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King County Water Quality Monitoring

King County has monitored the ecological health of the Cottage Lake Creek basin in a variety of ways including collecting and analyzing water and sediment samples. Water quality samples are collected monthly from station N484 in Cottage Lake Creek. Station N484 is located at the downstream side of the bridge on Avondale Road (near NE 151st Street). Water quality sampling at this site began in 1974 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. However, monthly water quality sampling resumed at this station in February 2013. Sediment samples have been collected from Cottage Creek as part of the Streams Sediment Monitoring Program starting in 1987. Benthic invertebrates have been sampled at various locations in the Cottage Creek basin beginning in 1995.

From time to time additional studies have been conducted in the Cottage Creek basin. Click here for information about Special Studies of Bear-Evans Creek.


Water Shed Image The Bear 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).


The Bear Creek Basin plan, completed in 1990, designated Regionally Significant Resource Areas (RSRAs) along Cottage Lake Creek (King County 1990). These areas exhibit high aquatic habitat and salmonid diversity and abundance and a demonstrated contribution to the regional fishery resource. Found in the basin are extensive freshwater mussel populations, freshwater sponges, river otters, crayfish and a good representation of aquatic insects. Because of its diversity, the Bear-Evans Creek basin was distinguished as one of the top six natural resource basins in King County in the Waterways 2000 program. King County and the City of Redmond have facilitated construction of numerous stream restoration projects identified in the Bear Creek Restoration Plan.

Since 1996 volunteers with the Salmon Watcher Program have been making observations at various locations in the watershed. In Cottage Lake Creek volunteers have consistently observed chinook salmon and sockeye salmon, and occasionally seen kokanee salmon, coho salmon, and cutthroat trout.

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. Cottage Lake 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 °C 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 statistics for the routine water quality data collected at this site to date.

Long-term Trends

A 34-year (1974 - 2007) trend analysis was conducted with water quality data collected from Cottage creek. As with most streams in WRIA 8, there has been a significant increase in water temperatures over this 34 year period. Conductivity, total nitrogen, nitrate-nitrogen, and total phosphorus also increased significantly while dissolved oxygen concentrations decreased.

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 view WQI scores for Cottage Lake Creek visit the Water Quality Index page. You can also access site specific WQI reports from the Streams List or the Water Quality Index links on the left.


Table 1. Routine monitoring summary statistics for this station from 1974 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)21810.67.414.510.51.0
Temperature (°C)34610.
Turbidity (NTU)3482.020.209.731.791.12
Conductivity (mSIEMS/cm)185141.355.0172.0143.021.8
Total Suspended Solids (mg/L)3484.160.0018.003.602.67
Ortho-Phosphorus (mg/L)3440.02690.00380.18000.02510.0143
Total Phosphorus (mg/L)3440.05280.02890.12500.04900.0158
Ammonia (mg/L)3470.03530.00290.17500.02200.0324
Nitrate (mg/L)3470.82220.22701.50000.82300.2169
Total Nitrogen (mg/L)2771.17680.39801.95001.18000.2739
Fecal Coliform(CFU/100ML)34820751520080852

Table 2. Storm water monitoring summary statistics for this station from 1974 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)
Temperature (°C)316.015.216.516.30.7
Turbidity (NTU)33.311.884.243.801.25
Conductivity (mSIEMS/cm)3152.7144.0160.0154.08.1
Total Suspended Solids (mg/L)38.134.9012.207.303.72
Ortho-Phosphorus (mg/L)0     
Total Phosphorus (mg/L)0     
Ammonia (mg/L)0     
Nitrate (mg/L)0     
Total Nitrogen (mg/L)0     
Fecal Coliform(CFU/100ML)0     


King County maintains two streamflow gauges in the Cottage Creek basin: Cottage Lake Creek at NE 132nd ST (02g), Cold Creek near Cottage Creek (02h). King County also maintains two water temperature gauges: Cottage Creek (02i), Cold Creek Below Spring (02k), and one precipitation gauge: Cottage Lake Rain Gauge (02w).

Stream Sediment

In 2006, four samples were collected along a four mile reach of Cottage Creek, below Cottage Lake and above its confluence with Bear Creek. Concentrations of dibenzofuran and total PAHs (combustion byproducts that can be found in vehicle exhaust) at station VV484 were at levels likely to cause adverse effects to sediment-dwelling animals; these chemicals exceeded the Cleanup Screening Level (CSL). Additionally, arsenic, lead and nickel exceeded the Sediment Cleanup Objective concentrations at this site, meaning that adverse effects to benthic animals are uncertain for these chemicals. Nickel concentrations at site MUSCOT01 also exceeded the SCO. However, additional data were collected (AVS/SEM ratio) which suggest that metals are not likely bioavailable at either site and therefore not likely to cause adverse effects despite, in some cases, high concentrations.

Station VV484 is located just downstream of Cottage Lake in a large wetland area. The sediments are fine grained and consist of approximately 23 percent mud and silt indicating that this is a depositional area where sediment associated contaminants can accumulate. It is unclear exactly what sources could have contributed to the elevated levels of these contaminants at this location, however, the station is located near 165th NE and stormwater runoff from the roadway may be contributing to the contaminant load.

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 Cottage Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Bear Creek Basin Planning

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.

Freshwater Mussel Assessment of Bear and Cottage Lake Creeks in 2001.

King County conducted stream habitat assessments in 2001 in Bear Creek and Cottage Lake Creek to characterize the habitat quality, primarily for salmonids. During this habitat assessment, notations were made of freshwater mussel locations, numbers, size, and mussel-bed substrate. This report is intended as a preliminary reconnaissance of the mussel presence in the Bear Creek Basin. Additional surveys were completed in 2004.