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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 time additional studies have been conducted at various streams in King County. (Click here for more information about Special Studies involving Thornton Creek .)

Thornton Creek Bacteria Study

Seattle Public Works conducted an intensive bacterial study that showed high levels of fecal coliform bacteria. As a consequence, the public was warned to stay out of Thornton Creek.


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 minded from the area. The North Branch drains approximately 4,446 acres of Shoreline and Seattle. The South Branch originates west of I-5 near the North Seattle Community College and drains approximately 2,333 acres 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.

Historical Context

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). 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. After the forests were removed, farms, orchards, and diaries dominated the area until mid-1900s.

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 watershed is now heavily developed with an estimated fifty percent of the surface area impervious. Currently, land use in is predominantly residential (fifty-three percent), twenty three percent roads, nine percent commercial and industrial, and four percent parks and golf courses, four percent schools, and four percent vacant.


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 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).

Since 1997 volunteers with the Salmon Watcher Program have been recording salmon observations at various locations in Thornton Creek. Volunteers have consistently seen coho in the creek. Less commonly spotted are 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 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. 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, 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 under the 2003 rules. 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. Thornton Creek is on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list list for violation of fecal coliform, dissolved oxygen and temperature standards (Category 5). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

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 dissolved oxygen. High conductivity can suggest 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 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)25210.57.315.710.51.3
Temperature (°C)38710.81.618.810.83.7
Turbidity (NTU)3554.610.1043.003.004.99
Conductivity (mSIEMS/cm)225217.267.2374.0232.043.7
Total Suspended Solids (mg/L)3909.480.60132.004.8515.80
Ortho-Phosphorus (mg/L)3900.03510.00530.12000.03240.0152
Total Phosphorus (mg/L)3900.07340.00740.40600.06570.0418
Ammonia (mg/L)3900.03400.00100.19400.02900.0212
Nitrate (mg/L)3881.10230.31102.03001.12000.2653
Total Nitrogen (mg/L)3111.37470.76502.40001.37000.2361
Fecal Coliform(CFU/100ML)38987114120004301294

Table 2. Storm water monitoring summary statistics for this station from 1971 to 2018
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 maintains a streamflow gauge on Thornton Creek: Thornton Creek Tributary 0030 near Mouth (58b).

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 for the years 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, with basin-wide sampling occurring 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 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 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.

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 Benthos webpage.

Special Studies

USGS Studies

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

In joint studies, USGS and Ecology evaluated pesticides in water, sediment, and fish tissue in Thornton Creek as part of a nationwide pesticide assessment and determined that instream pesticides may be a problem for fish in urban streams (Bortleson, et. al, 1997; Kerwin, 2001).

In August 1998, the USGS sampled several streams, including Thornton Creek, and rivers in the Puget Sound Basin as part of the National Water Quality Assessment Program (NAWQA) looking at multiple indicators (bacterial, viral, chemical) of sewage pollution. The USGS documented high densities of E. coli and the presence of coliphages in water samples from Thornton Creek (Embrey, 2001). Impacts from sewage pollution were clearly indicated on the basis of chemical indicators.

Thornton Creek Watershed Characterization Report

The Thornton Creek Watershed Management Committee under the direction of the 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. For more information check out their web page at:

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.

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

June 2013. Seattle Public Utilities (SPU), with assistance from the City of Shoreline and King County WLRD, 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.