This poem is about a lady and a knight, for sure, but isn't it also about a river? Everything we see here – islands and trees and castle and fields – is stretched out along the river. It's like the poem's spine. We even think this poem sounds like a river. It burbles and swirls and gushes and roars like a river.
No really, think about it. A river doesn't just have one sound, it has many, and so does this poem. We start the poem with a quiet, lazy, open sound, like a river running flat and wide: "On either side the river lie/ Long fields of barley and of rye" (lines 1-2). Do you feel how relaxed and calm the river sound is here? No hurry, just smooth water and soft sounds.
Then in the next stanza, things start to pick up speed, and so does the sound. Now it's like a river rushing down the rapids: "Willows whiten, aspens quiver,/ Little breezes dusk and shiver" (lines 10-11). Can you hear how those words hurry and dance along the line? We're in a different part of the river, and the sound has changed completely.
The poem does this again, and again, speeding up, rushing and crashing, and then slowing down again, at the end of the stanza, where the short little refrain bubbles along: "The Lady of Shallot." See that? Just like a river, speeding up slowing down, loud, quiet, fast, slow, over and over.