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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 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). 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 and was resumed in 2014. 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.

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


Water Shed Image Evans Creek is one of three sub-basins of the Bear-Evans Creek basin, which is approximately 32,100 acres. The three sub-basins are: Bear Creek at 14,300 acres, Cottage Lake Creek at 8,000 acres, and Evans Creek at 9,800 acres. In total there are over 100+ miles of streams. This includes approximately 12.4 miles from Bear Creek, 6.7 miles from Cottage Lake Creek, and 8.2 miles from Evans Creek (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, which allow 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. In 1985, the Bear-Evans basin consisted of 71% forest, 17% grass, 9% wetland, and 3% effective impervious surfaces (King County, 1989). Currently, total land use in the Evans basin is more than half developed, followed by forest and some wetlands. There is a small amount scrub and agriculture in the area. However, other land uses (barren land, grassland, and open water) comprise more of the land use than agriculture. 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% 54% 38$ 2% 5% >1%

A unique resource in the Bear-Evans basin is Cold Creek, a cold-water spring. This spring in 5 to 7 °C 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).


From 1996 to 2015, volunteers with the Salmon Watcher Program made observations at various locations in the watershed. Chinook and coho salmon sighting were rare, but sockeye had been observed more commonly.

Water Quality

Water quality samples are analyzed monthly for water 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. 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 16 ºC to be applied from September 15th to May 15th. Evans Creek and Bear Creek are on the Washington State Department of Ecology's (Ecology) 303(d) list for violations of bioassessment standards (Category 5). Both creeks also have two types of EPA-approved total maximum daily load (TMDL) plans in place and implemented: a Bear Evans Watershed Temperature and DO TMDL and Bear Evans Watershed Bacteria TMDL (Category 4a). Cottage Lake Creek has a Cottage Lake Total Phosphorus TMDL (Category 4a).

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 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 DO 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 FC 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 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 1971 to 2020
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)2347.93.812.37.91.5
Temperature (°C)35910.51.019.710.64.5
Turbidity (NTU)3613.590.2034.503.102.75
Conductivity (mSIEMS/cm)204138.771.3200.0136.528.9
Total Suspended Solids (mg/L)3615.510.8055.104.304.97
Ortho-Phosphorus (mg/L)3610.02690.00640.32000.02420.0195
Total Phosphorus (mg/L)3610.05860.02200.37100.05500.0260
Ammonia (mg/L)3610.01890.00610.08800.01550.0106
Nitrate (mg/L)3610.31990.01700.97800.28300.1733
Total Nitrogen (mg/L)2890.67470.33001.86000.64200.1772
Fecal Coliform(CFU/100ML)3611020290050220


King County maintains one stream gage on Evans Creek: Evans Creek at Union Hill Road (18a).

Stream Sediment

Six sediment 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 webpage. To see the benthic macroinvertebrate data for Evans Creek, please visit the Puget Sound Stream Benthos webpage.

Special Studies

2015 Reconnaissance Report for Evans Creek Tributary 0107

To better understand the relationship between stormwater, hydrology, and natural conditions in Tributary 0107, data on general ecological conditions, channel shape and form, and measurements of existing hydro-modifications were collected. Data were compared to the National Oceanic and Atmospheric Administration's (NOAA) Properly Functioning Conditions (PFC) for salmon streams. Results found that the tributary does not meet minimum PFC standards. There is an extensive amount of sand and lack of functioning riparian habitat. Little riparian habitat means there was not much large woody debris or pools.

Bear Creek Watershed Management Study

The Bear Creek Watershed Management Study, which was formerly called the Watershed-Scale Stormwater Plan, outlined recommendations to restore Bear Creek to existing and designated uses. The basin provides critical habitat and resources for fish and wildlife, so the goal of the study was to link stormwater management and salmon recovery in order to provide recommendations. The study recommends a ten-year effort to improve water quality and streams flows: (1) update and build new stormwater infrastructure on public land; (2) provide incentives for installing rain gardens, cisterns, and permeable pavement on private land; (3) restore habitat along streams and wetlands; and (4) support success of the study.

Bear Creek Basin 1995 Basin Plan

Adopted by the King County Council in 1005, the Bear Creek Basin Plan assessed the conditions of the Bear Creek drainage basin. The basin includes Big Bear and Evans Creeks basins. The Bear Creek basin is most productive basin within the Puget Sound area for spawning salmon given its size. It has acres of wetlands and freshwater mussel populations, which indicate good water quality. The basin is, however, affected by flooding, erosion, and sedimentation. This plan outlines strategies to protect its streams, wetalnds, and fishery habitat.