Water Quality Improvement Project
Duwamish-Green Basin:


The drainage basin of the Duwamish River covers 483 square miles, nearly 90 percent of which is also drained by the Green River. The Green River flows north and west from its headwaters in the Cascade foothills about 60 miles to its confluence with the Black River in Tukwila. The Green River turns into the Duwamish River at its confluence with the Black River. The Black River was radically altered when Lake Washington was lowered several feet during construction of the Lake Washington Ship Canal and Ballard Locks around 1916. Prior to construction of the ship canal, Lake Washington drained via the Black River through Renton and into the Green River. This natural drainage path was eliminated when the lake was lowered. The flows that now make up the Black River are largely from Springbrook Creek which flows north from the eastern Green River Valley in the Kent area.

  Duwamish River, upper area, Washington State.  Photo courtesy of Joe Mabel, Google Images.

© Joe Mabel, Google Images

Water quality issues

In 1981, studies determined that effluent from the Metro-Renton wastewater treatment plant impacted the lower Green-Duwamish River area. The treatment plant outfall was located in the Green River upstream of the confluence with Black River. The plant effluent particularly affected the Green River, upriver of the Duwamish River, during the critical late summer and fall periods, when the effluent made up a substantial portion of the flow in the river. The effluent led to increased water temperature (though not enough to violate water quality standards), low dissolved oxygen, toxic levels of residual chlorine, and increased nutrients.

Why this matters

Ammonia (NH4+) is one measure of nitrogen, a nutrient that can increase the growth of plants and algae in water. The presence of large concentrations of ammonia in a stream or lake can create a large oxygen demand. This demand is caused by the conversion of ammonia to nitrate, called "nitrification". High concentrations of nitrate in wastewater treatment plant effluent can cause algae to grow in large quantities. Dead and decaying algae can cause oxygen depletion problems, which in turn can kill fish and other aquatic organisms in streams. Higher-than-normal levels of nutrients in the water can also lead to changes in the water’s pH and clarity. In addition, increased algae and plants can be ugly, create odor problems when they die, decompose and interfere with recreational activities like boating and swimming.

Biological Oxygen Demand (BOD) is the amount of oxygen required by aerobic microorganisms (organisms that need oxygen to survive) to break down organic matter in water. It can be used to measure the amount of water pollution in a water body.

Lower Duwamish River photo, Washington State.  Photo courtesy of EPA via Google Images.  

© EPA, Google Images


Status of the project

To protect water quality, the decision was made to remove the point source (Metro-Renton wastewater treatment plant) from the Green River, and instead discharge the effluent to Puget Sound at Alki Point. Only emergency discharges to the Green River are allowed from the wastewater treatment plant. Discharges for maintenance were authorized only during high flow periods, and only for a few hours. Subsequently, the wasteload allocation (WLA) for the wastewater plant was set to zero for routine discharges. Ecology submitted a TMDL to EPA with an established WLA for the wastewater plant. EPA approved the TMDL on December 29, 1992.

The city of Seattle conducted bi-weekly monitoring at the three Duwamish River and Waterway stations for dissolved oxygen and biological oxygen demand (BOD 5-day). They also monitored water quality in the Green River for ammonia, total suspended solids, and heavy metals during the infrequent plant discharges.

Removal of the wastewater treatment plant effluent resulted in some improvement in Green River water quality below the plant outfall. The Green River showed some improvement for ammonia-nitrogen and heavy metals. The Duwamish portion met water quality standards for ammonia. However, both Green River segments continue to show some impairment for fecal coliform, dissolved oxygen, mercury, lead, and temperature.

Technical information

Duwamish Waterway Ammonia-Nitrogen TMDL (Ecology publication)

Related information

Green-Duwamish River Basin

WRIA 9: Duwamish-Green Watershed Information (Water website)

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Last updated October 2014
  Water resource inventory area (WRIA) 9 map, Washington State.


WRIA: #9 (Duwamish-Green)
County:  King

Water-body Names:
Duwamish Waterway
Duwamish River
Green River


# of TMDLs - 2

Approved by EPA

Contact Info:
Joan Nolan
Phone: 425-649-4425
Email: Joan.Nolan@ecy.wa.gov

Northwest Region
Department of Ecology
3190 160th Ave. SE
Bellevue, WA 98008-5452