Alliteration is the repetition of consonant sounds at the beginning of words in close proximity. While there might be some subtle instances of alliteration in "Brook," it's not a prominent literary device in the poem.
Here's the full text of "Brook" to show you:
> I chatter, chatter, as I flow
> To join the brimming river,
> For men may come and men may go,
> But I go on forever.
Do you have any other poems you'd like to explore for examples of alliteration?