Course
The Stillaguamish River is formed from the confluence of the North and South Forks, both of which rise in the Cascades. The North Fork rises as several branches in a remote area of Skagit County near Finney Peak, approximately 10 miles (16 km) north of Darrington. Collecting many tributary creeks, the river flows south, then west through the foothills along the Skagit-Snohomish county line. Tributaries of the North Fork Stillaguamish River include Boulder River and Deer Creek.
The South Fork (30 miles (48 km)) rises near in central Snohomish County, 20 miles (32 km) south of Darrington, and flows west and northwest by Silverton, Verlot, and Granite Falls.
The two forks unite at Arlington in northwestern Snohomish County. The combined stream flows west, entering the north end of Port Susan, an arm of Puget Sound, approximately 10 miles (16 km) west of Arlington. Between the North and South Forks there is a portion of the Mount Baker-Snoqualmie National Forest called the Boulder River Wilderness.
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