The river that John needs to cross to complete his quest is clearly a source of fear and trepidation for him. When he first comes across the river, his description of it makes clear his awe and fear of it: "There was the great river below, like a giant in the sun. It is very long, very wide. It could eat all the streams we kjnow and still be thirsty." In response to its vastness and awesomeness, John says "It was magic and I prayed." It is clear that there is a significant amount of internal conflict that occurs within John: "My magic did not help me at all and yet there was afire in my bowels, a fire in my mind." This "fire" drives him to build a raft to cross the river. Despite his fear of the river he nevertheless continues with his attempt to cross the river, for "It is better to lose one's life than one's spirit, if one is a priest and the son of a priest." And thus he sings his death song as he launches out onto the river.
It is clear that John really is able to cross the river by mostly luck. He obviously does not understand rivers or how they work, as he is surprised by the strength of the current: "That was magic, for the river itself is wide and calm." It is also clear that being on the river makes John feel horribly exposed and alone: "There was no strength in my knowledge anymore and I felt small and nakes as a new-hatched bird - alone upon the great river, the servant of the gods." This simile clearly is appropriate to the character of John, using an image from nature.
As John progresses on his journey, he appeals to the gods to help him reach his goal, and learns how to steer the raft to guide it to his destination. As he nears his goal, the raft tips over and John has to swim to the side, dragging himself up onto a great spike of rusted metal. It is left unexplained how John returned over the river after his discovery, but it is clear that the river is a symbol of the power of nature, of magic, and also a means by which John is made to feel his insignificance and trial in which he can prove himself.