One thing is for certain in this poem: sounds repeat themselves. The most basic form of sonic repetition in this poem is, of course, rhyme. If you've read our "Form and Meter" section, you know that each stanza in the poem has a rhyme scheme of ABAB (where each letter represents that line's end rhyme). This ensures that the first and third lines of each stanza are connected in some way (via sound, if not other things).
In addition to rhyme, however, the speaker repeats sounds in all sorts of other ways. His favorite techniques? Alliteration and internal rhyme are just two. You can go to just about any stanza in the poem and find several examples of alliteration. In the fourth stanza, for example, you've got the Th sound repeating in "there," thy," and "the," whereas in the sixth stanza we get the S of "soon," "summer," and "scream." In addition to the rampant alliteration there is internal rhyme galore here, too. Just check out the long I sounds in line 6 ("Might" and "flight"), the long E's in line 23 ("scream" and "reeds"), and the long I's of line 30 ("Guides," "sky," "thy," and "flight").
Okay, so we've established the fact that there's a lot of sonic repetition in this poem, but what effect does it have? "To a Waterfowl" is a poem about order. Recall that first the speaker describes the waterfowl us just coasting along, floating in the sky, with no rhyme or reason. Eventually, he realizes that in fact the waterfowl's course is being guided by a deeper "Power," and it is this Power that will also guide him through the life.
In other words, the poem starts with an idea of disorder, or the lack of order, only to end up by saying that the universe is a very ordered place. Consequently, we get a very well-ordered, well-patterned poem. Once you start digging, you'll realize that the sounds of this poem (not just the rhymes, but the vowels and the consonants) repeat themselves over and over again. The poem is like a well-woven basket, with lots of interconnecting strands.