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King County Water Quality Monitoring

King County monitors the ecological health of the Green-Duwamish River in a variety of ways including collecting and analyzing water, sediment, and benthic invertebrate samples. King County has been conducting monthly baseline water quality monitoring at several sites along the Green-Duwamish River beginning in the early 1970s.

Duwamish River - Station 0309 is located in the Duwamish River at the bridge on East Marginal Way in Allentown. Sampling at this site began in 1970 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. The County samples two additional stations (0305 and 0307) further downstream on the Duwamish River. These downstream sites are strongly influenced by tidal water and therefore are not included with the other streams in freshwater assessments.

Lower Green River - Two stations are located in the Lower Green River. Station 3106 is located at the bridge at Fort Dent Park downstream of the former Renton Treatment Plant outfall. Sampling at this site began in 1970 and continues today. Station 0311 is located a few hundred yards upstream from the former outfall at the Renton Junction Bridge on West Valley Road at Highway 1. Sampling at this site occurred from 1970 to 2008.

Middle Green River - There are two stations located in the Middle Green River. Station A319 is located upstream of the confluence of Soos Creek at the bridge on Black Diamond Road. Sampling at this site began in 1976 and continued until 2008 when budget cutbacks forced King County to reduce the breadth of its water quality monitoring program. Station B319 is located upstream of the confluence of Newaukum Creek at the bridge on Southeast Green Valley Road. Water quality sampling at this site began in 1972 and continues today.

From time to time special studies have been conducted at various streams in King County. Click here for information about Special Studies of Green - Duwamish River.


Water Shed Image The Green-Duwamish River watershed is located in southern King County and is the largest freshwater component in the Water Resources Inventory Area 9 (WRIA 9). The river flows for over 93 miles beginning at the crest of the Cascade Mountains and ending as it empties into Elliott Bay (Herrera 2005; Kerwin and Nelson 2000).

Historically, the White, Green, and Cedar Rivers flowed into the Duwamish River and drained an area of over 1,024,000 acres (Kerwin and Nelson 2000). Major alterations of the Green-Duwamish watershed have taken place over the last century resulting in many alterations to the drainage area. The WRIA 9 Habitat Limiting Factors and Reconnaissance Assessment Report, summarizes the major historical changes to the watershed as following:

  • 1851 - European settlement begins in the Duwamish River.
  • 1880-1910 - Logging occurs across much of the watershed and in the lower river valley; agricultural land use expands.
  • 1911 - White River is diverted from Green River to Puyallup River for flood control, reducing watershed area by 30 percent.
  • 1913 - City of Tacoma begins diverting water from Green River to provide water for homes and industry. Anadromous salmonids are blocked from Upper Green River Sub-watershed.
  • 1916 - Black and Cedar Rivers are diverted from Duwamish River to Lake Washington to improve navigation, further reducing watershed area by 40 percent from its original size.
  • 1900-1940 - Duwamish estuary tidelands are filled, drained and dredged to support growing industrial and port activities.
  • 1895-1980 - The Green/Duwamish River is channelized and diked for navigation and flood control.
  • 945-2000 - Residential, commercial, and industrial land uses expand, largely replacing farmlands and forests in the western half of the WRIA.
  • 1962 - Howard Hanson Dam is completed for flood control purposes.

The Green-Duwamish River watershed as it is today can be divided into four subwatersheds. The Green-Duwamish Estuary from the River Mile (RM) 0.0 at Elliot Bay/Harbor Island to RM 11.0 at Tukwila covers 14,080 acres. The Lower Green River from RM 11.0 at Tukwila to RM 32.0 at Auburn Narrows covers 40,832 acres. The Middle Green River from RM 32.0 at Auburn Narrows to RM 64.5 at the Howard Hanson Dam covers 113,600 acres. And the Upper Green River from RM 64.5 at the Howard Hanson Dam to the headwaters covers 140,608 acres.

Land use varies considerably throughout the length of the Green-Duwamish River watershed (Kerwin and Nelson 2000). Land in the Upper Green River sub-watershed is almost entirely used for forest production. The Middle Green sub-watershed is a mix of residential, commercial forestry, and agricultural land uses. The Lower Green River Sub-watershed is characterized as residential, industrial, and commercial land uses. And, the Green-Duwamish Estuary sub-watershed is split between residential and industrial uses.

For more detailed information about the Green-Duwamish River watershed, please visit the Green River Home Page.


Anadromous salmonids found in the Green-Duwamish watershed are coho, chinook, chum, sockeye, and pink salmon and coastal cutthroat, steelhead, and bull trout/Dolly Varden char (Kerwin and Nelson 2000). Non-native Atlantic salmon have recently been found in the watershed as well. Compared to other freshwater systems of Puget Sound, the Green River has not experienced the same level of decline in the naturally spawning adult chinook.

There are several reports that include comprehensive information about the fisheries and ecosystems of the Green-Duwamish River. More information about habitat protection and restoration, and the recovery of salmon species in the Green-Duwamish basin can be found at the Green River Home Page.

In addition, volunteers with the Salmon Watcher Program have been making observations at various locations within the Green-Duwamish River basin since 1997.

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 Duwamish River is now categorized as “Salmonid Rearing and Migration Only” habitat, the Lower Green as “Salmonid Spawning, Rearing, and Migration” habitat, and the Middle Green as “Core Summer Salmonid” habitat for aquatic life use. As part of the updated water quality standards, the Middle Green River has been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from September 15th through July 1st. For recreational use, the Duwamish is designated as “Secondary Contact” and the Lower/Middle Green as “Primary Contact”. The Duwamish Waterway and River is on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list for not meeting pH and water temperature standards (Category 5). The Green River is listed for violation of DO (Category 5), fecal coliform bacteria (Category 5), and temperature standards (Category 4a).

Water quality in the Duwamish River has been characterized as “fair”, the Lower Green as “fair to good”, and the Middle Green as “good to very good” (Metro 1990; Herrera 2005). 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 baseflow water quality data from the three upstream sampling locations (0311, A319, and B319) showed some significant changes in the water quality since 1979. Results show that there has been a significant decrease in dissolved oxygen, pH, and ortho-phosphorus at all three stations. Total suspended solids decreased significantly at station A319 and B319, and turbidity decreased significantly at station B319. Fecal coliform bacteria decreased significantly at stations 0311 and A319. Temperature increased significantly at station 0311 over this time period. Total phosphorus increased significantly at B319. For information about how water quality trends compare to other County streams, please visit the links above.

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 see how the Green-Duwamish River ratings compare to the other sites in the County monitoring program visit the Water Quality Index website.

Table 1. Routine monitoring summary statistics for this station from 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)26111.68.914.511.61.1
Temperature (°C)3869.61.318.18.74.2
Turbidity (NTU)3513.950.30119.001.1810.42
Conductivity (mSIEMS/cm)23156.530.0111.055.013.9
Total Suspended Solids (mg/L)3887.550.40160.002.3018.30
Ortho-Phosphorus (mg/L)3870.00570.00090.02900.00480.0041
Total Phosphorus (mg/L)3880.01910.00200.39300.01200.0290
Ammonia (mg/L)3880.01380.00200.07930.01100.0114
Nitrate (mg/L)3880.20890.03100.54000.20250.0829
Total Nitrogen (mg/L)3140.29860.05901.17000.28200.1359
Fecal Coliform(CFU/100ML)387120480627

Table 2. Storm water monitoring summary statistics for this station from 1972 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)4911.68.513.511.71.1
Temperature (°C)688.
Turbidity (NTU)6019.110.53543.002.1672.38
Conductivity (mSIEMS/cm)3853.427.
Total Suspended Solids (mg/L)6827.721.00566.004.3078.41
Ortho-Phosphorus (mg/L)680.00670.00200.06300.00480.0080
Total Phosphorus (mg/L)680.03850.00570.46000.01610.0686
Ammonia (mg/L)680.02130.00520.05950.01550.0140
Nitrate (mg/L)680.23170.07340.42300.21750.0804
Total Nitrogen (mg/L)660.39910.11902.50000.30400.3452
Fecal Coliform(CFU/100ML)684504201972


King County does not maintain stream gages on the mainstem of the Green-Duwamish River. The US Geological Survey (USGS) does operate five stream gages on the Green River: At the Howard A. Hansen Dam (12105900), at the purification plant near Palmer, WA (12106700), near Auburn, WA (12113000), near Kent, WA (12113344), and at Tukwila, WA (12113350).

King County operates three water temperature sensors on the Green River: at the Whitney Bridge (GRT 10), at the Interurban Ave. Bridge near Fort Dent (GRT 40), and the 42nd Ave Bridge near the Tukwila Community Center (GRT 41).

Stream Sediment

Four samples were collected along the Green River reach in 2010. The most upstream site is located at Flaming Geyser State Park (FG319) and the most downstream site is located at the Foster Links Golf Course downstream of the Black River confluence (FL319). Results indicate that Green River sediments did not exceed any sediment quality guidelines; however, there was difficulty collecting fine sediments at these sites and a large percentage of the particle size was sand (>80% on average). Total organic carbon (TOC) was also particularly low at these sites (average of 0.75% TOC).

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 of the Green River, please visit the Puget Sound Benthos webpage.

Special Studies

There are several extensive reports written about the Green - Duwamish River. For a listing of these reports, visit the Green - Duwamish River Watershed home page.