The two rivers in the book are described in symbolic ways. The Devon School sits between two rivers, the Devon and the the Naguamsett. The Devon is described as a place of fun, and it is the river into which Phineas leaps from his canoe. Later, it is the place in which the boys jump from a tall branch into the water and where Phineas has his accident.
The other river, the Naguamsett, is a saline river whose movements are mysterious. Gene describes it in the following way: "its movements were governed by unimaginable factors like the Gulf Stream, the Polar Ice Cap, and the moon." The boys do not use this river, which runs to the ocean. This second river, unlike the freshwater Devon, is not an idyllic place; it is a river fringed with muck and seaweed.
These two rivers represent boyhood (the Devon) and adulthood (the Naguamsett). The Devon is a seemingly innocent place in which Finny frolics until he experiences his fall. The other river, which is connected to the wider world and which also straddles the school, represents the less idyllic nature of adulthood. When Gene destroys the innocent nature of the boys' experiences on the Devon by shaking a tree branch and causing Finny to fall into the river, he moves from boyhood into adulthood. The idyllic nature of the Devon then ends forever for him.