Board Thread:The Panopticon/@comment-7302713-20130404114052/@comment-5473162-20130404115420

River actually says in The Time of Angels that she has pictures of all the Doctor's faces, but he never shows up in the right order. She initially assumed that the Tenth Doctor was a later Doctor before she realized how young he looked compared to "her" Doctor.

Put it this way. Forget everything you know about the Doctor's personal timeline. River meets a man she knows is the Doctor but she doesn't know which one and doesn't want to spoil anything. How is she supposed to know what's safe to discuss and what's not? That's where the whole "doing diaries" routine comes in :)