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King County Water Quality Monitoring

King County monitors the ecological health of Forbes Creek in a variety of ways, including collecting and analyzing water, sediment, and benthic invertebrate samples. Water quality samples are collected monthly near the mouth of the creek near the end of the pump access road on NE 106th Street. In the spring of 2016, the historical site, station 0456, went dry. Station 0456A was established a little further south where the flows pass through a culvert. Sampling began in 1979 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. However, routine water quality sampling at this site resumed in February 2013. Sediment samples have been collected from Forbes Creek as part of the Streams Sediment Monitoring Program starting in 1987. King County began collecting benthic macroinvertebrate samples in Forbes Creek in 2002.

From time to time, additional studies have been conducted on Forbes Creek . Click here for more information about Special Studies.


Water Shed Image The Forbes Creek basin comprises approximately 1,000 acres. The creek originates at an elevation of 240 feet above sea level and flows roughly 1.8 miles thorough the City of Kirkland, Washington and a wetland area before entering the north east corner of Lake Washington. \

More than three-quarters of total land use in the Forbes Creek basin is developed. The remaining land use is mostly (deciduous) forest, while agriculture is not present. Very little scrub, wetlands, and other land use (barren land) exist. The development is largely low intensity, open space, and medium intensity. Less than a quarter is a high intensity development. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Scrub Wetlands Other
Total 84% 13% <1% 2% <1%


Little is known about the existing fish population in Forbes Creek. The creek historically supported cutthroat trout and coho salmon have been introduced (Metro, 1990). From 1996 to 2015, volunteers with the Salmon Watcher Program made observations at river mile (RM) 0.2 and RM 0.9. Only one coho was observed in the creek in 2001 (King County, 2002).

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)./p>

State water quality standards were revised in 2003. Forbes Creek is now categorized as “Core Summer Salmonid Habitat” for aquatic life use, and “Extraordinary Contact” for recreational use. Forbes Creek is on the Washington State Department of Ecology’s (Ecology) 303(d) list for violation of DO, temperature, FC bacteria, and bioassessment 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.

Long-term Trends

A 25-year (1979 – 2007) trend analysis was conduced with baseflow data collected from Forbes Creek. This analysis showed some improvements in the water quality since 1979. Ortho-phosphorus, total nitrogen, nitrates, total suspended solids, and FC bacteria have all shown a significant decrease over this 29-year time period. However, stream temperatures and conductivity have increased, and DO concentrations have decreased significantly during this same time period.

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 Forbes Creek ratings compare with other stream sites, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 1979 to 2016
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)1938.41.512.78.92.3
Temperature (°C)33310.90.218.711.13.9
Turbidity (NTU)3354.660.3045.003.394.61
Conductivity (mSIEMS/cm)161203.986.0287.0209.536.9
Total Suspended Solids (mg/L)3356.721.00100.004.009.75
Ortho-Phosphorus (mg/L)3340.05290.00400.18900.04940.0206
Total Phosphorus (mg/L)3350.09250.00830.36200.08490.0392
Ammonia (mg/L)3340.04070.00700.55900.03310.0422
Nitrate (mg/L)3340.56320.01402.22000.51350.2707
Total Nitrogen (mg/L)2500.86550.42102.20000.81500.2676
Fecal Coliform(CFU/100ML)3344024200001501280

Table 2. Storm water monitoring summary statistics for this station from 1979 to 2016
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)369.
Temperature (°C)5210.
Turbidity (NTU)5313.873.5060.0010.5011.70
Conductivity (mSIEMS/cm)23124.665.1214.0124.035.0
Total Suspended Solids (mg/L)5325.902.60140.0015.2027.83
Ortho-Phosphorus (mg/L)530.04060.01830.10900.03280.0183
Total Phosphorus (mg/L)530.12220.04980.26400.11100.0550
Ammonia (mg/L)530.04320.01100.11000.03420.0254
Nitrate (mg/L)530.50450.26201.20000.48900.1855
Total Nitrogen (mg/L)511.02550.61301.61001.01000.1953
Fecal Coliform(CFU/100ML)521771100860011501863


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

Stream Sediment

Sediment samples were collected from three stations approximately a mile apart, along a three mile reach of Forbes Creek in the summer of 2005 (see map). Concentrations of bis(2-ethylhexyl)phthalate (a chemical found in some plastics) and nickel at site 0456 were above Sediment Cleanup Objective (SCO) concentrations. Additional data were collected (acid volatile sulfide/simultaneously extracted metals ratio) at these sites to better understand the toxicity potential of metals. This information indicates metals are not likely bioavailable at 0456.

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 Forbes Creek, please visit the Puget Sound Stream Benthos webpage.

Special Studies

Pyrethroid Survey of King County Stream Sediments 2014 Final Report

Pyrethroids are a group of synthetic insecticides commonly used in residential areas. Residential and commercial use has resulted in an increased detection in urban runoff, which can be harmful to aquatic life. In October 2014, a pyrethroid survey was conducted in sediments collected from six King County streams. These streams include: Juanita, Forbes, Thornton, Pipers, Springbrook, and Mill Creeks. Results found that bifenthrin was the only pyrethroid detected in the sediment samples; and most detected concentrations were below the reporting detection limit. Percent finds and TOC content, which varied substantially by creek, may be influencing bifenthrin detections. These results suggest that sediments may contain toxic levels of pyrethroids even if they are not detectable above detection limits in this study.

Water Resources Inventory Area (WRIA) 8

In WRIA 8, citizens, scientists, businesses, environmentalists, and governments are cooperating on protection and restoration projects and have developed a science-based plan to conserve salmon for future generations.