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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, which is 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, additional studies have been conducted on May Creek. Click here for information about Special Studies.


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 the City of Renton and City of Newcastle, Washington) 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.

In total, just over half of the basin is developed. The remaining land use is mostly forest land, specifically mixed forest. Agriculture, scrub, wetlands, and other (barren land, grassland, and open water) combined make up less than 5% of the basin. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Agriculture Developed Forest Scrub Wetlands Other
Total <1% 53% 43% <2% 1% >1%


May Creek supports five species of fish: Chinook, sockeye, coho and kokanee salmon, and steelhead and cutthroat trout (Kerwin, 2002; "Stream List," 2016). From 2000 to 2015, volunteers with the King County Salmon Watcher Program observed salmon in May Creek. Volunteers consistently saw sockeye salmon. Less commonly spotted were Chinook salmon, coho salmon, cutthroat trout, and kokanee salmon.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform (FC) 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 to May 15th. May Creek is on the Washington State Department of Ecology’s (Ecology) 303(d) list for violation of water temperature, FC bacteria, and bioassessment standards (Category 5).

See Table 2 below for routine monitoring summary statistics of water quality data collected to date. If stormwater data is available for this site, it will be shown as Table 3. Historical data reviews can be found in the annual reports produced by METRO/King County DNRP.

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 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, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 1972 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)35311.19.215.911.01.2
Temperature (°C)48210.2-0.417.910.23.8
Turbidity (NTU)4464.250.30302.002.0015.20
Conductivity (mSIEMS/cm)311160.510.0222.2158.034.1
Total Suspended Solids (mg/L)4828.420.50564.003.0029.92
Ortho-Phosphorus (mg/L)4800.02280.00500.18000.02080.0137
Total Phosphorus (mg/L)4820.04490.01620.33700.04180.0243
Ammonia (mg/L)4810.01830.00260.12500.01300.0167
Nitrate (mg/L)4811.07640.28502.56001.08000.3528
Total Nitrogen (mg/L)4121.29160.31802.79001.28000.3372
Fecal Coliform(CFU/100ML)4451701760065461

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2024
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 gages on May Creek:

  • Streamflow: mouth of May Creek (37a), Coal Creek Parkway(37b), May Creek Tributary 0285 (Honey Creek) (37c), 164th Ave Bridge (37E), and May Creek Tributary 0280 (37n)
  • Temperature: 148th Ave SE Bridge (37g)
  • Precipitation: lower May Creek (37u), upper May Creek (37v), and upper May Creek (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. Acid volatile sulfide/simultaneous extracted metals (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 webpage. To see the benthic macroinvertebrate data for May Creek, please visit the Puget Sound Stream Benthos webpage.

Special Studies

Final Adopted May Creek Basin Action Plan

King County and the Cities of Renton and Newcastle adopted the 2001 May Creek Basin Action Plan. The basin plan sets forward actions to reduce flooding in the May Creek basin; improve infrastructure for stormwater conveyance, streambank stability, and erosion; and protect and enhance habitat and water quality.

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.