Help with So Far

I’ve made it to the Grassland, and have figured that I need to go through the tree’s shadow to escape being chased and have found the rivulet, but I can’t figure out what exactly I should be doing with it.
It feels like I’m missing something obvious!


This world has a lot less puzzles than the other worlds. Beyond taking things, there is exactly one thing you need to do with the rivulet.

Slightly stronger hint:

This is a world you need to be in exactly twice, and the two times can happen right after each other.