This puzzle is on top of the mountain.
Map
River Obelisk
South #1
>
>
Look at the rest of the island for something that looks like a puzzle.
There is a yellow panel at the edge with a weirdly shaped line. It shows you what shape you should look for.
-
Stand next to the panel and look at the water coming from the waterfall. The little river become a puzzle from this perspective. Solve it like you would with any other puzzle.
Here's a screenshot in case you need it
-
Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
Look at something with water.
Look downwards along the side of the mountain.
Intro
This is the only hidden puzzle that the game helps you with, so this is a good place to start with the hidden puzzles. It shows how the hidden puzzles work and what you need to look for.
-
This is a good point to try and find the puzzle. It will still feel rewarding and you will learn how these hidden puzzles work.
Some more hints if you are stuck:
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE