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King County Water Quality Monitoring

King County monitors the ecological health of the Cedar River basin in a variety of ways including collecting and analyzing water and benthic invertebrate samples. Sampling at the upstream station A438, located at the bridge on East Jones Road at 196th Ave South East, began in 1972, was discontinued in 2009 after budget cutbacks forced King County to reduce the breadth of its water quality monitoring program, and then was resumed in 2014. Sampling at station 0438, located upstream at River Mile 1.46 at the bridge on Bronson Way, began in 1976, was discontinued in 1989, and resumed again in 2009. An additional water quality site was added in 1996 at the mouth of the river (X438), just above where the river enters Lake Washington. Sampling at X438 was discontinued in 2009. Benthic invertebrates have been sampled at various locations in tributaries to the lower Cedar River beginning in 1995.

From time to time special studies have been conducted in the Cedar River Basin.


Water Shed Image The Cedar River basin can be divided into two reaches: the Upper Cedar river from the Cascade Crest to the Landsburg Diversion Dam; and the Lower Cedar River from the Landsburg Dam to the mouth at the City of Renton.

Upper Cedar River

The Upper Cedar River encompasses roughly 79,951 acres and runs 25 miles from Meadow Mountain near Cascade Crest (elevation 5,414 feet) to the Diversion Dam (elevation 543 feet). The City of Seattle owns 79,452 acres of this part of the basin that is upstream of where the City of Seattle diverts a drinking water supply from the Landsburg Dam. The remaining 499 acres is owned by other parties(Kerwin 2002). This Upper Cedar River watershed is almost entirely forested with approximately 90 percent (71,588 acres) supporting commercial timber harvest for over 120 years (Kerwin 2002).

There are several major tributaries that drain into the Upper Cedar River, including Upper Rock Creek (2,917 acres) at River Mile (RM) 3.9, Williams Creek (1, 564 acres) at RM 29.6, Steele Creek (788 acres) at RM 31.7, and Upper Taylor Creek (10,950 acres at RM 29.5). Taylor Creek has the largest drainage area and provides approximately 15 percent of the total Upper Cedar River flows (Hart Crowser 1983; Kerwin 2002).

Lower Cedar River

The Lower Cedar River drains 42,240 acres and runs approximately 21.7 miles from below the Landsburg Dam to Lake Washington. Most of the Lower Cedar River is forested (61 percent), with 29 percent low to medium density development (Kerwin 2002; King County 1993). Less than 1 percent (according to a 1999 GIS analysis) is high-density development. The Lower Cedar River basin is primarily (90 percent) within the jurisdictional boundary of King County. The remaining jurisdictional area is within the cities of Renton (7.8 percent), Maple Valley (2.1 percent) and Kent (0.8 percent).

The aquatic habitat of the Lower Cedar River basin has been altered dramatically since the 1800s by human activity such as water supply dams, agriculture, coal mining, railroad construction, and development. The City of Seattle built the Landsburg Diversion Dam in 1901 to divert water for municipal use and the Masonry Dam further upstream in 1914 for water supply, flood control and hydroelectric power generation (City of Seattle 2000a). Prior to this there had been many alterations to the mainstem due to railroad construction and operation. Water withdrawal and flow regulation are primary causes for the reduction in the average mainstem channel width from 250 feet in 1865 to 170 feet by 1936. Further flood control structures constricted the average channel width an additional 35 percent compared to the 1936 condition to 110 feet. The Cedar River Current and Future Conditions Report (King County 1993) describes in depth historic flood control and management efforts in the Cedar River.

For more information about salmon habitat conservation in the watershed, please visit the Salmon Habitat Conservation page for the Lake Washington/Cedar/Sammamish River Watershed.


Chinook salmon, coho salmon, sockeye salmon, kokanee, winter steelhead, bull trout, and coastal cutthroat are known species to inhabit the Cedar River system (Kerwin 2002). Volunteers with the Salmon Watcher Program have been making observations at various locations within the Cedar River basin since 1997.

There are several reports that include comprehensive information about the fisheries and ecosystems of the Cedar River. The City of Seattle's Cedar River Habitat Conservation Plan includes extensive information about the Cedar River ecosystem (City of Seattle 2000a). In addition, the 2005 Proposed WRIA 8 Chinook Salmon Conservation Plan is a science-based plan developed through a collaboration of citizens, scientists, community, business, environmental groups, local elected officials, and public agency staff. The plan recommends actions to restore and protect salmon habitat, and provides an approach to implement these actions over the next ten years. Additional fisheries resource information can be found at the WRIA 8 salmon page.

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 bacteria (FC). 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. The Cedar River is now categorized as “Core Summer Salmonid Habitat” for aquatic life use. As part of the updated water quality standards, these lower portions of the river have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from the fall (September 15th) until spring (June 15th). For recreational use the river is designated “Extraordinary Contact Recreation” from the mouth to river mile (RM) 4.1, and “Extraordinary Primary Contact from RM 4.1 to the headwaters. The river is on the Washington Department of Ecology’s (Ecology) 303(d) list, Category 5, for violation of water temperature, fecal coliform bacteria, dissolved oxygen, and pH standards. These violations occurred in the Lower Cedar River watershed near it's outlet into Lake Washington. See Tables 1 and 2 for a summary statistics of the routine and storm water quality data collected to date.

Long-term Trends

A 36-year (1972 – 2007) trend analysis was conducted with water quality data collected from the upstream station (A438). A 12-year (1996 - 2007) trend analysis was conducted with X438 water quality data. As with most streams in WRIA 8, there has been a significant increase in water temperatures at A438 over the 36-year time period. Conductivity increased at both sites. At X438 total phosphorus concentrations increased and dissolved oxygen concentrations decreased. There have been some improvements in water quality as evidenced by the decrease in total suspended solids (A438), turbidity (A438), fecal coliforms (A438), ammonia (both sites), and ortho-phosphorus (A438). For information about water quality trends in other streams or current conditions in the river, visit the links at the top of the page.

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 the Cedar River sites 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 2009 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)11111.49.613.111.40.8
Temperature (°C)11110.13.519.19.53.6
Turbidity (NTU)764.960.50218.001.3424.91
Conductivity (mSIEMS/cm)11167.036.7105.066.615.8
Total Suspended Solids (mg/L)1129.480.70476.002.7545.11
Ortho-Phosphorus (mg/L)1120.00520.00140.01310.00490.0023
Total Phosphorus (mg/L)1120.01740.00510.36900.01210.0350
Ammonia (mg/L)1120.00780.00250.03660.00600.0056
Nitrate (mg/L)1120.22070.06650.66400.20700.0981
Total Nitrogen (mg/L)1120.31380.15001.59000.28350.1550
Fecal Coliform(CFU/100ML)1124114702661

Table 2. Storm water monitoring summary statistics for this station from 2009 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)811.711.012.511.40.7
Temperature (°C)89.25.915.58.63.4
Turbidity (NTU)0     
Conductivity (mSIEMS/cm)873.156.395.268.213.1
Total Suspended Solids (mg/L)83.781.5011.802.553.43
Ortho-Phosphorus (mg/L)80.00560.00360.00960.00530.0019
Total Phosphorus (mg/L)80.01500.00830.03250.01270.0078
Ammonia (mg/L)80.00860.00530.01360.00770.0036
Nitrate (mg/L)80.24840.19600.31700.23700.0390
Total Nitrogen (mg/L)80.34710.24100.47000.34300.0692
Fecal Coliform(CFU/100ML)81421443044172


King County maintains two streamflow gauges near the Cedar River, Lower Cedar Tributary 0308 above Jones RD (31e), and Lower Cedar Tributary 0306A above Fairwood R/D (31j).

Stream Sediment

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 tributaries to the Cedar River, please visit the Puget Sound Benthos webpage.

Special Studies

Cedar River Studies

Several extensive reports have been written about the Cedar River to address fisheries concerns.

Benthic Macroinvertebrate Studies

King County monitors stream health by collecting samples of benthic macroinvertebrates from selected streams. Benthic macroinvertebrates have been sampled at locations in the Cedar River basin as part of King County's Benthic Macroinvertebrate Program.

Water Resources Inventory Area (WRIA) 8

In WRIA 8, citizens, scientists, businesses, environmentalists and governments worked cooperatively on protection and restoration projects and have developed a science-based plan to conserve salmon today and for future generations. Visit the WRIA 8 Web page to view the latest information on projects in the Cedar River Basin.