The tomb entrance is opposite the Aqueduct Ruins camp, and you find it after the Trinity attack on the valley. Te path to the tom is very easy. Jump onto the fallen tree trunk and over onto the ledge. Inside, keep going down, pass the waterfall and drop down. On the right, you find a narrow passage that Lara can squeeze through.
Right past the passage, use the rope arrow to attach it to the post at the other side. Walk over the rope to the other side. In the round chamber, you find a Relic. By the relic, you can climb up and follow the path into the camp.
Approach the body of water (on the right, by the boat, there is a Document). First of all, you need to make it over to the isle. Approach the turning wheel, shoot a rope arrow towards the boat and jump into it, after you pull it closer. Cut off the rope and shoot an arrow towards the mechanism before the isle. After you get closer, cut off the rope and jump onto the stone platform, as you pass by. On the isle, you find several coins and a Document.
While on the isle, you can spot another boat under the waterfall (the screenshot), a bit ahead. Shoot a rope arrow and attach it to the mechanism. Then, jump into the boat, while it is passing you by and cut the rope. While on the boat, you need to look for another mechanism at the other side and shoot another rope arrow at the right moment (otherwise, you will crash against rocks). When you manage to stop the boat, start turning the wheel to pull yourself towards the platform. You can then jump out of the boat, up, where there are stairs and the book behind them. Along your path, you also find an Archivist Map and a Relic.
Reward: Ability "Anatomical Knowledge" (you can see hearts of animals and you deal more damage after you shoot them).