This Fluent Mam Monolith (2/7) can be found on one of the two islands at the north end of the river, near where the people are fishing. If you're returning here later, the nearest camp is the Fishing District Base Camp.
Examining it reveals a clue: "The waters below protect me from the waters above."
Selecting the monolith on the in-game map reveals the area in which the hidden treasure can be found: near the waterfall to the north.
You can go right for it or take a little detour to collect some condor feathers and a document. If you want to do that, climb the tree near the base camp and jump to grab a handhold on the rock wall. Climb onto the ledge above.
Turn left, and jump over to another set of handholds. Climb to the ledge above on the left.
Here you'll find a condor's nest, which you can loot for feathers and/or the Keep Calm and Carrion Challenge. Use the horizontal branch to swing over to a handhold on the wall beyond.
Then scramble up onto the ledge...
...to find the document (8/42) The First Emperor (Quechua/Founding Paititi). The tunnel leads to an area you can't access yet.
So dive into the water to get back down.
Survival cache #11 (above) and the jade ore marked on your map by one of the outcasts during the Dice with the Dead Mission can be found beneath the waterfall. As with all monolith riches, this cache only appears once you have actually deciphered the monolith. (It's so dark down there it's impossible to get a good screenshot. Use the annotated level map if you need help locating the cache.)
There's also more jade ore on a little ledge above the deposit in the wall.