The brook, the speaker of the poem, explains its origins in the first line of the poem, claiming to have “come from haunts of coot and hern,” meaning ponds or marshes frequented by coot and heron (two kinds of coastal and freshwater birds). This description of a location both gives the brook a starting point from which it can begin its journey, and is significant because it foregrounds nature—the “coot and hern,” along with their “haunts,” meaning their natural habitats. In this way, the first line hints at the brook’s attitude toward nature versus humankind; it is altogether focused on the natural world around it (of which it is also a part), and sees nature as powerful, important, and enduring. (Humans, in contrast, are just insignificant and temporary visitors—something the brook will explicitly spell out later.)

In the second line, the brook begins its journey with a big rush of energy. The word “sally” suggests that the brook surges forward enthusiastically, but the word can also have a militaristic meaning, suggesting that the brook is making a sudden raid or assault. While the brook isn’t exactly harsh and combative throughout the poem, the martial language emphasizes that the brook is nonetheless a powerful force to be reckoned with. This ties in with the broader idea that nature is powerful and enduring.

The brook is energetic and lively throughout the bulk of the poem. For instance, the word “sparkle” in the third line gives the brook a certain playfulness, and implies that sunlight is reflecting off of the water’s surface. In the fourth line, the word "bicker" means that the brook is making a pleasant trickling sound as it flows into the valley; however, the other, and perhaps more common, meaning of the word bicker—to squabble or argue—subtly gives the brook a more human quality, setting the brook up to be an extended metaphor for human life. In this part of that journey, with its quickness and energy, the brook is like a young child.

The first stanza showcases the structure and meter that persists for the rest of the poem. As a ballad, "The Brook" is broken up into stanzas of four lines, which breaks the poem up into more digestible chunks. The lines are written in common meter—a commonly used meter that alternates between lines of iambic tetrameter and iambic trimeter, or four metric feet per line and three metric feet per line, each with an unstressed-stressed pattern of syllables. However, as the first stanza shows, Tennyson put a little twist on common meter:

I come from haunts of coot and hern:
I make a sudden sally
And sparkle out among the fern,
To bicker down a valley.

Note how the second and fourth lines of the poem diverge slightly from common meter by ending in an extra unstressed syllable. In other words, the second and fourth lines of the poem are written in iambic trimeter, or three metric feet of unstressed-stressed syllables—with an extra unstressed syllable floating at the end. This is called a feminine ending, and is actually quite common in poetry. The purpose of feminine endings in "The Brook" is manifold. For now, notice how the feminine endings actually draw attention to the masculine (stressed) endings of the first and third lines: "hern" and "fern." In giving these words special emphasis, the poem emphasizes the importance of the "hern" and "fern" themselves, as elements of nature. In other words, the feminine endings in this stanza actually underscore the poem's broader claim that nature's power and importance is unparalleled.