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King County Water Quality Monitoring

King County monitors the ecological health of Idylwood Creek in a variety of ways, including collecting and analyzing water, sediment, and benthic invertebrate samples. Water quality samples have been collected monthly at Station A620 located at the footbridge in Idylwood Park. Sampling at this site began in 1995 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Sampling at this site resumed in February 2013. Sediment samples have been collected from Idylwood Creek as part of the Streams Sediment Monitoring Program starting in 1987. King County began collecting benthic macroinvertebrate samples in Idylwood Creek in 2002.

From time to time, additional studies have been conducted on Idylwood Creek. Click here for information about Special Studies.


Water Shed Image The Idylwood Creek watershed is located at the northwest end of Lake Sammamish in the City of Redmond, Washington. Headwaters of the creek originate in the City of Bellevue, Washington. The creek drains into Lake Sammamish roughly 50 feet north of the swimming beach area in Idylwood Park.

Land use in the basin is almost entirely developed with not much forest and no agriculture. There is also no scrub or wetlands and less than 1% of other land use (open water). Development is mostly low to medium intensity and evergreen forest is the most common type of forest present. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Other
Total 93% <7% <1%


From 2000 to 2015, volunteers with the Salmon Watcher Program made observations at various locations within the Idylwood Creek basin. Sockeye salmon were observed in Idylwood Creek, though there presence was very rare ("Stream List," 2016). No fish were observed in Idylwood Creek during the 2004 observation period (King County WLRD, April 2005).

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. Idylwood 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, portions of Idylwood Creek have been assigned an additional "Supplemental Spawning and Incubation Protection" temperature criteria of 16 ºC. The creek is on the Washington State Department of Ecology's (Ecology) 303(d) list for violation of DO, 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.

To view charts of current water quality data, please visit the Data Download webpage.

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 Idylwood Creek ratings compare with other stream sites, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 1995 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)28110.77.912.910.81.1
Temperature (°C)40111.71.720.112.33.6
Turbidity (NTU)3015.070.50460.001.4030.80
Conductivity (mSIEMS/cm)259220.745.0340.0234.058.0
Total Suspended Solids (mg/L)30112.340.501180.001.5088.47
Ortho-Phosphorus (mg/L)3010.02080.00750.09900.01960.0105
Total Phosphorus (mg/L)3010.04200.01490.63100.03330.0491
Ammonia (mg/L)3010.02280.00380.38800.01800.0274
Nitrate (mg/L)3010.52520.09761.32000.48700.1918
Total Nitrogen (mg/L)3010.72310.32602.89000.63100.3261
Fecal Coliform(CFU/100ML)3664782180001901447

Table 2. Storm water monitoring summary statistics for this station from 1995 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4810.48.712.310.31.1
Temperature (°C)5711.45.916.711.12.9
Turbidity (NTU)5825.041.50434.007.3175.75
Conductivity (mSIEMS/cm)44118.347.0174.0124.034.5
Total Suspended Solids (mg/L)5847.950.70769.0010.05136.65
Ortho-Phosphorus (mg/L)580.03350.00650.16600.02800.0239
Total Phosphorus (mg/L)580.10040.03230.73500.05560.1221
Ammonia (mg/L)580.02560.01100.08320.02050.0139
Nitrate (mg/L)580.57340.21901.80000.47900.3003
Total Nitrogen (mg/L)580.95000.59302.26000.83150.3518
Fecal Coliform(CFU/100ML)58162180140009052264


King County is not currently operating any stream, rain, or temperature gages in Idylwood Creek.

Stream Sediment

Sediment data were collected from Idylwood Creek as part of the Stream 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 Idylwood Creek sediments exceeded on sediment quality guideline (nickel). Of the 27 streams monitored in King County, Idylwood Creek had the 7th lowest metals concentrations.

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 Idylwood Creek, please visit the Puget Sound Stream Benthos webpage.

Special Studies

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 for future generations.