Meeting Jiahto at the Toto Lake is a part of Sidon of Zora main quest in Zelda: Tears of the Kingdom.
You obtain this task after completing the ‘Meet Sidon at Ploymus Mountain’ objective in which Link meets the King of Zora, Sidon, at the Mipha Court.
Sidon tells Link that he has to stay confined in Mipha Court because he needs to protect the region’s water supply. There is sludge falling from the sky in Zora and Sidon has to make use of his water manipulation abilities to keep the water clean from the sludge.
Sidon also expresses his concern about the sludge and suggests that it might be due to the Sky Islands and ruins. He tells Link that he has employed a historian named Jiahto to study these Sky Islands, ruins, and any historical findings related to them.
Sidon also suspects that they might have some relation with the disappearance of Princess Zelda. He asks Link to meet Historian Jiahto who might have some information leading to Zelda. He is currently carrying out an investigation at Toto Lake and Link has to make his way there to converse with him.
Before leaving to find Jiahto, Sidon also tells Link about the Skyview Tower that was built at the request of Purah for the purpose of making a map of Zora.
Finding Jiahto is quite easy and after you have done that, he assigns you another quest ‘The Broken Slate’ which is another main quest in The Legend of Zelda: Tears of the Kingdom. So before reaching this quest, you must know where to find Jiahto.
Where to find Jiahto at Toto Lake in Zelda: TotK
As mentioned by Sidon in Zelda: Totk, Jiahto is at Toto Lake which can easily be reached by continuing northwest from Mipha Court where you met Sidon. Toto Lake is marked on the map below.
To go there, climb the rocky mountains and soon you will see a stone structure on top of which you will see Dunma. Paraglide your way down to this structure and converse with Dunma, Jiahto’s guard.
Dunma will talk to you about how Jiahto always investigates dangerous things and then she will show you the entrance of the cave where Jiahtio is investigating the ruins.
Paraglide down to the cave and enter it. You will see Jiahto looking at a broken slate there.
Talk to him and he will tell you that this slate was written by Zora ancestors and he cannot read it due to it being broken.
He will express his wish to fix this slate and this will begin the next main quest of Tears of the Kingdom, ‘The Broken Slate’, in which you have to find the missing piece of the slate.