The river is moving. The blackbird must be flying. (Section XII)
If you wanted to argue that the poem has some kind of forward-moving narrative, here's a place to look. A-ha. He uses the present tense after using the past tense for most of the poem. Maybe the poem is set in the springtime, but the speaker spends most of it reflecting on the winter that has passed. The snow has melted and the river is finally starting to move again.
It was evening all afternoon. It was snowing And it was going to snow. (Section XIII)
The final section moves back into the past again, but with a nod to the future ("it was going to snow"). To say "It was evening all afternoon" is to put the cart before the horse, chronologically speaking. It's kind of like saying, "It was tomorrow all of today." There are no clear boundaries between time periods in this poem.