In A Separate Peace, what is the point of the rivers being described as they are?

The author is using the rivers as symbols. The Devon
River, in Gene's mind, is fondly remembered. It is the river into which he and Finny
jumped many times during the halcyon summer before they were drawn into the war. It is
significant that the Devon is a freshwater river, as it represents for Gene a time of
innocence and freedom, when he and his companions were still boys, too young to be
considered as fodder for war. The Devon is the river of the carefree summer, and even
after Finny's accident, it does not lose its appea.; it is symbolic of serenity, and a
time of peace.

In contrast, the Naguamsett River is "an
ugly, marshy, saline river." It is influenced by the forces of the world outside of the
sanctuary of Devon School, like "the Gulf Stream, the Polar Ice Cap, and the moon." The
Naguamsett does not have the pristine and untouched insulation of its counterpart, the
Devon River. Dark and forbidding, it is a symbol of the war, and the outside world gone
mad.