You know what? Screw it, I'm just going to have the underground river emerge in the forest and go into the sea. I don't see any problems with it, and it's not as if the river can't have another one branch off from it to a pond or something.
It works, nothing says it won't work, and it's flexible. I like it.

Thanks for the help, guys!
I'll be sure to drop by more often as I work on more maps for future sessions and campaigns!