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French and Pilchuck watersheds: dissolved oxygen and temperature TMDLs

Although the water temperatures and dissoslved oxygent (DO) levels are thought to be good in the Pilchuck River watershed for most of the year, high water temperatures and low DO values have been recorded in a number of areas during the warm weather months. Lower French Creek experiences similar problems.

French Creek and the Pilchuck River are considered core summer salmonid habitat. This means their waters should not exceed 16°C (60.8°F) measured as a 7-day average daily maximum (7DADM) value. Dissolved oxygen (DO) values should not be below 9.5 milligrams per liter (mg/L).

We are working with the U.S. Environmental Protection Agency (EPA) and local governments to determine the causes of the warm temperatures, which also affect DO levels in both water bodies. Maintaining cooler water temperatures will improve DO levels in the water, improving habitat for fish and other aquatic life.

Water quality issues

The French Creek and Pilchuck River watersheds are part of the Snohomish watershed. They help support important salmonid resources, agricultural uses, and recreational activities. The Pilchuck River is the largest tributary to the Snohomish River. Both watersheds are at the outer boundary of tidal influence in the Snohomish River.

Each water body is unique. For example, water quality and fish passage in French Creek are significantly affected by the French Slough pump station and numerous changes to the watershed in the floodplain area. The Pilchuck River also experienced structural changes in its lower reaches and its upstream is affected by municipal wastewater discharges from the city of Granite Falls.

What we have done

We began a combined temperature/DO total maximum daily load (TMDL) project for the French Creek and Pilchuck River watersheds in 2012. During the initial phases of the project, we worked with the EPA and a scientific consulting firm to design the study. During the summer of 2012 we conducted extensive field studies.

As part of the 2013 data assessment, we determined that some of the data collected in lower French Creek was not usable because stream flows were extremely low in that area. When looking at our Pilchuck River data, we determined that additional flow and groundwater data was needed and we collected that data during the summers of 2014 and 2016.

Status of the project

We are currently examined options on how to modify our technical approach for modeling water quality in lower French Creek in summer 2017. We completed modeling of Pilchuck River water quality in July 2017. We are now drafting the Pilchuck River temperature/DO TMDL. We expect to form an advisory group in early 2018.

Following advisory group meetings, we will gather public comment. We expect to complete modeling for the French Creek temperature/DO TMDL by July 2018, with drafting of the TMDL and advisory group to follow.

Why this matters

Oxygen dissolved in water is vital for fish and aquatic life to "breathe” to survive. It is more difficult to transfer oxygen from water to blood than it is to transfer oxygen from air to blood. Therefore, it is critical that an adequate amount of oxygen is maintained in the water for this transfer to take place efficiently and to sustain aquatic life. Oxygen is also necessary to help decompose organic matter in the water and bottom sediments as well as for other biological and chemical processes.

Water temperature influences what types of organisms can live in a water body. Cooler water can hold more dissolved oxygen that fish and other aquatic life need to breathe. Warmer water holds less dissolved oxygen. Many fish need cold, clean water to survive.