Although Merwin is freewheeling here—holding to no formal patterns or constraints—he is using a couple of technical tricks that are consistent throughout the poem. Just looking at the poem before even reading it, you can see that the lines are long. He's also doing this consistent indent thing every other line, which kind of sweeps us along from one line to the next. It also helps organize the punctuation-less poem, giving our eyeballs (and our brains) a little break as they glide from one line to the next.
As we get into the poem (starting with that "if" clause), we're waiting for a comma to come along, but… it never does! In fact, there's no punctuation whatsoever in this poem. Be sure to take a deep breath before you attempt to read this puppy out loud. The effect, much like the long, sweeping lines, is that the poem has a continuous, fluid, and uninterrupted flow. Merwin has to be careful (and he is most of the time) to break his lines at a natural place so it isn't weird and choppy to read through. The result is smooth moves for the most part, and a quick, though dense, read.