The Boulder River originates in the Cascade Range. It flows northwest and then northeast through a portion of Boulder River Wilderness to join the Stillaguamish River. The Stillaguamish empties into Port Susan, part of Puget Sound. The river's source is the Craig Lakes, two remote lakes located high on the slopes of Three Fingers. After exiting the lakes the river plunges over Craig Lakes Falls as it descends into the Boulder River Wilderness.