The Stillaguamish River is a significant waterway in northwestern Washington state, originating in the Cascade Mountains and flowing westward to empty into Puget Sound. The river is formed by the confluence of its North and South Forks near Arlington, with the main stem stretching approximately 45 miles to its mouth at Port Susan. The North Fork begins at the base of Three Fingers Mountain, while the South Fork originates near Barlow Pass. Both forks flow through steep, forested canyons before joining in the lowlands. Major tributaries include the Pilchuck Creek and Canyon Creek.

The Stillaguamish offers diverse paddling opportunities across its watershed. The upper reaches of both forks feature challenging whitewater runs suitable for experienced paddlers, with Class III to V rapids. The South Fork's Robe Canyon section is particularly renowned among expert kayakers for its intense Class V whitewater. Downstream, the river transitions to more moderate Class II-III rapids before becoming a meandering lowland river. For intermediate paddlers, the Silly Stilly section of the South Fork from Boardman Creek to Verlot provides an exciting Class III run. Novice paddlers can enjoy the gentler flows of the lower river and estuary.

The river is typically paddled from late fall through spring when water levels are highest, though some sections are runnable year-round. The river valley supports diverse ecosystems, from alpine forests to estuarine wetlands. Wildlife includes salmon, steelhead, bald eagles, and black bears. The estuary at the river's mouth is an important habitat for migratory birds.

Key access points include Deer Creek and Verlot on the South Fork, and Darrington and Oso on the North Fork. The town of Arlington sits at the confluence of the forks. Historical sites along the river include old logging camps and Native American fishing grounds.

Safety considerations include cold water, logjams, and rapidly changing conditions, especially in the upper canyon sections. Some areas have specific regulations regarding fishing and boating. For multi-day trips, paddlers can combine sections of the North and South Forks or continue down the main stem to the estuary. The river's proximity to the Mountain Loop Highway makes it accessible for both day trips and longer expeditions exploring the surrounding Cascade wilderness.