Why would you call "Dover Beach" a nature poem?

Whether or not you would call "Dover Beach," by Matthew Arnold, a "nature poem" depends upon your definition. Certainly, Arnold uses natural imagery as the backdrop to what he is trying to convey. Natural elements such as the "calm" sea, the "full tide," and the "fair moon" serve to describe the sort of evening the speaker is witnessing in the "tranquil bay." The sea, however, is not described for its own sake in this poem; it is not an ode to the beauty of nature. Rather, the sea, whose waves "bring / the eternal note of sadness in," is used as a metaphor for faith, which, Arnold fears, is "retreating." While faith was once "full" and "lay like the folds of a bright girdle furled," it has now withdrawn, leaving the people of England "here as on a darkling plain... where ignorant armies clash by night."
Arnold's "Dover Beach" is usually considered to be an expression of the crisis of faith experienced by Victorian England, where discoveries in anthropology and geology began to unseat long-held beliefs about the Bible and Christianity. So, rather than being a "nature poem," "Dover Beach" is a poem which uses vivid descriptions of nature as a vehicle to convey the poet's uncertainties about faith.