Cedar River Site 0438

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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. Since 1972 water quality samples have been collected monthly from an upstream station (A438), located at the bridge on East Jones Road at 196th Avenue South East. An additional water quality site was added in 1996 at the mouth of the river (X438), just above where the river enters Lake Washington. 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.

Cedar River Watershed

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 (City of Seattle 2000a; 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 please visit the Cedar River watershed page.

Fisheries

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 was designated a “Class AA” water body under the 1997 rules. As the 2003 rules become effective, the Cedar River from the mouth at Lake Washington to Chester Morse Lake is 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 “Primary 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 2004 Washington Department of Ecology’s (Ecology) 303(d) list for violation of water temperature and fecal coliform bacteria (FC) standards. See Table 1 for a summary of water quality violations in the creek during the most recent water year.

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with water quality data collected from the upstream station (A438). As with most streams in WRIA 8, there has been a significant increase in water temperatures over this 25-year time period. Total phosphorus concentrations and pH have also shown increasing trends. Parameters that decreased significantly were; total suspended solids, ortho-phosphorus, ammonia, and nitrate-nitrogen. 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. During the last six water years the station at the mouth of the Cedar River (X438) has been rated “moderate” concern primarily due to high FC, low summer DO, and high nutrients. The upstream station (A438) has alternated between “low” and “moderate” concern ratings during the last five years due to high FC. To see how these ratings compare with other stream sites, visit the Water Quality Index page.

A review of the 2008 data shows the following violations of State water quality standards:

 

Hydrology

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).

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.