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King County Water Quality Monitoring

King County monitors the ecological health of the Evans Creek basin in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. Two sites water quality monitoring stations are located on Evans Creek; station B484 is located just above the confluence of Evans Creek with Bear Creek at the bridge on Union Hill Rd (100 yards west of 188th Ave NE), and station S484 is located upstream at 50th Street. Monitoring at station B484 began in 1971 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Sampling resumed at this station in February 2013. Water quality monitoring at station S484 ran from 1981 - 2008. Sediment samples have been collected from Evans Creek as part of the Streams Sediment Monitoring Program starting in 1987. Benthic invertebrates have been sampled at various locations in the Evans Creek basin beginning in 1995.


Water Shed Image The Bear-Evans Creek basin is comprised of approximately 32,100 acres that includes three sub-basins: Bear Creek at 14,300 acres, Cottage Lake Creek at 8,000 acres, and Evans Creek at 9,800 acres. All together there are over 100+ miles of streams including Bear Creek at approximately 12.4 miles, Cottage Lake Creek at approximately 6.7 miles, and Evans Creek running approximately 8.2 miles (Metro 1990; King County 1989). There are nine lakes and over 2000 acres of wetlands. Local jurisdictions within the basin include: unincorporated King County, unincorporated Snohomish County, City of Redmond, City of Sammamish , and the City of Woodinville.

The headwaters originate at elevations of 180 feet above sea level (Bear Creek) and 100 feet above sea level (Evans Creek). The confluence of the two creeks is at 50 feet above sea level. Bear Creek empties into the Sammamish River on the north side of State Route 520 in the City of Redmond. Widespread permeable gravel and sand fill the valleys of Evans and Bear creeks, allowing them to absorb much of the water from local storm events and the inflowing tributaries (King County 1990; Kerwin 2001).

Land use in the watershed has changed markedly in the past 150 years as development in the area has increased. What was once primarily forest has become a mix of forest, grass, and impervious surfaces. In 1985 the Bear-Evans basin consisted of 71 percent forest, 17 percent grass, 9 percent wetland, and 3 percent effective impervious surfaces (King County 1989).

A unique resource in the Bear-Evans basin is Cold Creek, a cold-water spring. This spring in 5 to 7 degrees centigrade colder than the rest of Bear Creek and is partially responsible for the cooler water temperatures of the Sammamish River downstream of it's confluence during summer and early fall (Kerwin 2001).


Since 1996 volunteers with the Salmon Watcher Program have been making observations at various locations in the watershed. There have not been enough fish observed in Evans Creek to be considered common, but chinook and sockeye have been observed on occasion.

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. Evans 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 Cottage Lake creek have been assigned an additional "Supplemental Spawning and Incubation Protection" temperature criteria of 13 ºC to be applied from September 15th to May 15th. Bear Creek, Cottage Lake Creek and Evans Creek are all on the Washington Department of Ecology's (Ecology) 303(d) list, Category 4a, for having an approved TMDL plan for temperature, dissolved oxygen, and fecal coliform bacteria. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

Long-term Trends

A 25-year (1979 - 2004) trend analysis was conducted with water quality data collected from Evans creek. As with most streams in WRIA 8, there has been a significant increase in water temperatures over this 25-year period. Conductivity increased significantly at both sites and dissolved oxygen decreased at both sites. There have been some improvements in water quality as evidenced by the decrease in total suspended solids (TSS), phosphorus concentrations (ortho and total), and fecal coliform bacteria. Ammonia and total-nitrogen concentrations also decreased. Water in Evans 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 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 1981 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)1838.22.512.58.51.8
Temperature (°C)3039.81.017.510.03.4
Turbidity (NTU)3062.060.1018.901.701.79
Conductivity (mSIEMS/cm)150123.273.0177.0118.530.3
Total Suspended Solids (mg/L)3064.190.0057.302.307.49
Ortho-Phosphorus (mg/L)3070.02700.00290.09000.02420.0129
Total Phosphorus (mg/L)3070.05240.01970.25000.04900.0243
Ammonia (mg/L)3070.01770.00430.08100.01500.0108
Nitrate (mg/L)3070.53970.08241.21000.53250.1892
Total Nitrogen (mg/L)2380.81490.28101.94000.80350.2029
Fecal Coliform(CFU/100ML)3061471600033438


King County maintains three streamflow gauges in the Evans basin: Evans Creek at Union Hill Road (18a), Northridge Evans Creek #4 (Redmond Block South) (18b) Northridge Evans Creek #4 (Redmond Block Interior) (18c).

Stream Sediment

Six samples were collected along a six mile reach of Evans Creek in 2006. The concentration of chromium at BB484 was above levels likely to cause adverse effects in sediment-dwelling animals; chromium exceeded the Cleanup Screening Level (CSL) at this site. Concentrations of arsenic and nickel exceeded the Sediment Cleanup Objective (SCO) at this site, meaning adverse effects in benthic animals are uncertain for these metals. Additional data (AVS/SEM ratio) at BB484 indicates metals at this site are likely to be bioavailable. Nickel concentration at site SS484 also exceeded the SCO, but AVS/SEM analysis suggests that metals are not bioavailable at this site.

Station BB484 is located near 196th street in Evans Creek Park. Particle size information indicates that sediments primarily consist of sands and gravels with no fine particles. This indicates that the creek is dynamic and that sediments move quickly through this area and also suggests that contaminants here are likely from an on-going upstream source. However, it is not clear what that ongoing source would be. The immediate area is within the park and is undeveloped.

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 Evans Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Numerous studies have been conducted in conjunction with basin planning and salmon conservation efforts. King County produced the Bear Creek Basin Current and Future Conditions Analysis Report in 1989 and the Bear Creek Basin Plan in July 1990.For more information about work in the Bear-Evans Creek Basin please visit the Sammamish Watershed page.