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King County Water Quality Monitoring

King County monitors the ecological health of the Rock Creek basin in a variety of ways, including collecting and analyzing water quality samples. Since 2014, water quality samples have been collected monthly at one station, LSIN1, in Rock Creek. Station LSIN1 is located near the mouth of the creek where it cross SE 312th Street.

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


A major tributary to Lake Sawyer, the fourth largest natural lake in King County, the Rock Creek basin has flows from Black Diamond Lake, Jones Lake, and Ginder Lake. The basin comprises 54% of Lake Sawyer, which is approximately 13 square miles, and contains 24 wetlands (King County, 2000). It enters at the southeastern portion of the Lake Sawyer in the Lake Sawyer Regional Park with a drainage almost entirely in the City of Black Diamond, Washington.

Land use has had a significant impact on the Rock Creek basin. Historically, the basin’s aquatic habitat has been degraded by human activity, animal intrusion, and point and nonpoint pollution. Currently, about half of land use is forested and one-fifth is developed. Forested areas are mostly mixed forest and developed land is mostly low intensity development and open space. Agriculture, which is entirely pasture/hay, comprises a relatively small portion of land use. Scrub, wetlands, and other land uses (barren land, grassland, and open water) combined make up a greater portion of land use than developed land. See Table 1 below for more details on land use.

Table 1. Total land use in the basin


The Rock Creek basin is known to have the following salmonids: Chinook, sockeye, and coho ("Stream List," 2016). Late winter run Coho salmon use Lake Sawyer to reach spawning grounds in Rock Creek.

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. Rock Creek is now categorized as "Core Summer Salmonid Habitat" for aquatic life use and "Primary Contact" for recreational use. As part of the updated water quality standards, portions of Rock Creek have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. Rock Creek is not currently listed on the Washington State Department of Ecology's (Ecology) METRO/King County DNRP.

To view charts of current water quality data, please visit the Data Download webpage.

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 WQI ratings for other stream sites, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 2014 to 2020
Agriculture Developed Forest Scrub Wetlands Other
Total 2% 19% 50% 8% 7% 14%
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)667.
Temperature (°C)6610.80.921.09.35.4
Turbidity (NTU)652.820.9235.801.744.62
Conductivity (mSIEMS/cm)66278.079.8586.0258.0104.9
Total Suspended Solids (mg/L)652.690.6018.101.353.57
Ortho-Phosphorus (mg/L)650.01600.00390.04550.01190.0114
Total Phosphorus (mg/L)650.05300.01660.18800.03810.0380
Ammonia (mg/L)650.02110.00260.17900.01640.0228
Nitrate (mg/L)650.23460.01001.07000.16700.2208
Total Nitrogen (mg/L)650.62730.36001.43000.58900.2274
Fecal Coliform(CFU/100ML)65635140029173

Table 2. Storm water monitoring summary statistics for this station from 2014 to 2020
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)
Temperature (°C)29.48.310.59.41.6
Turbidity (NTU)0     
Conductivity (mSIEMS/cm)2245.0203.0287.0245.059.4
Total Suspended Solids (mg/L)37.401.1013.707.408.91
Ortho-Phosphorus (mg/L)30.01500.01380.01720.01400.0019
Total Phosphorus (mg/L)30.09850.03780.20700.05070.0942
Ammonia (mg/L)30.02170.00760.03840.01910.0156
Nitrate (mg/L)30.01600.01600.01600.0160 
Total Nitrogen (mg/L)30.84170.36801.70000.45700.7447
Fecal Coliform(CFU/100ML)328576540


King County maintains one streamflow gage in Rock Creek: Lake Sawyer in the Soos Creek Watershed (09b).

Stream Sediment

Stream sediment data is not currently collected for Rock Creek.

Special Studies

City of Kent Rock Creek Watershed Habitat Conservation Plan

The City of Kent, Washington owns and operates its own water supply system. To ensure it's obligation to supply water and enhance fish habitat, the City of Kent submitted applications for Endangered Species Act incidental take permits. The applications included a final environmental impact statement, habitat conservation plan, and implementing agreement.

Lake Sawyer Management Plan

Since the mid-1980s, Lake Sawyer has experienced water quality degradation due to watershed loading. Modeling of future water quality indicates development within the watershed leading to further degradation. King County Surface Water Management, with assistance from ENTRANCO and funding by the Washington State Department of Ecology, published a study that details a water quality.