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King County Water Quality Monitoring

King County has monitored the ecological health of Coal Creek in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Station 0442 is at the former USGS gauging station located near Coal Creek Parkway and 119th Avenue South. Water quality samples were collected monthly at this station from 1972 to 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Sampling at this station resumed in February 2013. Sediment quality samples have been collected from Coal Creek. Benthic invertebrates have been sampled from various sites in Coal Creek since 2002. Sediment samples have been collected from Coal Creek since 1987.

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


Water Shed Image Coal Creek is located along the southern limit of the City of Bellevue. The headwaters of Coal Creek originate in the steep terrain of Cougar Mountain at an elevation of about 1,400 feet. The creek flows for about 7 miles through a series of steep, narrow ravines before entering Lake Washington along the eastern shoreline at Newport Shores. The drainage basin for the creek is roughly 4,550 acres and contains one large tributary - Newport Hills Tributary (Tetra Tech/KCM 2005). The watershed is within the shared jurisdiction of the City of Bellevue, King County and the City of Newcastle.

Extensive coal mining activities took place in the basin beginning in the late 1800s. The mining activities resulted in changed stream courses, channelized reaches, and mine tailing dumped along stream banks (McDonald 1987; Kerwin 2001). Headwater streams of the south fork of Coal Creek originate from a caved-in section of mine that seems to combine with an extensive mine drainage system (Skelly and Loy 1985).

Urban development within the basin in the last century has further altered the creek's natural hydrologic characteristics, increasing the frequency, duration, and peak of flood events, stream bank erosion, and streambed sedimentation (Kerwin 2001). The channel was diverted southward in the late 1940s, then northward again in the late 1950s, because of the construction of an airstrip in the delta area of the stream. In the 1960s, as a feature of the residential development, two large canals were excavated just south of the stream mouth to allow moorage and waterfront amenities for inland properties. The stream has also been crossed several times by large municipal water and sewer mains.

A comprehensive basin plan for Coal Creek was produced by King County and the City of Bellevue in 1987 (King County 1987). As part of this planning effort a hydrologic model was developed. It was found that base flows in Coal Creek are augmented by approximately ten percent by flows from mine tunnels. Coal Creek was also found to have extensive sedimentation problems from stream bank erosion and the occasional catastrophic failure of tailing slopes that remain from the old coal mining activities in the creek's headwaters and landslides of the steep slopes above the creek. Since 1997, the City of Bellevue has maintained two sediment retention ponds in the system, one immediately upstream of Interstate Highway 405 and another immediately upstream of Coal Creek Parkway. While the sediment ponds are helping to control excessive delta formation through the reduction of large particles, smaller particles and fine silts and clays remain suspended and wash downstream (Tetra Tech/KCM 2005). Thus the ponds provide no protection for spawning and rearing habitat (Kerwin 2001).

Currently, land use in the Coal Creek basin is predominantly single-family and multi-family residences and parks, including the Cougar Mountain County Park in the headwaters and Coal Creek Regional Park (Kerwin 2001). The basin has 15 percent impervious area within the 100-foot riparian area surrounding the creek.


The Coal Creek basin has few returning adult salmon of any species, although substantial supplementation of coho has occurred (King County 2001b, 2001c, April 2005; Kerwin 2001). The Washington Department of Fish and Wildlife reported planting between 9,000-13,800 zero age coho into Coal Creek from 1994-1997. Coho, chinook, sockeye, steelhead, and trout have been noted in the basin (Kerwin 2001).

Since 1997 volunteers with the Salmon Watcher Program have been recording salmon observations at various locations in Coal Creek. Volunteers have consistently seen coho salmon in the creek. Less commonly spotted are chinook and sockeye salmon.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen, pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform 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.)

State water quality standards were revised in 2003. Coal Creek is now designated "Core Summer Salmonid Habitat" for aquatic life use and "Extraordinary Contact" for recreational use. As part of the updated water quality standards, the lower portion of Coal creek has been assigned an additional "Supplemental Spawning and Incubation Protection" temperature criteria of 13 °C to be applied from September 15th to May 15th. Coal Creek is on the Washington Department of Ecology's (Ecology) 303(d) list, Category 5, for violations of the dissolved oxygen standard. See Table 1 for a summary statistics for the routine water quality data collected at this site during the most recent water year. Wet weather summary statistics are shown in Table 2.

Coal Creek has historically had high dissolved oxygen concentrations and low temperatures (Metro 1990). However, sedimentation was a serious problem in the basin due to highly unstable soils. In 1987 King County and the City of Bellevue cooperatively produced a comprehensive basin plan for the creek (King County and City of Bellevue 1987).

Long-term Trends

A 36-year (1972 - 2007) trend analysis was conducted with baseflow data collected from Coal Creek. This analysis showed some improvements in the water quality over this time period. Phosphorus, fecal coliform bacteria, and total suspended solids have all shown significant decreases. However, in stream temperatures increased and dissolved oxygen and pH 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 view WQI scores for Coal Creek visit the Water Quality Index page. You can also access site specific WQI reports from the Streams List or the Water Quality Index links on the left.


Table 1. Routine monitoring summary statistics for this station from 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)21011.09.014.810.91.2
Temperature (°C)34310.
Turbidity (NTU)34312.740.401023.002.8960.96
Conductivity (mSIEMS/cm)177453.9110.0765.0442.0171.8
Total Suspended Solids (mg/L)34227.580.002132.002.60138.59
Ortho-Phosphorus (mg/L)3410.01820.00230.23600.01520.0176
Total Phosphorus (mg/L)3430.05060.00831.55800.03010.1042
Ammonia (mg/L)3420.02070.00100.15000.01550.0192
Nitrate (mg/L)3410.78370.11403.30000.53500.6276
Total Nitrogen (mg/L)2640.90410.20303.20000.68450.6183
Fecal Coliform(CFU/100ML)341376020000701342

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)3810.79.213.410.41.0
Temperature (°C)5410.65.916.110.52.6
Turbidity (NTU)5481.791.90999.0034.00173.19
Conductivity (mSIEMS/cm)33178.671.0453.0167.069.0
Total Suspended Solids (mg/L)54163.133.301690.0074.70291.35
Ortho-Phosphorus (mg/L)540.02930.00830.21000.01950.0375
Total Phosphorus (mg/L)540.18930.02601.39000.11150.2396
Ammonia (mg/L)540.02890.01000.12700.02060.0239
Nitrate (mg/L)540.91350.21503.79000.63750.6865
Total Nitrogen (mg/L)531.48200.53805.77001.23000.9404
Fecal Coliform(CFU/100ML)53164838110007302072


King County maintains one stream flow gauge on Coal Creek: Coal Creek Stream Gauge (06a).

Stream Sediment

Sediment data were collected from Coal Creek as part of the Stream Monitoring Program starting in 1987. Data were compiled for the years 1987 through 2002. Data analysis identified no significant trends for any of the parameters tested.

Basin-wide sampling took place in 2007, including four sites along the Coal Creek reach. Concentrations of arsenic at sites A442 and D442 exceeded the Sediment Cleanup Objective (SCO), meaning effects to sediment-dwelling animals are possible, but uncertain. At site A442, concentrations of nickel and bis(2-ethylhexyl)phthalate (an ubiquitous chemical found in many plastics) also exceeded SCO. AVS/SEM analysis suggests that metals were bioavailable at site D442, but not at site A442.

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 Coal Creek, please visit the Puget Sound Benthos webpage.

Special Studies

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 Coal Creek is part of this WRIA 8 planning process.