The puzzle is relatively far from the obelisk.
Map
River Obelisk
North #1
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It's in the marsh.
It's IN the marsh.
The familiar panel is in there for a reason.
Down the first set of stairs near the entrance, you need to move the bridge so that the color of the water matches the color of the pipe.
Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
It's in one of the underwater rooms.
The one just after the first bridge.
Show hints
Intro
There are a few like this one in the same area.
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE