The two rivers, the Devon with its clear, clean water and the Naguamsett with its ugly, marshy waters are symbolic of the relationship of Gene and Phineas at different points in the narrative. Having committed his act of envy, Gene finds himself at the borders of the Naguamsett shortly thereafter.
Gene repeats his vision of the forest that borders Devon, saying that he believed that the forest went on for thousands of miles, unbroken, until it finally ended in an untouched, pristine stand of pine trees in northern Canada. This vision of nature seems to symbolize an idea of youth and innocence and it is one that Gene comes to realize was false, just a youthful fancy, but he repeats the idea anyway and seems to treasure it despite its romantic sentimentality.
The central symbol of nature in the novel is the tree, which provides a place for the boys to come together and enjoy fellowship. This is one of the reasons that the injury that occurs during one of these outings is so disturbing. The symbol of fellowship suddenly becomes associated with a violation of that fellowship.