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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 1972 water quality samples have been collected monthly near the mouth of May Creek at station 0440 located at the gaging 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. King County began collecting benthic macroinvertebrate samples in May Creek in 2002.

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.


Water Shed Image 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.


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 now categorized as “Core Summer Salmonid Habitat” for aquatic life use, and “Extraordinary 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 2012 Washington Department of Ecology’s (Ecology) 303(d) list for violation of fecal coliform bacteria (FC) standards (Category 5). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

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. 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 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)27911.19.215.911.01.2
Temperature (°C)40810.1-0.417.910.23.7
Turbidity (NTU)3734.400.30302.002.0016.37
Conductivity (mSIEMS/cm)237159.710.0214.0158.033.8
Total Suspended Solids (mg/L)4098.570.50564.003.0031.45
Ortho-Phosphorus (mg/L)4070.02300.00500.18000.02100.0142
Total Phosphorus (mg/L)4090.04530.01620.33700.04170.0254
Ammonia (mg/L)4080.02000.00320.12500.01500.0174
Nitrate (mg/L)4081.13990.32402.56001.13000.3252
Total Nitrogen (mg/L)3391.35920.31802.79001.32000.3135
Fecal Coliform(CFU/100ML)4071772760066479

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4310.
Temperature (°C)6810.
Turbidity (NTU)6033.382.50733.0012.00100.90
Conductivity (mSIEMS/cm)38113.152.0174.0107.027.4
Total Suspended Solids (mg/L)6877.144.801710.0028.50217.98
Ortho-Phosphorus (mg/L)680.02350.00940.12200.02190.0163
Total Phosphorus (mg/L)680.12650.03201.35000.09460.1647
Ammonia (mg/L)680.03750.00930.18400.03000.0293
Nitrate (mg/L)680.92540.41102.22000.78150.4138
Total Nitrogen (mg/L)671.49890.35004.55001.32000.6269
Fecal Coliform(CFU/100ML)6813866666004851637


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 Sediment Monitoring Program from 1987 to 2002. Data analysis identified no significant trends for any of the parameters tested.

Sampling of the legacy site, 0440, continued through 2010, with basin wide sampling occurring in 2008. Results indicate that sediments in the May Creek basin exceeded the Sediment Cleanup Objective (SCO) for nickel at sites 0440, C440 and E440. Arsenic concentrations exceeded the SCO at site D440. SCO concentrations for 4-methylphenol and phenol were exceeded in sediment at 0440. No chemicals exceeded SCO concentrations at sites A440, B440 and F440. AVS/SEM ratios suggest that metals are not bioavailable at E440, so adverse effects to benthic animals are unlikely at this site.

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).

For more information about benthic macroinvertebrates and King County’s Benthic Macroinvertebrate Program, please visit the King County Stream Bug Monitoring web page. To see the benthic macroinvertebrate data for May Creek, please visit the Puget Sound Benthos webpage.

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.