"the same river across which the ferryman had once taken him"
The ferryman as a guide for the river symbolizes a guide on the spiritual path. As a link between the ordinary world and enlightenment, the ferryman shows Siddhartha how to find enlightenment within himself. Just as a Guru or teacher appears in the life of a student when he is ready, the ferryman guides only when the seeker is ready to explore the path.
The first time Vasudeva meets Siddhartha, he only wants to cross the river. Years later, Siddhartha searches for knowledge from the river, and Vasudeva guides him in his attempts to hear the sounds of the river. Siddhartha himself becomes a ferryman, a guide, a conduit for seekers, after he attains enlightenment, and his guide, Vasudeva, retires into the forest.