No site Tardis Data Core , ele fala sobre isso:
Upon saving the Doctor's life (after nearly killing him herself) by giving up her remaining regenerations, River ended up in the best hospital in the universe. It was there that the Doctor gave her the diary. This was done to make sure she kept track of their meetings to keep their timelines from becoming corrupted.
Então:
In the 52nd century, in Stormcage, River was writing about one of her outings with the Doctor before taking a phone call from Winston Churchill and escaping to help her husband.
After the Doctor reset the universe at the cost of his own erasure from existence, River temporarily gave her mother her diary as a wedding present to bring him back. Later, the Doctor returned the diary to River, telling her that all the writing had returned, but he hadn't peeked; foreknowledge is dangerous.
A última linha soletra. Se o Doutor tiver as informações contidas no diário de River, ele terá conhecimento que pode ser perigoso não apenas para si mesmo, mas para o Universo como um todo.