Water Quality Improvement Project
Snohomish River Estuary:


The study area for this water quality improvement project included the Quilceda, Allen, Woods Creek, and the Sultan and Pilchuck River watersheds, which drain into the mainstem Snohomish River. Within the study area there are also four drainage systems controlled by pumping stations: French Creek, the Marshland, Deadwater Slough, and Swan Trail Slough.

The lower Snohomish River and its sloughs are an important habitat for adult and juvenile salmon transitioning from salt to fresh water before they migrate from one environment to the other. In addition to these waters, the lower four parts of the river, sloughs, and near-shore estuary are important juvenile rearing areas. Portions of the Skykomish and Snoqualmie river systems provide excellent spawning habitat for chinook, coho, pink, and chum salmon both in the main channels and tributaries.

Water quality issues

Monitoring showed that the waters in the Snohomish Estuary system did not meet water quality standards for dissolved oxygen. The Snohomish River Estuary waters were placed on Washington State's 1996 Section 303(d) list of impaired waters for ammonia and biological oxygen demand (BOD).

Waterfall on the Snohomish River, Washington State.  Photo courtesy of Ralph Svrjcek, WA Department of Ecology.

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.

What was done

Local governments and dischargers made changes and adjustments as needed to divert effluent discharges to other appropriate locations, improve wastewater treatment plants (WWTP), and update comprehensive plans. Real-time water quality monitoring helped affected facilities like the Everett facility to take interim storage measures in case the Snohomish River approached critical flows before treatment upgrades were completed.

Technical information

Snohomish River Estuary Total Maximum Daily Load: Submittal Report (Ecology publication)

Snohomish River Estuary Dry Season TMDL Study Phase I - Water Quality Model Calibration (Ecology publication)

Related information

WRIA 7: Lower Snohomish Watershed Information (Water web site)


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Last updated June 2012
  Snohomish Watershed map


WRIA: #7 (Snohomish)
County: Snohomish

Water-body Names:
Snohomish River estuary


# of TMDLs: 3

TMDL approved by EPA

Contact Info:
Ralph Svrjcek
Phone: 425-649-7165
Email: Ralph.Svrjcek@ecy.wa.gov

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