I think you mean immediately. River has a journal of all the Doctor's faces but not in order, she knew 10 was early in his timeline because she spends most of her time with 11 but she didn't know the exact time of when they first met. She probably assumed 10 would know a bit about her. She was more surprised by the fact that 10 didn't know her at all.
Her first act was to try to sync diaries, so she must have presumed he was a later incarnation. The younger version of 11 hasn't started keeping a diary, while the older version has. The older River from the Library would remember that much.
Obviously, the real answer is that Moffat assumed 10 would have further adventures with a younger River, but that never materialized due to Tennant leaving the role.