The Stillaguamish River is a river in the northwestern region of the U.S. state of Washington. It is mainly composed of two forks, the longer North Fork Stillaguamish and the South Fork Stillaguamish. The two forks join near Arlington. From there the Stillaguamish River proper flows for 22 miles (35 km) to Puget Sound. The river's watershed drains part of the Cascade Range north of Seattle.

Arlington is a city in northern Snohomish County, Washington, United States, part of the Seattle metropolitan area. The city lies on the Stillaguamish River in the western foothills of the Cascade Range, adjacent to the city of Marysville. It is approximately 10 miles (16 km) north of Everett, the county seat, and 40 miles (64 km) north of Seattle, the state's largest city. As of the 2020 U.S. census, Arlington had a population of 19,868; its estimated population is 20,075 as of 2021.