The poem begins with a quiet loss: the rising tide covers a sandbar, and only odd, stumbling waves hint that it was ever there. Along the banks, currents pull debris loose and carry it off to be buried in the marsh mud. Everything here is being erased or hidden, slowly and without fuss.
Then the speaker steps into the scene, sitting in an old army tent and wishing the "green heat" would explain why life runs so close beneath the surface of the earth. It's a question about hidden energy, and the speaker doesn't get an answer. Instead the wind picks up and takes over the afternoon, and the speaker gives in to it, thinking "what a wind thinks / miles past land."
The mood is drowsy, watchful, and a little wistful. The speaker stops reaching for explanations and lets the mind drift outward, away from the shore and the self, which feels like relief.