Intanet seems to think 山椒 is just a lesser used name (and says don't confuse it with sansho pepper as that's from a related plant - but other places say 'sure, it's sansho') - dunno about 川椒. I reckon it's all pretty much variations on the theme of prickly ash... I can tell you those first characters are mountain and river, literally! But that information doesn't help you in the kitchen does it.