If there's a moral to “Upon the Sweeping Flood” by Joyce Carol Oates, it's that environment plays the most important part in determining our characters. That certainly seems to be the case with Walter Stuart, the protagonist of the story. When we're first introduced to him, he comes across as an ordinary guy, a decent, mild-mannered man, who works hard and loves his family. In fact, as the story begins he's on his way home in the middle of a violent storm to get home to his wife and two daughters. It's patently obvious that he cares very deeply for them.

But after Walter makes the big mistake of picking up two young hitchhikers, his whole personality seems to undergo a radical change. These two youngsters, a brother and sister, are violent, unpredictable, and behave in ways that suggest that they are almost unbalanced.

At first, Stuart tries to be helpful to the youngsters. For a while, he is the calm in the eye of this rapidly developing storm. But it isn't very long before his personality has been adversely affected by the kids and their increasingly unhinged behavior. He's internalized their craziness to such an extent that he's now as unhinged as they are.

He joins the boy in attacking a nest of snakes for no good reason before attacking the boy himself, again without any rhyme or reason. Then he turns his attention to the boy's sister, whom he also tries to attack. As the rescue boat mercifully comes into view, it's clear that Stuart has been completely deranged by his experiences of the storm and of spending time in the company of two such mentally-unstable youngsters.