Sammamish River Temperature & Dissolved Oxygen TMDL

Water quality issues

The Sammamish River is a highly modified waterbody with multiple water quality problems. It was originally called Squak Slough and wound about 30 miles through a large wetland complex to Lake Washington. When the outflow of Lake Washington was rerouted to the Lake Washington Ship Canal constructed in 1916, the level of the river dropped about 10 feet. This led to the floodplain becoming fertile croplands. The river and some tributaries were dredged and straightened to further improve drainage for the new agricultural area and reduce flooding. In 1964, a weir was installed near the outflow of Lake Sammamish to the Sammamish River.

Water quality in the Sammamish River is poor during summer months. Much of the river lacks shade and large portions are heavily infested by the invasive weed Egeria densa, or Brazilian Elodea. Along with relatively low flows that occur during dry summer months, these factors contribute to the observed high water temperatures and low dissolved oxygen levels in the river.

What we have done

Our study of the Sammamish River builds on earlier studies by local governments and the Muckleshoot Tribe to help pinpoint problem areas and develop solutions to improve the river’s water quality. Field work for the Sammamish River temperature and dissolved oxygen Total Maximum Daily Load (TMDL) started in late summer 2014. We prepared a Quality Assurance Project Plan (QAPP) in early 2015 and collected data in the summer of 2015.

The 2015 field work included two intensive surveys of groundwater and surface waters. We evaluated water volume, temperature, oxygen levels, nutrients, and several other water quality parameters at more than 20 locations. We also performed several surveys of aquatic vegetation, including identification of Brazilian Elodea. We installed two real-time telemetry stations to measure dissolved oxygen and temperature continuously through summer 2015 through the fall of 2016.

Status of the project

Field work for the study has ended. We expect to complete our data review and quality control in 2017. Because of the complex nature of the river, we are currently reevaluating the technical approach we will use to complete this TMDL. We expect to have a better understanding of our next steps in the fall of 2017. Although field work is completed and data review underway, we have not yet assigned staff to build the water quality models that will provide the scientific basis for the TMDL. That decision is likely to occur in spring 2018.

Why this matters

Oxygen dissolved in healthy water is vital for fish and aquatic life to “breathe” and survive. Therefore, it is critical that an adequate amount of oxygen is maintained in water to sustain aquatic life. Oxygen is also needed to help decompose organic matter in water and bottom sediments, as well as for other important biological and chemical processes.

Water temperature influences the health of many organisms that live in a water body. Cooler water holds more dissolved oxygen, which fish and other aquatic life need to breathe. Warmer water holds less dissolved oxygen and makes cold-water species like salmon more vulnerable to disease and predation. One of the most important ways to cool water temperature is to shade the water body by adding or retaining streamside vegetation that blocks the sun.