To complete the thirsty gods tomb in Mission of the San Juan in Shadow of the Tomb Raider cut the two rafts loose. Climb up to the raft near the ledge and make your way around to the upper area. Use the lever to turn the pillar so that the water flows into the weight on the right. Jump down and use a rope arrow to attach the rope wrapped beam to the pulley. Climb back up. Use the lever to turn the pillar so that the water flows onto the wheel until the gate is open.
Then turn the pillar back so that the water flows onto the weight again. Make your way over to the gate by swimming around the left side of the area to avoid any piranhas. Climb around to complete the thirsty gods tomb. .
As found on Youtube