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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 on Cottage Creek. Click here for more information about Special Studies.


Cottage Lake Creek is one of three sub-basins of The Bear Creek basin, which is 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. 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).

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

Total 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). Today, developed and forest dominates land use in Cottage Creek. Development is mostly low intensity and open space. The most abundant type of forest is evergreen. Agriculture, scrub, wetlands, and other (barren land, grassland, open water) contribute a only a very small portion to land use. 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% 56% 36% <1% 5% >1%


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.

Volunteers with the Salmon Watcher Program made observations at various locations in the watershed from 1996 to 2015. In Cottage Lake Creek, volunteers consistently observed Chinook salmon and sockeye salmon, and occasionally kokanee salmon, coho salmon, and cutthroat trout.

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. 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. 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 34-year (1974 - 2007) trend analysis was conducted with water quality data collected from Cottage creek. As with most streams in Water Resource Inventory Area (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 DO concentrations decreased.

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 view WQI scores for Cottage Lake Creek visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 2018 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)
Temperature (°C)613.110.316.412.92.2
Turbidity (NTU)61.861.222.881.670.59
Conductivity (mSIEMS/cm)6160.5149.0173.0161.09.2
Total Suspended Solids (mg/L)62.431.734.242.150.92
Ortho-Phosphorus (mg/L)60.01950.01760.02230.01890.0019
Total Phosphorus (mg/L)60.03870.02140.05330.03700.0119
Ammonia (mg/L)60.01170.00930.01360.01220.0016
Nitrate (mg/L)60.54650.48300.66200.52450.0629
Total Nitrogen (mg/L)60.84150.74100.95900.83550.0971
Fecal Coliform(CFU/100ML)61171733077112


King County operates one stream gage on Cottage Creek: Cottage Creek at NE 159th Street (02L).

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 (SCO) 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 the metals are not likely bioavailable at either site. Consequently, these chemicals are 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% 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 webpage. To see the benthic macroinvertebrate data for Cottage Creek, please visit the Puget Sound Stream 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.

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.