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King County Water Quality Monitoring

King County monitors the ecological health of Thornton Creek in a variety of ways, including collecting and analyzing water, sediment, and benthic invertebrate samples. Station 0434 is located at the mouth of Thornton Creek located one block south of Matthews Beach. Water quality samples have been collected monthly at the mouth since 1971. Beginning in 1987, sediment quality samples have been collected from Thornton Creek. King County began collecting benthic macroinvertebrate samples Thornton Creek in 2002.

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


Water Shed Image Subbasins of Thornton Creek include the mainstem, North Branch, and South Branch (Maple Leaf Creek) (Thornton Creek Watershed Characterization Report, 2000; Kerwin, 2001). The headwaters of the North Branch originate near Ronald Bog, which along with Twin Ponds one mile downstream, were ponds created in the 1950s when peat deposits were mined from the area. The North Branch drains approximately 4,446 acres of the Cities of Shoreline and Seattle, Washington. The South Branch originates west of Interstate-5 near the North Seattle Community College and drains approximately 2,333 acres of the City of Seattle. The creek and its tributaries flow over 15 miles and drain approximately 7,402 acres before entering the northern end of Lake Washington at Matthews Beach Park.

Prior to European settlement, Native Americans lived around Lake Washington. One of the eighteen historic home sites was identified near the mouth of Thornton Creek (Thornton Creek Watershed Characterization Report, 2000; Kerwin, 2001; Buerge, 1984). After European settlement, extensive logging took place in the watershed in the late 1880s. Sawmills were located at the mouth, in the upper South Branch near the current site of Northgate Mall, near Pinehurst School, and on the North Branch downstream of the Jackson Park Golf Course. Farms, orchards, and dairies dominated the area, after the removal of forests, until the mid-1990s.

Following World War II, the population in the Thornton Creek basin increased dramatically from 2,898 in 1920 to 17,500 in 1940. The population nearly tripled in the next decade reaching 43,680 by 1950 (Miholovich, 1977). In 1990, the population had reached 69,000 and in 2000 it was 75,400 (Thornton Creek Watershed Characterization Report, 2000; Kerwin, 2001). The introduction of Europeans marked a significant change for the Native Americans that lived around Thornton Creek, as well as land use.

Total land use in the Thornton Creek basin consists almost entirely of developed land. Developed land ranges from low to high intensity and open space, although about half is low intensity. The remaining land use is forest, scrub, wetlands, and other (open water). There is no agriculture in the basin. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Scrub Wetlands Other
Total 96% 3% <1% <1% <1%

Flooding is an issue in this watershed where much of the development took place prior to the promulgation of flood and pollutant control regulations. Notable flood-prone areas include the confluence, the mainstem, the upper North Branch at Ronald Bog, and at Jackson Park Golf Course (Thornton Creek Watershed Characterization Report, 2000; Kerwin, 2001). Most stormwater is conveyed to the creek either through storm drains along busy streets and commercial districts or through open ditches in residential areas. A 72- to 90-inch by-pass pipe diverts up to 350 cubic feet per second (cfs) storm flows from just below the confluence and drains it directly into Lake Washington.


Several sporadic salmonid surveys have been conducted primarily in the mainstem of Thornton Creek. Salmonid species present in the creek include Chinook salmon, coho salmon, coastal cutthroat trout, steelhead, and rainbow trout. The most common species encountered during surveys has been coastal cutthroat that have been occasionally observed in high densities (King County, 2002; Kerwin, 2001).

From 1997 to 2015, volunteers with the Salmon Watcher Program recorded salmon observations at various locations in Thornton Creek. Volunteers consistently saw coho in the creek. Less commonly spotted were Chinook, sockeye, and cutthroat trout.

Stocking has occurred in Thornton Creek since the 1950s (Kerwin, 2001). Between 1952 and 1990, an average of about 15,000 coho fry per year were released into the creek. Since 1990, local schools have released up to 200 salmon fry/school/year (mostly coho) at three locations including the river mouth, below the confluence of the North and South branches, and in South Branch at River Mile (RM) 0.3. Private property owners operated egg incubators on the South Branch tributary, obtained from the Issaquah Hatchery. In 1977, about 17,000 Chinook fry were released into Thornton Creek. Between 1979 and 1982, a total of 50,000 - 130,000 Cedar River sockeye fry/fingerlings were released into the creek.

Poor fish habitat has been identified as lacking throughout the creek, including deep pools, off-channel habitat, instream complexity, riparian cover, large woody debris, and refuge habitat (Kerwin, 2001). A fish passage assessment was conducted by Washington Trout on Thornton Creek in 1999, sponsored by Seattle Public Utilities (SPU). Loss of hydrologic connectivity to the floodplain resulting from stream bank hardening is also identified as a problem.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids (TSS), 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. Thornton Creek is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Primary 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 to May 15th. Thornton Creek is on the Washington State Department of Ecology’s (Ecology) 303(d) list list for violation of water temperature, DO, and FC bacteria 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 37-year (1971 – 2007) trend analysis was conducted with water quality data from station 0434 in Thornton Creek. Results indicated that water quality might have declined over this 37-year period with significant increases in water temperatures, total phosphorus, turbidity, conductivity, and decreased DO. High conductivity suggests the presence of unidentified dissolved charged substances in the water. Water at the mouth of Thornton Creek is becoming more acidic as indicated by the significant decrease in pH (the pH remains within acceptable range relative to the state standards). Decreased ortho-phosphorus, nitrate-nitrogen and total nitrogen indicate some improvements in water quality in the same 37-year period.

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 1971 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)32810.47.315.710.51.2
Temperature (°C)46311.01.620.810.93.8
Turbidity (NTU)4304.500.1043.003.004.75
Conductivity (mSIEMS/cm)301219.967.2374.0233.542.0
Total Suspended Solids (mg/L)4659.000.60132.004.8014.78
Ortho-Phosphorus (mg/L)4650.03520.00530.12000.03270.0148
Total Phosphorus (mg/L)4650.07320.00740.40600.06670.0391
Ammonia (mg/L)4650.03370.00100.19400.02900.0206
Nitrate (mg/L)4631.04850.31102.03001.06000.2789
Total Nitrogen (mg/L)3861.31100.74302.40001.30000.2603
Fecal Coliform(CFU/100ML)42984914120004201284

Table 2. Storm water monitoring summary statistics for this station from 1971 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4510.27.812.810.61.4
Temperature (°C)7110.84.217.410.43.2
Turbidity (NTU)6419.433.40117.0013.9017.80
Conductivity (mSIEMS/cm)30125.552.7205.0131.042.6
Total Suspended Solids (mg/L)7257.725.30294.0030.1563.51
Ortho-Phosphorus (mg/L)720.03170.00650.08500.02680.0161
Total Phosphorus (mg/L)720.15850.05210.56700.12850.1026
Ammonia (mg/L)720.07460.01200.24200.05820.0510
Nitrate (mg/L)720.65350.32301.36000.60750.2289
Total Nitrogen (mg/L)701.33360.81902.91001.21000.4158
Fecal Coliform(CFU/100ML)7147931504100032005849


King County operates one stream gage on Thornton Creek: Thornton Creek near mouth (58A). Before September 30, 2013, the U.S. Geological Survey operated this gage. Seattle Public Utilities also operates several stream gages.

Stream Sediment

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

Sampling of the legacy site, 0434, later moved to site A434, continued through 2010. Basin-wide sampling occurred in 2004 (see map). Results indicate that sediments in the Thornton Creek basin exceeded the Sediment Cleanup Objective (SCO) for nickel at three sites (0434, A434 and WW434), but acid volatile sulfide/simultaneously extracted metals (AVS/SEM) ratios suggest that metals may be available only at site WW434. Concentrations of bis(2-ethylhexyl)phthalate (a chemical found in some plastics) exceeded the SCO at A434 and VV434. Concentrations of 4,4`-DDE (a degradation by-product of the pesticide DDT) and dieldrin (an organochlorine pesticide) were above the SCO at site A434. The presence of DDE and dieldrin in the sediments at 0456 is troubling because production of these pesticides ceased in the 1970s. These types of pesticides are persistent in the environment and can also bioaccumulate in the food chain and show up at elevated levels in fish that live in both in this creek and Lake Washington.

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).

King County has not conducted regular benthic macroinvertebrate sampling on Thornton Creek. However, the City of Seattle has extensively monitored benthos populations in Thornton Creek. To see this data for Thornton 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.

Investigation of Bacteria Sources in the Thornton Creek Watershed Seattle, Washington

In June 2013, SPU, with assistance from the City of Shoreline and King County Water and Land Resource Division, conducted an intensive bacterial study on Thornton Creek. This study identifies localized areas that appear to have higher amounts of bacteria entering the stream. SPU stormwater investigators are continuing to locate sources of bacteria entering the creek, and will be using the study results to direct future source tracing work.

City of Seattle State of the Waters 2007

The City of Seattle has undergone extensive development in the last 150 years. Using decades worth of existing research, the City of Seattle assessed the conditions of Seattle's waterbodies (watercourses, small lakes, large lakes, estuaries, and marine systems). Of interest was stream hydrology, water quality, physical habitat, and biological communities because these characteristics indicate how well watersheds can perform environmental services. The five major watercourses focused on in the report were: Fauntleroy Creek, Longfellow Creek, Pipers Creek, Taylor Creek, and Thornton Creek.

Thornton Creek Watershed Characterization Report

The Thornton Creek Watershed Management Committee under the direction of Seattle Public Utilities prepared a Thornton Creek Watershed Characterization Report in November 2000.

Thornton Creek Alliance

The Thornton Creek Alliance is a grassroots, nonprofit organization dedicated to preserving and restoring an ecological balance in the creek.

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.