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King County Water Quality Monitoring

King County monitors the ecological health of Lyon 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 Lyon Creek at station 0430. The station is located at the bridge inside the Lake Forest Park Civic Club property. Sampling at this site began in 1975 and continues today. Sediment samples have been collected from Lyon Creek as part of the Streams Sediment Monitoring Program starting in 1987. King County began collecting benthic macroinvertebrate samples in Lyon Creek in 2002.

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


Water Shed Image Lyon Creek originates from wetlands in south Snohomish County and flows 3.8 miles through Lake Forest Park before draining into the northwest corner of Lake Washington. The drainage basin is approximately 2,600 acres in size, one of the smallest of the Lake Washington tributary systems. The upper drainage basin is composed of a westerly fork within the City of Mountlake Terrace, Washington and an easterly fork in the City of Brier, Washington. The west fork of Lyon Creek traverses three wetlands within the city limits of Mountlake Terrace. Two of the three wetlands are on City park property. Lake Forest Park recently purchased the third wetland area that is now City park property that drains into a tributary of Lyon Creek just north of Ballinger Way at 195th Steet (Kerwin, 2002). The creek is placed in Ecology's Noncore Salmon/Trout Aquatic Use Category, which sets criterion for the protection of spawning, core rearing, and migration of salmon and trout, and other associated aquatic life.

Land use in the basin is predominantly developed as much of the land was developed in the late 1970s (Kerwin, 2002). A majority of the development is low and medium intensity. Some of the land in the area is forest and wetlands, however, there is no agriculture or any other types. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Wetlands
Total 86% 13% <1%

When development was occurring in the 1970s, no stormwater retention/detention facilities were constructed to mitigate the effects of greater basin wide total impervious area on the stream system. Peak flows increased from an average of 0.5 cubic feet per second (cfs) in the early 1970s to 30 cfs in the late 1970s. A 1981 Comprehensive Drainage Plan (Entranco, 1981) was prepared to address downstream flooding that resulted from the much-altered hydrology.


From 2000 to 2015, volunteers with the Salmon Watcher Program recorded salmon observations at river mile (RM) 0.1. Sockeye and coho salmon were observed in the creek though coho sightings were very rare. Historically, cutthroat and rainbow trout had also been observed. The suitability of Lyon Creek in terms of salmonid habitat has been impacted by high storm flows. High storm flows have resulted in degraded substrate and lack of spawning habitat.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids (TSS), 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. Lyon 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. The creek is on the Washington State Department of Ecology's (Ecology) 303(d) list for violation of DO, FC bacteria, water temperature, and bioassessment standards (Category 5).

p>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 data collected from Lyon Creek. The analysis showed some improvements in water quality since 1979 with significant decreases in turbidity, TSS, ortho-phosphorus, and FC bacteria. However, as with most streams in the County monitoring program, a significant increase in conductivity occurred during this same time period. Conductivity is a measure of water’s capacity to convey an electric current and is related to the total amount of dissolved charged substances, or ions, in the water. This can suggest the presence of unidentified material in the water. Water in Lyon Creek is becoming more acidic as indicated by the significant decrease in pH values. However, the pH remains within acceptable range relative to the State standards.

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

Table 1. Routine monitoring summary statistics for this station from 1971 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)33510.63.413.510.61.2
Temperature (°C)47110.40.818.110.33.7
Turbidity (NTU)4354.190.20125.002.028.33
Conductivity (mSIEMS/cm)305212.166.3288.0222.541.8
Total Suspended Solids (mg/L)4709.030.00362.003.1024.61
Ortho-Phosphorus (mg/L)4700.02620.00210.18700.02400.0134
Total Phosphorus (mg/L)4700.05350.00740.46600.04700.0378
Ammonia (mg/L)4700.02050.00100.32000.01410.0244
Nitrate (mg/L)4681.08020.32602.80001.05500.3058
Total Nitrogen (mg/L)3991.30590.71602.60001.28000.2972
Fecal Coliform(CFU/100ML)4375766132002501196

Table 2. Storm water monitoring summary statistics for this station from 1971 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)5810.68.713.110.71.1
Temperature (°C)7310.
Turbidity (NTU)6719.693.20116.0013.0021.45
Conductivity (mSIEMS/cm)41121.753.3192.0131.039.2
Total Suspended Solids (mg/L)7551.794.10292.0034.6058.84
Ortho-Phosphorus (mg/L)750.02760.00910.08180.02300.0160
Total Phosphorus (mg/L)750.13400.03550.61800.09320.1055
Ammonia (mg/L)750.03120.01000.08190.02800.0182
Nitrate (mg/L)750.78160.34301.75000.68400.3210
Total Nitrogen (mg/L)731.37320.79703.26001.20000.4683
Fecal Coliform(CFU/100ML)752539601600021002501


King County maintains a streamflow gage on Lyon Creek: near the mouth in Lake Forest Park (34a).

Stream Sediment

Sediment samples were collected from four stations, approximately a mile apart, along a four mile reach of Lyon Creek in the summer of 2005 (see map). The concentrations of nickel at two sites (I430 and KK430) were at levels that suggest adverse effects to aquatic animals living in the sediments are possible, but uncertain. They exceeded the Sediment Cleanup Objective (SCO) for nickel.

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

Special Studies

Small Streams Toxicity/Pesticide Study

As part of King County's Small Streams Toxicity/Pesticide Study, pesticide concentrations and toxicity were measured in Lyon Creek in 1999 and 2000. The County collected samples for analysis of pesticides, other organics, metals, suspended solids, and toxicity to two test species (Ceriodaphnia dubia and Selenastrum capricornutum). A total of 24 pesticides were detected in samples collected from Lyon Creek. Toxicity to S. capricornutum was observed in the Lyon creek samples during spring and early fall. In the spring sampling event, much of the toxicity appeared to be associated with copper. The cause of toxicity during the rest of the year was unclear.

Water Resources Inventory Area (WRIA) 8

In WRIA 8, citizens, scientists, businesses, environmentalists and governments are cooperating on habitat protection and salmon restoration projects and have developed a science-based plan to conserve salmon today and for future generations. Visit the WRIA 8 webpage to see how this creek is part of this WRIA 8 planning process.