Mill Creek Site A315

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King County Water Quality Monitoring

King County monitors the ecological health of Mill Creek in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Station A315 is located at the bridge at 68th Ave and South 261st Street. Water quality samples have been collected monthly from this station near the mouth of Mill Creek since 1979. Beginning in 1987, sediment quality samples have also been collected from this site. In addition, King County monitors stream health by collecting samples of benthic invertebrates from selected streams. In 2002 and 2003 benthic invertebrates were sampled in Mill Creek from a variety of locations.

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

Mill Creek Watershed

The Mill Creek watershed (sometimes referred to as Hill Creek in literature) is located in South King County in Water Resource Inventory 9 (WRIA 9). The watershed is roughly 22 square miles in size and includes portions of the cities of Kent, Auburn, Algona, and Federal Way (Kerwin and Nelson 2000; Herrera 2005). The creek originates from Lake Doloff and Lake Geneva in the west Green River Valley and flows through the steep Preasley Canyon before entering the Green River at RM 23.9. The creek is approximately 8.35 miles long. (Note: There is another Mill Creek located within WRIA9 that drains into Springbrook Creek further downstream on the Green River).

Historically, Mill Creek has served as an important flood storage area as water is conveyed from nearby wetlands to the Green River (Kerwin and Nelson 2000). The creek has provided important refuge for fish during periods of high winter flows. Several studies related to flood control have been produced in the last 40 years (Kerwin and Nelson 2000). Land use in the Mill Creek watershed consists of forested and residential land in the upper basin, and residential and agricultural in the lower basin (Herrera 2005).

For more information about the Mill Creek watershed, visit the WRIA 9 Web site for the Lower Green River Subwatershed.

Fisheries

Coho, chum and winter steelhead have been observed spawning in Mill Creek, and juvenile coho, chum, winter steelhead, cutthroat and chinook have been captured in the creek (Kerwin and Nelson, 2000). The Lower Green River Baseline Habitat Survey Report (Anchor Environmental 2004) provides detailed information about fisheries habitat conditions in the Mill Creek area.

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. Mill (Hill) Creek was considered a “Class A” water body under the 1997 rules. As the 2003 become effective Mill Creek is categorized as “Salmonid Spawning Rearing and Migration” for aquatic life use, and “Primary Contact” for recreational use. The creek is on Ecology’s 2004 303(d) list for violation of temperature, dissolved oxygen, and fecal coliform bacteria standards. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

Water quality conditions in Mill Creek have historically been characterized as poor due to high temperatures, low dissolved oxygen, high nutrient concentrations, and high fecal coliform bacteria counts (Metro 1989).

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with water quality data collected from the mouth of Mill Creek. This analysis shows some improvements in water quality since 1979 as turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, and fecal coliform bacteria have all shown a significant decrease and dissolved oxygen concentrations have increased significantly in this 25 year time period. In contrast, pH values have shown a significant decrease but have remained within acceptable range relative to the state standards. While these trends indicate improvements in water quality since 1979, another water quality data assessment conducted for the Green-Duwamish River in 2003 found that Mill (Hill) Creek still had low dissolved oxygen concentrations and high ortho-phosphorus and fecal coliform bacteria counts relative to the rest of the Green-Duwamish watershed (Herrera 2005).

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 five water years Mill Creek stations have been rated “high” concern. High concern ratings were due primarily to high bacteria and nutrient values, and low dissolved oxygen concentrations. To see how these ratings compare with other stream sites, visit the Water Quality Index page.

A review of the 2008 data shows the following violations of State water quality standards:

 

Stream Sediment

Sediment data were collected from Mill Creek as part of the Streams 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 Mill Creek sediments exceeded two sediment quality guidelines (arsenic and lead). Of the 27 streams monitored in King County, Mill Creek had the 7th highest metals concentration.

Benthic Invertebrates

The County also 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 both 2002 and 2003, benthic invertebrate samples were collected, analyzed, and benthic invertebrate index scores (BIBI) were calculated for Mill Creek.

Results from both years indicate that conditions for benthic invertebrates in Mill Creek were very poor. The samples were dominated by species that are tolerant to degraded conditions. Very few individuals that are long lived or are sensitive to degraded conditions were present. Longer-lived species tend to take longer to reproduce and along with sensitive species are among the first to disappear when a stream ecosystem is altered by human activity such as urbanization.

Special Studies

Green-Duwamish Water Quality Assessment

In 2001 King County initiated a separate comprehensive study of the Green-Duwamish Watershed, called the Green-Duwamish Watershed Water Quality Assessment (GDWQA). Several reports evaluate data collected as part of the GDWQA. Mill Creek is included in this analysis. More information about the GDWQA, as well as the reports, can be found at the Green -Duwamish Water Quality Assessment Web site.