At first we might think we're in a Disney story with Snow White and her dwarfs, but then we realize the speaker is just using some common romantic imagery in the setting to keep with the tradition of the ballad. The natural scenery of the park is a typical place for two lovers to meet (or, in this case, to break up), and so we have a setting that puts us firmly in this the folk ballad tradition.
The setting does more than just give us a sense of history and genre, though. If it weren't for the setting, we wouldn't really see the girl's message to the speaker as clearly as we do. She uses the willow trees and the grass growing on the weirs as similes for how the speaker should approach life. So the setting becomes a kind of means towards the end of trying to snap some sense into the speaker. ("Look around you! Can't you just chill like the trees are chilling?") But even with the girl's nifty use of her surroundings, the speaker is young and foolish and just doesn't get it. He's as blind to the setting as he is to her wishes. Maybe it's fitting then that his eyes get filled with regretful, mega-sad tears.