tolkien treated the recurring image as material for his mythology. númenor is an island kingdom destroyed by the sea because of its pride. its drowning grew around the same image: a mountain of water moving across the land.
i have in this dream a terrible recurrent dream (beginning with memory) of the great wave, towering up, and coming in ineluctably over the trees and green fields.
tolkien later gave the dream to one of his characters. in the lord of the rings, faramir dreams of the great wave over númenor. tolkien wrote that he had given faramir his own dream. he also reported that the nightmare stopped after he wrote it into the work.
the end of the nightmare is tolkien’s report, not a controlled result. however, his letters show how recording a dream can change its place in waking life. he named the image, described it, and gave it to a story.