Your condition is cute, but doesn't seem like much of a condition at all?
About names, there'll be something. Maybe "Post Office" and "Runner"? That would go a bit into the direction i imagined earlier before the card's name was set to River. But for now we got an image, and images are nice, and the name can be changed when the card itself is somehow set on.
You are right about this. I don't like the idea of trashing it, but probably i need another condition.
I thought about an Overpay: "Gain a River from the River pile per $ you overpayed" Doesn't seem that exciting to me, though. Besides, Overpay isn't exactly my favourite mechanic...
Alternatively, a restricted on-gain: "When you gain this, you may put a card from your hand on top of your deck. If you do, gain a River from the River pile." This does allow a few cute options, but you still have to spend $4 on each River you want to get (as you get only one per LV). Also, at the start of the game, this means either wasting a possible $5 turn for a Village and River, or using a $4 turn and harming your next one. On the other hand, hey, there may be some combos here, but i don't mind that.
Hmm. You know, Black Market allows outright buying non-Supply cards. Perhaps this could access River (and only River) through that type of mechanic, even if it was relegated to the Buy phase?
That's something i considered: "While this is in play, you may buy River cards from the River pile during your buy phase.". It's not bad, but i'm a bit worried that River would end up costing $3*/$2* or something like that, and that would make it look like Peddler, a supply card. If it costs $0*, on the other hand, it looks a bit strange that you'd "buy" it. I mean, it neither costs something (like BM cards) nor is it in the supply (like other buyable $0s are).
werothegreat's idea isn't bad either, though i wouldn't have it "spend" the buys. Diadem doesn't "spend" your actions, either. Sure, there is only one Diadem (edge case: Counterfeit), but you don't want to gain massive amounts of River, anyway. The only thing i don't like about the option is that it doesn't really feel different from Hermit. On some boards, it's going to be the same.