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King County Water Quality Monitoring

King County monitors the ecological health of North Creek in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Station 0474 is located near the mouth of North Creek on the upstream side of the freeway bridge. Water quality samples have been collected monthly at this station since 1976. In 1999, the County began collecting water quality samples at an additional site on North Creek (D474) located at the bridge on NE 205th near 120th NE. However, regular water quality sampling at this site was discontinued in 2008 due to budget cutbacks. Beginning in 1987, sediment quality samples have been collected from North Creek. King County began collecting benthic macroinvertebrate samples in North Creek in 2003.

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


Water Shed Image North Creek originates in highly urbanized south Everett within the Everett Mall area and flows 12.6 miles southward through the City of Mill Creek before draining into the Sammamish River near Bothell. The drainage basin is roughly 19,000 acres and includes Silver Lake, Ruggs Lake and Thomas Lake.

The rapidly urbanizing North Creek basin contains approximately 49 percent impervious surface area (King County WLD, 2001a; Kerwin, 2002). The headwaters of North Creek were originally dominated by forested wetlands but are currently dominated by commercial and multi-family residential development. The upper-middle mainstem includes the City of Mill Creek, a rapidly growing community. Land use in the lower-middle reaches is mostly rural residential and contains mostly intact riparian forest. Below the Silver/Tambark confluence near Maltby Road, North Creek is mostly channelized, and flows through wetlands, industrial parks, and rural residential areas.


Historically, North Creek supported runs of chinook, sockeye, kokanee, and coho salmon and steelhead and coastal cutthroat trout. Since 1997 volunteers with the Salmon Watcher Program have been recording salmon observations at various locations in North Creek. Volunteers have consistently seen chinook, coho, kokanee and sockeye in the creek. Less commonly spotted are chum salmon.

The habitat qualities of segments of North Creek were evaluated by King County from August to November 1999 (see Special Studies section below). In general, upstream habitat was better quality than downstream habitat. The lower segments were rated low to medium-low habitat quality because of limited in-stream habitat complexity (i.e., the creek is channelized, no forest cover, etc.).

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. North Creek is now categorized as “Core Summer Salmonid” for aquatic life use and “Extraordinary Contact” for recreational use. As part of the updated water quality standards, the creek has been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from September 15th through May 15th. North Creek is on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list for violation of temperature (Category 5), dissolved oxygen (Category 5), and fecal coliform bacteria standards (Category 4a). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

To view charts of current data for North Creek, visit the links above.

A substantial portion of North Creek is located in Snohomish County. Snohomish County has been monitoring two upstream sites since 1992 and found bacteria counts and temperatures that consistently violated State criteria (Snohomish County 2000). In addition, Snohomish County found concentrations of lead, mercury, copper and zinc in water samples were in violation of State standards.

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with water quality data from station 0474 in North Creek. Results indicated that water quality might have declined over this 25-year period with significant increases in water temperatures and conductivity. High conductivity can suggest the presence of unidentified dissolved charged substances in the water. Water at the mouth of North Creek is becoming less acidic as indicated by the significant increase in pH. (The pH remains within acceptable range relative to the state standards.) Decreased total suspended solids (TSS), nutrients (ortho- and total phosphorus, ammonia and total nitrogen), and bacteria levels indicate some improvements in water quality in the same 25-year 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 these ratings compare with other stream sites, visit the Water Quality Index page.

Table 1. Routine monitoring summary statistics for this station from 1971 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)25110.
Temperature (°C)38010.
Turbidity (NTU)3464.020.2032.603.442.79
Conductivity (mSIEMS/cm)221168.694.3217.0170.028.4
Total Suspended Solids (mg/L)3826.631.3082.204.806.50
Ortho-Phosphorus (mg/L)3810.04150.00700.23000.03870.0202
Total Phosphorus (mg/L)3810.07620.02350.21700.07250.0255
Ammonia (mg/L)3810.02930.00700.12600.02500.0163
Nitrate (mg/L)3810.78840.24701.89000.77000.2145
Total Nitrogen (mg/L)3031.11990.45102.07001.08000.2165
Fecal Coliform(CFU/100ML)3814600160001601307

Table 2. Storm water monitoring summary statistics for this station from 1971 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4110.17.912.410.01.3
Temperature (°C)589.
Turbidity (NTU)5112.562.5077.808.1012.00
Conductivity (mSIEMS/cm)29145.866.5195.0150.032.0
Total Suspended Solids (mg/L)5923.933.3073.3018.2016.50
Ortho-Phosphorus (mg/L)590.04230.01890.15100.03700.0235
Total Phosphorus (mg/L)590.12480.04950.29600.12000.0568
Ammonia (mg/L)590.04160.01100.16700.03460.0297
Nitrate (mg/L)590.70430.40401.60000.66300.2094
Total Nitrogen (mg/L)571.26920.86902.30001.24000.2470
Fecal Coliform(CFU/100ML)59134240130006202032


King County maintains two temperature gauges on North Creek: North Creek located on west side of creek north of Hwy. 522 bridge (45A) and North Creek located on north-west side underneath 185th St. bridge near Kennard Corner at RM5 (45B) and one streamflow gauge on North Creek at County Line, Snohomish County gage (45Nc)

Stream Sediment

Sediment samples were collected from ten stations approximately a mile apart, along a ten mile reach of North Creek in the summer of 2005 (see Map). The most downstream site, 0474, was sampled every year from 2004-2010 as a legacy site. Chemical concentrations of arsenic, dieldrin (an organochlorine pesticide) and bis(2-ethylhexyl)phthalate two stations (TT474 and L474) were above levels where adverse effects in aquatic animals living in the sediments are possible, although uncertain; as they exceeded Sediment Cleanup Objectives (SCO) . Concentrations of nickel at sites RR474 and TT474 also exceeded the SCO level.

Station TT474 is located within North Creek Park in a wetland area. Because the wetland is a depositional area, the sediment consists of primarily fine grain muds and silts. This area can accumulate sediments as well as contaminants. However, additional data (AVS/SEM ratio) suggested that arsenic and nickel were probably not bioavailable and therefore not likely to cause adverse effects at TT474 despite their elevated concentrations.

Downstream at L474, the sediment also consists primarily of mud and silts, indicating a depositional area. The AVS/SEM ratio suggested arsenic was not bioavailable and therefore not likely to cause adverse effects even though it is present at elevated levels. Station L474 is adjacent to 240 St. SE so chemicals detected at this site likely reflect contaminants migrating off the roadway before draining into the creek.

The AVS/SEM ratio at site RR474, where nickel exceeded SCO levels, suggests that metals are bioavailable at this site.The elevated concentrations of dieldrin and bis(2-ethylhexyl)phthalate at both TT474 and L474 are of concern. The presence dieldrin in the sediments at 0456 is troubling because production of this pesticide ceased in the 1970s. This type of pesticide (like DDT) is persistent in the environment and can also bio-magnify in the food chain and show up at elevated levels in fish that live in both in this creek and in Lake Washington. Bis(2-ethylhexyl)phthalate is a ubiquitous chemical found in many plastic consumer products. It can break down over time, and while still somewhat persistent, it is not as long lived as chlorinated compounds (PCBs and DDT) or metals. As such, the elevated levels of this compound present at these locations suggest there may be an ongoing source within the watershed.

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 North Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Habitat Assessment

From August to November 1999 King County conducted habitat assessments on North, Swamp, and Little Bear creeks (King County WLRD, May 2001a). These streams are the three major northern tributaries to the Sammamish River. The North Creek habitat assessment took place from the mouth of the creek to the McCollum Park segment near 128th SE in Snohomish County. Click here to download the Habitat Inventory and Assessment for North, Swamp, and Bear Creeks (PDF format).

The results of these habitat assessments indicate that the channel and habitat structure of a number of the segments in all three streams are frequently degraded relative to values from published "properly functioning conditions" for the Puget Sound or the Pacific Northwest region. These data provide important baseline information for any restoration projects that might occur in the basins, as well as for monitoring changes in habitat quality. These assessments may be used for limiting factor analysis for the threatened chinook salmon as well as other salmonid species in these basins.

North Creek Streamkeepers

The Adopt-A-Stream/Streamkeepers have conducted a public awareness campaign to further protect North Creek by building awareness and developing stewardship activities, and promoting conservation easements. This project was partially funded in 1999 by a King County Waterworks Grant.

Small Streams Toxicity/Pesticide Study

In 1999, 2000, 2001, and 2002 King County conducted a pesticide study on selected creeks. North Creek was part of this study in 2002. The County collected samples for pesticides, other organics, metals, suspended solids, and toxicity (Ceriodaphnia dubia, and Selenastrum capricornutum). For more information, visit the pesticide study Web site.

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 today and for future generations. Visit the WRIA 8 Web page to see how this creek is part of this WRIA 8 planning process.