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Date of Award

Fall 2023

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Mitchell, Robert J.

Second Advisor

Pfeiffer, Allison

Third Advisor

Yearsley, John R.


The Stillaguamish River is a snow-and-rain mixed basin and the fifth largest river in the Puget Sound basin. Elevations in the 1700 km2 Stillaguamish River basin reach roughly 2000 m and historically a snowpack is sustained above 1000 m. Snowmelt in the basin is important for sustaining spring and summer streamflow and buffering stream temperatures. Stream temperature increases are of significant concern because of the threatened Chinook salmon (Oncorhynchus tshawytscha) population.

I reexamined projected stream temperatures in the Stillaguamish River by forcing the coupled Distributed Hydrology Soil Vegetation Model and River Basin Model with dynamically downscaled meteorological forcings from the Weather Research and Forecasting model and projected changes in the entire basin, including the Pilchuck subbasin and mainstem through 2099 by applying 12 dynamically downscaled Global Climate Models with high emission scenarios of RCP 8.5. Using an updated version of the River Basin Model, I applied tributary-specific calibration parameters and calibrated modeled streamflow and temperature using historical gauges and field measurements. My model calibrations and projections are consistent with other modeling studies in the Stillaguamish and other western Cascade watersheds.

Snow covered area in the basin is projected to decrease by 74%, and summer streamflow decreases for the primary locations and at-risk tributaries are projected to be 48% and 53%, respectively for July and August at the end of the century. With the decreases in snowpack and streamflow, stream temperatures reach their peak earlier in the year, in July instead of August which was historically the warmest month. Stream temperatures are projected to increase by 14% on average for the larger primary reaches and 21% for the smaller at-risk tributaries by the 2080s for July and August. The greatest stream temperature increases are in mountainous reaches due to a reduced snowpack. The warmest stream temperatures are projected to occur in late summer along the mainstem of the Stillaguamish River. By the end of the century, six of nine locations examined will exceed the seven-day average daily maximum adult Chinook salmon lethality threshold (22.0 °C). These results indicate that continued work on climate adaptation actions and research will be required to improve Chinook salmon resiliency in the Stillaguamish River as the climate warms.




Hydrology, Climate Change, Stillaguamish River, Streamflow, Stream Temperature, Chinook Salmon, Western Washington, Modeling, Water Quantity, Water Quality


Western Washington University

OCLC Number


Subject – LCSH

Hydrological forecasting--Washington (State)--Stillaguamish River Watershed; Climatic changes--Washington (State)--Stillaguamish River Watershed--Forecasting; Chinook salmon--Climatic factors--Washington (State)--Stillaguamish River Watershed; Chinook salmon--Effect of temperature on--Washington (State)--Stillaguamish River Watershed; Water temperature--Washington (State)--Stillaguamish River Watershed--Mathematical models; Streamflow--Washington (State)--Stillaguamish River Watershed--Mathematical models; Stillaguamish River Watershed (Wash.)

Geographic Coverage

Stillaguamish River Watershed (Wash.)




masters theses




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