Proceed through Mission 3: The Root Of All Evil until you exit a cave into a large open area, with a fallen tree at the cave's exit. Look to the left to find a tan donut life raft on the shore of the river. Interact with it to push the life raft into the water. Jump on it, and ride it on the river for twenty seconds to get the "White Rider" achievement. Crouch to avoid getting knocked off.