May Creek Site 0440

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King County Water Quality Monitoring

King County monitors the ecological health of May Creek in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Since 1976 water quality samples have been collected monthly near the mouth of May Creek at station 0440 located at the gauging station near the bridge on Lake Washington Blvd and SE 80th Street. Sediment samples have been collected from May Creek as part of the Streams Sediment Monitoring Program starting in 1987. In 2002 and 2003 benthic macroinvertebrates were sampled from May Creek at three locations.

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

May Creek Watershed

The May Creek basin encompasses roughly 8,960 acres and includes 26 miles of mapped streams, two small lakes, and over 400 acres of wetlands. Headwater streams come off steep, forested ravines from the north, east, and south (Kerwin 2001). The basin can be divided roughly into two halves. The upper, eastern portion of the basin is characterized by less dense residential and agricultural development, and includes a significant portion of the undeveloped parkland on Cougar Mountain. Above May Canyon the creek lies in a formerly dredged, straightened channel at the center of a wide, very low gradient valley. The lower, western portion of the basin is inside the Urban Growth Boundary (primarily within the jurisdiction of Renton and Newcastle) and is fairly dense urban residential development. The mainstem of the May Creek is in parkland and runs through a steep, narrow, wooded canyon before flowing into east Lake Washington.

About fifty percent of the basin is still forested but residential development is rapidly increasing. The effective impervious area is expected to increase from 7-10 percent to more than twelve percent with future build out (Kerwin 2001). The May Creek Basin Action plan was adopted in 2001 by King County and the cities of Renton and Newcastle.

Fisheries

The basin supports five species of salmonids: chinook, sockeye, and coho salmon, and steelhead and cutthroat trout (Kerwin, 2002). Since 2000, volunteers with the Volunteer Salmon Watchers Program have been observing salmon in May Creek. Volunteers have consistently seen sockeye salmon in the creek. Less commonly spotted are chinook salmon, coho salmon, cutthroat trout, and kokanee salmon.

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. May Creek is considered a Class AA water body under the 1997 rules. As the 2003 rules become effective May Creek is 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 through May 15th. May Creek is on the 2004 Washington Department of Ecology’s (Ecology) 303(d) list for violation of fecal coliform bacteria (FC) and temperature standards.

In 1988 - 1989, water quality in May Creek was characterized as “fair” (Metro 1990) due to relatively high sediment metals concentrations, high nitrate and ammonia concentrations, high turbidity and total suspended solids. The combined agriculture-related disturbances in the upper reaches and erosion in the canyon area were considered the source of sediment build-up at the mouth of the stream.

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with baseflow water quality data from May Creek. Results indicated that water quality has declined over this 25-year period with significant increases in water temperatures, conductivity and ammonia-nitrogen. Water in May Creek is becoming more acidic as indicated by the significant decrease in pH. However, the pH remains within acceptable range relative to the state standards. Decreased total suspended solids (TSS), and decreased nutrient concentrations (ortho-phosphorus, nitrate and total nitrogen) 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. During the last six water years May Creek was rated “moderate” concern. To see how these ratings compare with other stream sites, visit the Water Quality Index page.

A review of the water year 2007 data shows the following violations of State water quality standards:

 

Hydrology

King County maintains nine stream gauges on May Creek. Five are streamflow gauges: May Creek at the mouth (37a), May Creek at Coal Creek PKWY (37b), May Creek Tributary 0285 (Honey Creek) (37c), May Creek at 164th Ave Bridge (37E), and May Creek Tributary 0280 (37n). One is a temperature gauge: May Creek at 148th Ave SE Bridge (37g), and three are precipitation gauges: Lower May Creek Rain Gauge (37u), Upper May Creek Rain Gauge (37v), and Upper May Creek Rain Gauge (37W).

Stream Sediment

Sediment data were collected from May 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 May Creek sediments exceeded one sediment quality guideline (nickel). Of the 27 streams monitored in King County, May Creek had the 9th 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 May Creek.

BIBI scores from both years indicate that conditions in May Creek were fair for benthic invertebrates. Almost 40 percent of the species identified in the creek were tolerant of degraded condition. A total of six individuals of species that are long lived were present, and one individual of a species that is considered intolerant of degraded conditions was 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

King County and the cities of Renton and Newcastle adopted the 2001 May Creek Basin Action Plan.

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.