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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. Water quality samples have been collected monthly at the mouth of the creek since station since 1971. Station 0434 is located one block south of Matthews Beach. Beginning in 1987, sediment quality samples have been collected from Thornton Creek. In 2002 King County began a benthic invertebrate monitoring program that included samples collected from Thornton Creek.

From time to time additional studies have been conducted at various streams in King County. Click here for information about Special Studies of Thornton Creek.

Watershed

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

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.

Fisheries

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 was considered a “Class AA” water body under the 1997 rules and categorized as “Core Salmon Migration and Rearing 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 2004 Washington Department of Ecology’s (Ecology) 303(d) list for violation of fecal coliform, dissolved oxygen and temperature standards. 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. During the last seven water years station 0434 at the mouth of Thornton Creek were rated “high” concern. High concern ratings were due primarily to high fecal coliform bacteria levels, low dissolved oxygen, high temperatures, high TSS, and high nutrients. 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 2013

Table 2. Storm water monitoring summary statistics for this station from 1971 to 2013

Hydrology

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 Streams Sediment Monitoring Program starting in 1987. Data were compiled and analyzed for the years 1987 through 2002. Data were analyzed for trends, correlations, and were compared to sediment quality guidelines. No significant trends were identified during data analysis for any of the parameters tested. Results indicate that sediments in the South Branch of Thornton Creek exceeded sediment quality guidelines for three metals (lead, nickel, and zinc). Of the 27 streams monitored in King County, Thornton Creek had the third highest metals concentrations.

Benthic Invertebrates

The county monitors stream health by collecting samples of benthic invertebrates from selected streams as part of King County’s Benthic Invertebrate Program. Benthic invertebrates are an important link in the food chain for fish in the creek and are an excellent indicator of stream health.

In 2002 and 2003, benthic invertebrate samples were collected, analyzed, and benthic invertebrate index scores (BIBI) were calculated for the South Branch of Thornton Creek. BIBI scores for both years indicated that conditions in Thornton Creek for benthic invertebrates were very poor. Over 80 percent (82% in 2002 and 98% in 2003) of the invertebrate community was tolerant of degraded conditions. More individuals of sensitive species (15) were present in 2002 than in 2003 (0) and only one individual of a long-lived species was collected during the two years of study. Longer lived species tend to take longer to reproduce and they, along with sensitive species, are among the first to disappear when a stream ecosystem is altered by human activity such as urbanization.

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: http://www.scn.org/tca/

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.