorus (mg/L)540.04860.02690.12000.04580.0152
Total Phosphorus (mg/L)540.10460.04900.40800.08520.0631
Ammonia (mg/L)540.03120.01000.09560.02300.0238
Nitrate (mg/L)541.65650.55402.43001.81500.5176
Total Nitrogen (mg/L)521.98990.95602.89002.05000.4070
Fecal Coliform(CFU/100ML)547091157002351239