Played by Alex Kingston, River Song is one of the most fascinating characters in Doctor Who – and here's her complete timeline and a recommended viewing order. River Song is the time-traveling archeologist who has one of the closest relationships with the Doctor in the entire show. The two even married, albeit in an alternate timeline that was erased from existence. What makes River Song unique is that she and the Doctor meet one another in a completely different sequence. They frequently have to check diaries to figure out where they are in one another's personal timelines, ensuring they don't accidentally drop any "spoilers" for each other.

https://screenrant.com/doctor-who-river-song-chronological-order-timeline-explained/