In the short story "On the Rainy River," the fictional Tim O'Brien must decide between draft-dodging to Canada or going to war in Vietnam. At the center of his choice is Rainy River, a natural border that separates Canada from his home in Minnesota.
By the end of the story O'Brien admits, "I was a coward, I went to war." So, the choice of going to the Vietnam war symbolizes Tim's lack of moral courage. He fears disappointing his family and living a life of exile more than he does fighting in an immoral war and possibly dying for false-patriotism.
So, the choice of going to Canada symbolizes the exiled life O'Brien might have led as a draft-dodger and ex-patriot. There, O'Brien would have had to turn his back on his family and country. The antithesis of O'Brien's end to the story might have been, "I was a hero, I went to Canada."
Water symbolizes baptism, a rite of passage, and indoctrination in the novel. In this case, Tim does not partake of its waters: he fails to cross it into Canada. The Rainy River is a counter-symbol to the Song Tra Bong in Vietnam. Other characters, once in Vietnam, do partake of its baptismal waters: Mary Anne becomes a lethal killer after swimming in it. Kiowa is swallowed by its mud and stench in the shit field. Morty Phillips swallows a mouthful of it and dies, and Bowker, the professional soldier, kills himself because of its stench. The Song Tra Bong is both a rite of passage and a sirens' song, and once baptized by it, one longs to return to it (to bury Kiowa's moccasins) and be tortured by it (Bowker's suicide). And lest we not forget that O'Brien could have been saved from the whole Vietnam experience by a swim across the Rainy River.