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King County Water Quality Monitoring

King County monitors the ecological health of Juanita Creek in a variety of ways, including collecting and analyzing water, sediment, and benthic invertebrate samples. Water quality samples have been collected monthly from two sites along Juanita Creek. Station 0446 is located at the U.S. Geological Survey (USGS) gaging station north of Juanita Park and has been monitored since 1975. Station C446 is located at the bridge on NE 128th, east of 100th NE. Monitoring at this station began in 1976 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Sediment samples have been collected from Juanita Creek as part of the Streams Sediment Monitoring Program starting in 1987. King County began collecting benthic macroinvertebrate samples in Juanita Creek in 2010.

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


Water Shed Image The mainstem of Juanita Creek originates east of Interstate 405. It flows approximately five miles west and south where it enters Lake Washington on the west side of Juanita Beach Park. The Juanita Creek drainage basin is roughly 4,000 acres. There are three main tributaries flowing into Juanita Creek, an upper West (Simonds Tributary), a lower West, and a lower East (Totem Lake Tributary).

Land use in the area has changed rapidly over the last 20 years. In 1981, only 40% of the basin was characterized as being "urban/suburban" (PSCOG, 1981). Since then, much of the basin has been developed and is now described as "highly developed" (Greater Lake Washington Technical Committee, August 22, 2001). Development ranges from low to high intensity, low being the most common, and open space. A smaller portion of the basin is deciduous, evergreen, and mixed forest. An even smaller amount of land is scrub, wetlands, and other (open water). There is no agricultural land use. See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Scrub Wetlands Other
Total 88% 10% <1% 1% <1%


Historically, cutthroat trout have been found throughout the basin along with small populations of coho and Chinook salmon. Electrofishing conducted in 1998 found both coho salmon and cutthroat trout (Kerwin, 2001). Both species are believed to use the entire length of the mainstem of Juanita Creek from Lake Washington to Interstate 405. Volunteers from the Salmon Watcher Program consistently sighted kokanee, sockeye, and coho salmon. Cutthroat trout were also spotted on occasion.

In 1989, Juanita Creek was rated as "fair" for salmon habitat suitability condition (METRO, September 1990). Removal of riparian vegetation during development in the basin was cited as a primary factor leading to declining habitat conditions. In-stream habitat complexity is significantly degraded and measures of complexity factors (e.g. large woody debris, channel form, pool quality) show lower frequencies than values determined for "natural" conditions (Kerwin, 2001).

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. Juanita 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. Both sampling sites in Juanita Creek are listed on the Washington State Department of Ecology’s (Ecology) 303(d) list list for violation of water temperature, DO, and FC bacteria 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.

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with baseflow water quality data showed some significant changes in the water quality since 1979. Water quality has declined over this 25-year period with significant increases in water temperatures and conductivity at both sampling sites. Other indications of declining water quality are a decrease in DO at the mouth (0446) and a decrease in pH upstream (C446). Decreased total suspended solids (TSS), and decreased nutrient concentrations (ortho-phosphorus and total nitrogen) at both sites indicate some improvements in water quality in the same 25-year period. In addition, FC bacteria decreased at the mouth of the creek and nitrate-nitrogen decreased at the upstream station.

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

Table 1. Routine monitoring summary statistics for this station from 1975 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)33810.46.513.110.41.2
Temperature (°C)58511.
Turbidity (NTU)4465.800.10120.002.2710.88
Conductivity (mSIEMS/cm)310187.456.3256.0197.936.0
Total Suspended Solids (mg/L)48117.490.502486.702.80117.27
Ortho-Phosphorus (mg/L)4800.02930.00220.11800.02670.0152
Total Phosphorus (mg/L)4810.06450.01101.30900.05100.0705
Ammonia (mg/L)4810.02850.00600.34000.02100.0294
Nitrate (mg/L)4801.16370.27302.32001.15000.3779
Total Nitrogen (mg/L)3851.36030.66302.50001.35000.3151
Fecal Coliform(CFU/100ML)5625790110002701142

Table 2. Storm water monitoring summary statistics for this station from 1975 to 2024
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4210.68.413.910.61.3
Temperature (°C)5810.44.217.410.13.2
Turbidity (NTU)5127.423.30365.0014.0053.32
Conductivity (mSIEMS/cm)30118.956.3180.0120.034.6
Total Suspended Solids (mg/L)5970.864.50608.0036.80103.02
Ortho-Phosphorus (mg/L)590.02870.01270.12200.02270.0187
Total Phosphorus (mg/L)590.14280.03250.73500.09500.1275
Ammonia (mg/L)590.04290.01200.17100.03600.0311
Nitrate (mg/L)590.68160.31901.40000.65000.2336
Total Nitrogen (mg/L)571.22420.75202.39001.18000.3014
Fecal Coliform(CFU/100ML)59209695800017001925


King County maintains three streamflow gages on Juanita Creek: Juanita Creek at the mouth (27a), Juanita Tributary 0236 above Simonds R/D (27e), and Juanita Tributary 0241 above High Woodlands (27f). The County also maintains one water temperature gage: Juanita Creek Rain Gage (27u).

Stream Sediment

Sediment samples were collected from four stations approximately a mile apart, along a four mile reach of Juanita Creek in the summer of 2005 (see map). Nickel concentrations in samples at all four sites were above the Sediment Cleanup Objective (SCO). Acid volatile sulfide/simultaneously extracted metals (AVS/SEM) ratios suggest metals are bioavailable at all sites, except the most downstream site (0466).

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

Special Studies

Swimming Beach Monitoring

Juanita Creek is monitored weekly in the summer from mid-May through Labor Day weekend for fecal coliform bacteria pollution and algal toxins. The source of the bacteria can either be human or animal though human source bacteria can present a high risk for illness. Algal toxins are potentially lethal to humans and animals.

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.

Pyrethroid Survey of King County Stream Sediments 2014 Final Report

Pyrethroids are a group of synthetic insecticides commonly used in residential areas. Residential and commercial use has resulted in an increased detection in urban runoff, which can be harmful to aquatic life. In October 2014, a pyrethroid survey was conducted in sediments collected from six King County streams. These streams include: Juanita, Forbes, Thornton, Pipers, Springbrook, and Mill Creeks. Results found that bifenthrin was the only pyrethroid detected in the sediment samples; and most detected concentrations were below the reporting detection limit. Percent finds and TOC content, which varied substantially by creek, may be influencing bifenthrin detections. These results suggest that sediments may contain toxic levels of pyrethroids even if they are not detectable above detection limits in this study.

Habitat Inventory and Assessment of Juanita Creek in 2000

In August 2000, habitat on Juanita Creek was assessed using methods derived from standard assessment protocols. The goals of the assessment project for Juanita Creek were threefold: (1) characterize instream and riparian habitat quality - primarily for salmonids; (2) establish a baseline for future evaluation of trends in habitat quality and watershed function; and (3) provide information for process of prioritizing areas for restoration and preservation. The habitat assessment results indicate that channel and habitat structure in a number of the Juanita Creek segments are degraded relative to values from published "properly functioning conditions" for the Puget Sound or the Pacific Northwest region. This decreased quality of slow water rearing habitat may limit juvenile carrying capacity as well as hinder upstream migration by adult salmon.

Streams Toxicity/Pesticide Study

In 1999, King County conducted a pesticide study on Juanita, Lyon, and Lewis creeks. The County collected water samples for analysis of pesticides, other organic compounds, metals, suspended solids, and toxicity (Ceriodaphnia dubia - a zooplankton, and Selenastrum capricornutum- an algae). A total of 24 pesticides were detected over the course of the year in Juanita Creek. Toxicity to S. capricornutum was observed in the Juanita creek samples during spring, summer, and early fall. Toxicity observed during the spring sampling event appeared to be associated with copper. The cause of toxicity during the rest of the year was unclear; however, it was associated with the particulate fraction of the samples.