How would you describe the musical quality of "The Brook" by Alfred Lord Tennyson?

Tennyson's use of onomatopoeia throughout this poem helps to create the effect of a brook bubbling, like the titular brook, down a valley. Tennyson uses alliteration—a "sudden sally" and a "sparkle"—to create images that appeal to the sense of hearing, as well as to create a visual picture. The alliteration on "s," and particularly the word "sparkle," seems to echo the splashing sound a brook would make if it were moving quickly and lightly, indeed in a "hurry" down the valley. Other onomatopoeic words that connote the sounds of a stream include "chatter," "bubble," and "babble."
The poem's rhythm and meter help to contribute to this effect, too. The short stanzas and near-hypnotic rhythm help the poem "draw [us] along" as readers with the brook as it flows. Repetition and parallelism, too—"and here and there," "the brimming river"—replicate the sameness of an ever-moving, repetitive stream repeating the same motions over and over as it flows.
The idea of the brook as a musical accompaniment appears explicitly toward the end of the poem, as the speaker suggests that it makes "the netted sunbeam dance." Words like "slip" and "slide" also suggest dancing, a sort of rhythmic motion which would accompany the continuous sound of the brook.

Tennyson's "The Brook" has a musical quality that imitates a flowing stream. The alternating lines rhyme, creating a musical sense, and many of the words that end the lines are repeated, such as "flow" and "go." The repetition of words and rhymes creates the lyrical quality of a brook that is constantly running and that has a repetitive quality in the sound of its water moving downstream. 
In addition, the lines in the poem are very short, running to three or four syllables rather than to the ten syllables that many of the poems used at the time. This creates the kinds of quickly moving sounds that a brook might make as it tumbles downstream. In addition, many of the lines in the poem use alliteration, or the repetition of sounds at the beginning of words, such as "sudden sally" and "men may." These sounds imitate the rushing sound of water in the brook.