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King County Water Quality Monitoring

King County monitors the ecological health of Lewis Creek in a variety of ways, including collecting and analyzing water, sediment, and benthic invertebrate samples. Water quality samples have been collected monthly near the mouth of the creek at Station A617. The station is located at a bridge near the intersection of 187th Ave SE and 185th Pl. SE. Water quality sampling began in 1995 and continued until 2008 when King County was forced to reduce its monitoring program due to budget cutbacks. Sampling at this site resumed in February 2013. Sediment samples have been collected from Lewis Creek as part of the Streams Sediment Monitoring Program starting in 1987. King County began collecting benthic macroinvertebrate samples in Lewis Creek in 2010.

From time to time special studies have been conducted at various streams in King County. Click here for information about Special Studies of Lewis Creek.


Water Shed Image

The mainstem of Lewis Creek is a relatively short 1.5 miles with numerous branched tributaries (Kerwin, 2001). The mainstem originates in a low gradient area above the Eastgate region of Bellevue on the northeast side of Cougar Mountain. The creek then flows through a steep, wooded ravine before entering another low gradient reach downstream of Interstate 90 where it drains into the southern end of Lake Sammamish.

The basin encompasses 1,209 acres. The dominant land use in the basin is developed followed by forest. Development is mostly low to medium intensity and open space; while mixed forest is the most common type of forest. Scrub and other land use (grassland) only make up a very small percent of the land use. Agriculture and wetlands are not present.

Total land use in the basin

Developed Forest Scrub Other
Total 71% 27% <2% <1%


Numerous fisheries studies have been done on Lewis Creek (The Watershed Company, 1988, 1989; URS Corp., 1987; King County, 1995; Ostergaard, 1998; King County, 2000c). A culvert located at River Mile (RM) 0.75 at Interstate 90 is a barrier to fish migration. Below this barrier, in the lower reaches of the creek, kokanee, sockeye, coho and cutthroat trout were observed during a recent habitat and barrier assessment done by the Washington Department of Fish and Wildlife (WDFW, 2000). King County staff and volunteer salmon watchers have also observed adult coho, sockeye, kokanee, and chinook in the last seven years (King County, 2000c, 2001b, 2001c, June 2002). A fish habitat and population assessment conducted in 1992 by the Watershed Company noted the presence of cutthroat trout, steelhead trout, kokanee, and coho (Kerwin, 2001).

Beginning in 1997, volunteers with the Salmon Watcher Program have made observations in Lewis Creek. Kokanee salmon have been observed in the creek. Less commonly seen are coho, sockeye, and chum salmon.

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. Lewis 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 Lewis Creek have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. The creek is on the Washington State Department of Ecology’s (Ecology) 303(d) list for violation of water temperature, DO, and FC bacteria standards (Category 5). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

Studies conducted as part of the Lakemont Development process found exceedences of State water quality criteria for lead, zinc, and FC bacteria (Herrera, 1995).

To view charts of current water quality conditions in the creek, please visit the links at the top of the page.

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 1995 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)21411.18.414.811.11.3
Temperature (°C)24110.31.519.510.33.8
Turbidity (NTU)2423.840.41163.001.3512.08
Conductivity (mSIEMS/cm)192161.774.4363.0163.534.2
Total Suspended Solids (mg/L)2428.320.50482.001.6935.76
Ortho-Phosphorus (mg/L)2420.03790.00940.07930.03210.0170
Total Phosphorus (mg/L)2420.05830.01560.34400.05210.0359
Ammonia (mg/L)2420.01360.00230.04960.00980.0112
Nitrate (mg/L)2420.81500.28301.95000.71450.3436
Total Nitrogen (mg/L)2421.00050.40302.62000.91500.3794
Fecal Coliform(CFU/100ML)2391772360059364

Table 2. Storm water monitoring summary statistics for this station from 1995 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)3910.99.513.810.71.0
Temperature (°C)5010.55.716.210.22.7
Turbidity (NTU)50100.502.541990.0013.55341.66
Conductivity (mSIEMS/cm)34101.456.0132.099.517.9
Total Suspended Solids (mg/L)50283.503.405060.0026.70989.83
Ortho-Phosphorus (mg/L)500.03560.01530.23100.02660.0321
Total Phosphorus (mg/L)500.26260.03727.24000.09621.0127
Ammonia (mg/L)500.03200.01000.12700.02000.0281
Nitrate (mg/L)500.95530.37302.21000.78450.4884
Total Nitrogen (mg/L)501.50640.70006.76001.22500.9682
Fecal Coliform(CFU/100ML)50152850290004454234


King County maintains a streamflow gage: Lewis Creek at West Lake Sammamish Parkway SE (63a) and a precipitation gage: Lewis Creek Rain Gage (63u) on Lewis Creek.

Stream Sediment

Sediment data were collected from Lewis Creek as part of the Stream Monitoring Program starting in 1987. Data were compiled and analyzed for the years 1987 through 2002. Data were analyzed for trends, correlations, and were compared to sediment quality guidelines. No significant trends were identified during data analysis for any of the parameters tested. Results indicate that Lewis Creek sediments did not exceed any sediment quality guidelines. Of the 27 streams monitored in King County, Lewis Creek had the 3rd lowest metals concentration.

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 Lewis Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Lakemont Boulevard

Public concern over the 1998 construction of Lakemont Boulevard in the sensitive Lewis Creek corridor led to major environmental controls. Bellevue and King County staff worked together to control stormwater road runoff. Several reports have been written in relation to this project including:

Small Streams Toxicity/Pesticide Study

As part of King County's Small Streams Toxicity/Pesticide Study, pesticide concentrations and toxicity were measured in Lewis Creek in 1999 and 2000. The County collected samples for analysis of pesticides, other organics, metals, suspended solids, and toxicity (Ceriodaphnia dubia, and Selenastrum capricornutum). For more information, please visit the pesticide study website.

Habitat Assessment

Washington Department of Fish and Wildlife conducted a fish barrier evaluation and habitat assessment on Lewis Creek in 2000.