When you've opened the second salvage crate, turn around and move down into the little pool with the rope-wrapped float.
Position the float beneath the climbing wall on the tall rock column. You can do this by dragging it across the pool using a rope arrow or by pushing it. Just stand close and press the Melee button to give it a shove.
You may need to tweak the positioning by pushing/pulling it a few times.
Once the float is in position, jump from the edge of the pool onto the float.
Then jump and latch onto the climbing wall.
Climb up and around to the right to reach the top of the column.
Here you'll find a little cairn (1/5) with a metal helmet on top. Stand near it and press interact to dismantle the marker and unlock the Cairn Raider Challenge: Find and excavate 5 of these for bonus XP.
NOTE: If you're having trouble positioning the float in order to reach the climbing wall, there's a second float that might make things easier. Return up the hill to the wooden walkway and drop down into the stream bed below. Shoot out the plank barrier and push the second float over the edge into the pool. Drop down after it.
Now you can arrange the 2 floats like stepping stones to reach the climbing wall.
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