The fifth largest tributary draining into Puget Sound, the Stillaguamish River is part of a watershed that covers 700 square miles from sea level to an elevation of 6,854 feet. The North Fork and South Fork of the river join in Arlington, where they form the Lower Mainstem that travels 18 miles to Stanwood, where it meets Puget Sound. Forestry is the dominant land use, followed by rural residential development.
The climate tends toward cool winters with high rainfall, which varies from 30 inches annually in the lowlands to 150 inches at the high peaks. Low flows occur during the summer, between July and October.
- Counties: Skagit, Snohomish
- National Estuary Programs: Puget Sound
- Other Watersheds Upstream: None
- Other Watersheds Downstream: Puget Sound