Fog still hung in veils upon the crumbling rock-wall, and to their left mist shrouded the River: they could hear it rushing and foaming over the sharp shelves and stony teeth of Sarn Gebir, but they could not see it.
- The Fellowship of the Ring (Book 2), Chapter 9
The Sarn Gebir are the rapids of the river Anduin as it flows through the Emyn Muil north of the Argonath. Their pointy stones and eyots make them untraversable, which led the Gondorians to create a small portage-way along the western banks of the rapids to allow for travel with smaller boats from Rhovanion to Osgiliath.