Jump up to grab the handhold above.

Traverse to the left around the corner. Pull up on the beam and jump to grab the next beam ahead.

Pull up and turn right to face the waterwheel. Jump up to grab the small handhold.

Then jump to grab the next handhold on the right.

Traverse around the corner and pull up on the beam.

Take a look below and to the left. You should be seeing a metal ring on the back wall of a ledge. Angle Lara to that ledge and jump there.

Use the Grapple to catch the ring and step off the edge. Climb down the cable line until you see another ledge below.

Swing and jump there. Turn right and walk on the beam. Turn left and jump to grab the horizontal bar. This is a mechanism that Lara's weight lowers and opens the gate below. More water flows into the room, so release the bar drop into the pool.