The Lost River begins in Kinsman Notch, one of the major passes through the White Mountains. As it flows through the notch, it passes through Lost River Gorge, an area where enormous boulders falling off the flanking walls of the notch at the close of the last Ice Age have covered the river, creating a network of boulder caves. The gorge is owned by the Society for the Protection of New Hampshire Forests and is operated as a tourist attraction, with trails and ladders accessing many of the caves.