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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 special studies have been conducted at various streams in King County. Special studies on Forbes Creek are listed at the bottom of this page.


Water Shed Image The Forbes Creek basin comprises approximately 1000 acres. The creek originates at an elevation of 240 feet above sea level and flows roughly 1.8 miles thorough the City of Kirkland and a wetland area before entering the north east corner of Lake Washington. Land use in the basin is primarily mixed residential (WSDOT 2003).


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). Volunteers with the Salmon Watcher Program have made observations at river mile 0.2 and at river mile 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 bacteria (FC). 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. 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 Department of Ecology’s (Ecology) 303(d) list (Category 5) for violation of dissolved oxygen, temperature, and fecal coliform bacteria standards (Category 5). The creek is also listed for bioassessment scores that indicate that biological integrity is degraded. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

Forbes Creek has historically had low dissolved oxygen concentrations and high temperatures (Metro 1990). Because the monitoring station is located near a large wetland with slow moving water and high levels of organic matter, low dissolved oxygen and higher temperatures are not surprising.

To view charts of current water quality conditions in the creek, please visit the links at the top of the page.

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 fecal coliform bacteria have all shown a significant decrease over this 29-year time period. However, stream temperatures and conductivity have increased, and dissolved oxygen 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, visit the Water Quality Index Web page.

For more information about long-term trends in other streams or to view charts of current water quality conditions in Forbes Creek, please visit the links at the top of the page.

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 maintains a water temperature gauge on Forbes Creek (20a).

Stream Sediment

Sediment samples were collected from three stations approximately a mile apart, along a three miles 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 (AVS/SEM 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 web page. To see the benthic macroinvertebrate data for Forbes Creek, please visit the Puget Sound Benthos webpage.

Special Studies

In October of 2002 about 50 4th grade students from Alexander Graham Bell Elementary School collected water samples from several sites within Forbes Creek (and Juanita Creek). The students then studied the water quality constituents. This project was made possible through the creative thinking of 4th Grade teachers at Alexander Graham Bell, and a partnership with the City of Kirkland. The Department of Public Works Storm/Surface Water Division provided the water quality testing kits used by the students. This project was partially funded by a King County Waterworks Grant.

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. Visit the WRIA 8 Web page to see how Forbes Creek is part of this WRIA 8 planning process.