I am a fanatic about making backups. And I have restored my entire game from backup before. However, this would overwrite everything that happened in Sunset Valley in the two weeks since I had last made a backup! I really didn't want to lose that gameplay.
I went looking around, and found this article on the official Sims 3 forum. It turns out that you can indeed ONLY restore one neighborhood from backup. In this case, I was able to successfully restore Riverview with my two-week-old backup, while leaving Sunset Valley unchanged.