By the mountains— near the river Murmuring lowly, murmuring ever, — (lines 25-6)
We think this is a good example of how this poem is tied into the rhythms of nature. Poe doesn't just happen to mention a river, he gives us a real feeling for how it sounds. In these lines, it almost becomes a character in the poem, speaking quietly to us as it rolls by. This isn't designed to make us scared or upset, instead it gives us a kind of quiet sense of the world we're exploring. It's like a little detour on this sad journey, a peaceful little bit of nature poetry that lets us shift gears for a second.
By the grey woods,— by the swamp Where the toad and the newt encamp— By the dismal tarns and pools (lines 27-9)
This is definitely not about the happy or beautiful side of nature. These lines are all about the icky, slithery, unpleasant side of the natural world. We've heard some beautiful descriptions in this poem, but now we're just getting pulled through the muck. Everything is grey and dirty and unhappy. Gross, we know, but this is a Poe poem after all, so there has to be some of that.