We know of three incarnations of that person, although only one of them used the name River Song. These three incarnations are:
- 1. The original Melody Pond, seen as a baby in "A Good Man Goes to War" & as a young girl in "The Impossible Astronaut"/"Day of the Moon". We saw her begin her first regeneration in New York at the end of "Day of the Moon".
- 2. Mels (short for Melody), seen as a girl & as a young adult in "Let's Kill Hitler". We saw her regenerate in that episode & she said that the last time she'd done so had been in New York. The surname used by this incarnation hasn't been revealed.
- 3. River Song, who used up all her remaining regenerations in "Let's Kill Hitler" to save the Doctor's life, after she herself had poisoned him. We saw her die, again saving the Doctor's life, in "Forest of the Dead" but her "data ghost" was saved to the Library's planet-sized hard drive. We saw a manifestation of that "data ghost" in "The Name of the Doctor".