The symbol of the brook is mentioned a total of 11 times in chapter 16 of The Scarlet Letter. The constant mentioning of it nearly equates this symbol to a motif, as there is more that just one reason why this small body of water is so influential to the chapter.
Picture the scenery where Hester and Dimmesdale decided to meet and speak secretly. It is deep in the forest, with huge stones and tall trees surrounding their hiding place. They sat in the middle of all this, in the depths of the growing and mysterious vegetation of the forest. The forest is already considered a fantastic (supernatural) place due to the mysterious life forms that live in it. However, Hawthorne extends this enigmatic feature by personifying the very natural elements surrounding the couple:
All these giant trees and boulders of granite seemed intent on making a mystery of the course of this small brook; fearing, perhaps, that, with its never-ceasing loquacity, it should whisper tales out of the heart of the old forest whence it flowed, or mirror its revelations on the smooth surface of a pool.
While brooks are candidly known as "babbling brooks" due to the speech-like sounds that their waters make when they splash against the rocks and the wind, who else -other than the brook- needs to do some serious talking to Dimmesdale? Hester.
While we are not told whether Hester was ever demanding of Dimmesdale when she first found out that she was pregnant with his child, we can definitely conclude that she is not one to hold things back. Therefore, could it be possible that her "never-ceasing loquacity" may have already told Dimmesdale too much about her emotions for him, or "whispered tales out of the heart of the old forest whence it flowed"?
The answer to that will be found toward the end of the novel, when we will see the product of the conversation manifest.
Meanwhile- back to the brook. Hester nervously awaits Dimmesdale, and encourages Pearl to play by the brook. Pearl cries out a very condescending phrase, presumably aimed at it:
"O brook!O foolish and tiresome little brook!" cried Pearl, after listening awhile to its talk. "Why art thou so sad? Pluck up a spirit, and do not be all the time sighing and murmuring!"
While Pearl is both passive and aggressive, this phrase could very well be a combination of the two: a button-pushing way to warn her mother to quit looking for what she won't find in Dimmesdale.
The metaphor extends, explaining that "the brook" had been through so many changes and issues that it has not stopped "talking about it". Moreover, Hawthorne states that the brook resembles Pearl (another hint) but that, when Pearl sees herself in it reflection she appears cheerful, and happier than the brook itself. This is yet another similarity to the situation of mother and daughter.
Continuing with the symbolic connection, Hester sends off Pearl to go somewhere where she "cannot hear the brook speak" as she has to talk to Dimmesdale. However, Pearl goes away singing and teasing her mother. To this, the narrator adds:
the little stream would not be comforted, and still kept telling its unintelligible secret of some very mournful mystery that had happened--or making a prophetic lamentation about something that was yet to happen--within the verge of the dismal forest
Again, this is no different than from would unfold towards the end of the novel.
Now, since the conversation between Dimmesdale and Hester does not take place in chapter 16, we cannot jump into an analysis and comparison between what we see on chapter 16 versus the actual dialogue.
For this reason we can safely conclude that the brook is a representation of the emotional state of Hester--a woman who yearns desperately the freedom to manifest and express herself and her love.
Sadly, she continues to be repressed by the "things in the forest" that keep looking down and witnessing her sad, never-ending search for a response from Dimmesdale.
The brook serves a couple of purposes. First, it symbolizes nature's sadness over Hester, Pearl, and Dimmesdale's situation. When Hester and Pearl are walking through the forest, Pearl mentions that the brook "sounds" sad. It is not until Hester and Dimmesdale are together and Hester's letter removed that the light (nature's approval) breaks through the treetops and shines upon Hester.
Secondly, the brook serves as a division between the couple and Pearl. After Hester and Dimmesdale have had an opportunity to talk, Pearl shows up on the other side of the brook, and her disapproval of Dimmesdale and Hester (without her letter) is obvious. The years of Dimmesdale's guilt and secrecy divide him and Hester (the parents) from Pearl (the physical evidence of the couple's sin), and she does not cross the divide or amend her disapproval until Hester returns the letter to her chest.