Would that be RIVera, or Riviera? Either way, it's awful.
River is bad enough, but pinning an A on the end for no good reason makes it ludicrous. As for choosing the gender ... really, if you take a word name and stick on a typically female ending, what do you expect people to do? Cry out in rapture 'At last! An -a name for a boy that isn't
Isabella!'?
The only combo that comes to mind is
Niall Rivera; that is, assuming you want it to be a male name.