The speaker steps into a swamp they haven't visited in a long time, and the poem moves in short, hesitant glimpses, much like walking through uncertain ground. First comes a single alder standing at the water's edge. Then a admission: this is somewhere the speaker "had not been," and in the meantime the place has "grown natural," as if it has gone back to being fully itself without anyone watching.
The last image is the strangest. A "bit of rainbow" is "maimed as a dead leg." Judging by the setting, this could be an oily sheen on the water or a scrap of colored light, but the poem doesn't say. Whatever it is, the comparison turns beauty into something injured and limp. The mood is quiet, uneasy, and a little awed. The speaker seems both drawn to the wildness and unsettled by it, noticing that nature carries on in its own way, with beauty that can look damaged and unfamiliar.