The falling action of a story is normally defined as what occurs after the climax or high point of the story, leading towards the ending and the resolution of what happens. Given this definition, the falling action of "By the Waters of Babylon" occurs after the epiphany or moment of insight that the narrator experiences after discovering the "dead god". Thus the falling action consists of his return back to his father and his tribal grouping before his conversation with his father and the resolution of the story:
I had no fear after that - I had no fear going home, though twice I fought off the dogs and I was hunted for two days by the Forest People.
Thus we are brought from the climax of the story through to the end of the story, where John the narrator shares his new-found knowledge with his father and they decide what they are going to do about it.
The falling action would be when he's is on his way home, with no worries of any sort. Also, when he gets home and talks to his father about his journey and tells what he discovered and learned