A person is driving home at night after visiting a river swollen with floodwater. That's all the poem tells us, and then it ends on a striking image: the forest, seen in the dark, is "full of moths."
The poem never says how the speaker feels, so the feeling has to come from the pictures. The river suggests something large and a little unsettling, water pushed past its usual limits. The drive home suggests a return to safety. But the last line doesn't settle things. Moths are small, fragile, and drawn to light, and I'd guess they flutter through the headlights as the car passes. Judging by the wording, the forest seems to hold a huge, hidden life that only shows itself in the beam.
The mood is quiet, hushed, and a bit wondering. There's an edge of unease from the flood and the darkness, but the ending feels more like awe. The speaker seems to be noticing how much is going on in nature that we rarely see.