Post-second Siren, where do I go?

[Relle]

I got Refresh, I understand they want me to purify some poison rivers, and I know I saw a river I couldn't get past, but I don't remember where it is.

[Nibbie]

Since you can't swim down yet, refreshing water isn't too important right now. You are actually heading as far east as you can get if you have refresh. See, refresh works on more than just inanimate objects. Hopefully that points you in the right direction.