This happens often enough.
Either (a) the student moves to the new university with the supervisor, in which case supervision just continues as before in the new establishment, or (b) the student stays and the supervisor goes.
In situation (b), unless the student will be submitting extremely soon after the supervisor leaves, they are allocated a new supervisor in the department. Even if there is no one who actually works in the same research area, a nominal supervisor is needed for administrative purposes. In general, the old supervisor continues to supervise the thesis - so in your case you'll just email/post drafts back and forth, and spend more time on the phone, while getting day to day support if you need it from the new supervisor allocated in the department. If the new supervisor also has some knowledge of the area they may provide useful input too.
Talk to your supervisor about your concerns, and ask her what she envisages. However, if you're already at the stage of doing revisions you should be fine. It'd be more awkward if you were doing a scientific project and were still in the early experimental stages.