She always knows where he is, now.
She sits at her desk and works through paperwork that tells her where he's been, accounts from people who have met him, details of the worlds he's changed, and does not go to him.
She doesn't need to follow him, yet. She will always know where to find him, so long as her lives run beneath his skin.
They're part of each other, now. River can't change that. After all that's happened, she can find no reason why she would.
He is her pole star, her guiding light, and she will be his.