Modified Rondeau
Alright folks, let's talk some form and meter. "The Tide Rises, the Tide Falls" is a rondeau (we'll get to that in just a second here) that has no fixed meter for every, single line of the poem. Take a look at line 2 to get a sense of what we mean:
the twilight darkens, the curlew calls.
This line is more or less written in a pattern of beats called "iambic tetrameter," where each line contains four (tetra- means four) iambs. Now, we said "more of less" because, well, the third beat in this line contains two unstressed syllables, followed by a stressed syllable (remember, an iamb has only one unstressed syllable). This beat is known as an anapest. Still, even though it interrupts all the iambs going on here, the line is still considered iambic tetrameter.
Line 11 gives us another great example of an iambic tetrameter line that is, ahem, interrupted:
The morning breaks; the steeds in their stalls.
In this line, the first three beats are iambs and the fourth is an anapest. Now, even though these two lines might give you the impression that the poem is written mostly in iambic tetrameter, you also have to consider the meter of the refrain, which is totally different:
the Tide rises, the tide falls.
This line almost defies classification. It starts with an iamb, then switches to a trochee (the reverse of an iamb), then another iamb, and then one additional stressed syllable. Alternatively, you could say that the line starts with an iamb, then switches to a dactyl (a stressed syllable, followed by two unstressed syllables), and finally concludes with a spondee. Whew, that's a lot to take in. Let it sit for a minute…
Okay, that's long enough. If your head is still swimming in -ameters, though, we've got good news. Even though this poem doesn't have a fixed or regular meter, it does have a very regular rhyme scheme that offsets the irregularity of the meter. The scheme is as follows: AABBA AACCA AADDA, where each letter stands for that line's end rhyme. It turns out that this poem is actually a slightly modified rondeau, a type of poem that contains three stanzas, 10-15 lines, a refrain ("the tide rises, the tidefalls"), and a rhyme scheme that is kind of like the one we just showed you.
Alright then, now it's time to make some sense out of all this stuff. This poem is a mixture of both regularity (rhyme scheme, number of lines in each stanza, a refrain) and irregularity (no set metrical pattern), of stability and instability. The poem's mixture of both regular and irregular elements mimics the rising and falling of the tides mentioned in the refrain and the ups and downs of life.
At the same time, "The Tide Rises, the Tide Falls" is a poem about the permanence of nature, and the shortness or fleeting-ness of human life. The traveler's death seems unexpected and then, to add insult to injury, his footprints are completely erased. In a way, the poem's constantly-changing meter emphasizes this theme of impermanence, while the more regular elements emphasize the fact that even though people come and go, certain things will always remain the same (the motions of the tides, for instance).