Climb out of the water on the small, red-and-white marbled ledge to the left of the waterfalls when facing them. Pull up onto the taller block next to it.
Turn right, walk Lara to the edge, and use the Look button to check Lara's position. Turn slightly left or right as needed so she's squarely facing the angled block on the other side of the pool.
Now hop back once and take a running jump without grabbing...
...to land on the edge of that angled block.
Turn right and take another running jump to the next flat block.
Slide down the short slope on the left to a ledge with some shotgun shells.
Grab the ammo, turn around, and take a carefully angled standing jump back onto the ledge to the left of the slope.
Take another running jump across the water to grab the ledge below the rock ladder.
Climb the handholds and dismount on the left. This is the ledge where you first entered the canyon.