When do desperados appear? I gather there are several up-turned cards that you can choose from for harvesting. Is he in with those up-turned cards? Or does he only show up once you've used a harvest-card and you're flipping over a new card to replace?
The latter. There are two desperado cards, one per river, and if it gets turned up as a replacement for a harvest card, it gets played immediately, then another harvest card gets turned up. Or possibly the second desperado card. At the end of your turn, there will always be four face up harvest cards.
As for the turned up harvest cards, you don't choose among them, you can only choose the first one (furthest from the draw pile), or send it to the end of the line of face up cards (nearest the draw pile). So you can do the current available harvest, or not harvest at all.