Your starting position might not be near a river, but other than that, you *can* make sure your other cities are on rivers.They apply the same bonus. One applies to river cities, which can't be all your cities, unless you're really lucky or editing the map. The other applies to all your cities that reach at least size 6. You can make all your cities reach size 6. You can'tmake all your cities riverside, until they add the digging-the-Panama-Canal wonder.
You don't need to edit a map. You just focus on river sites, just like how some people will focus on coastal sites or particular luxury sites. Rivers have so many advantages anyway (free gold + irrigated farms + Watermill/Garden/Hydro Plant + defensive options) that on vanilla games I will ideally try to found my cities on rivers if at all possible.
Maybe this is a case of chicken or the egg. Do you choose your city sites first and then choose a pantheon based on it? Or do you choose some of your city sites based on your pantheon belief? Or a combination of both?
By random chance, it is unlikely that all your cities will have wine/incense or gold/silver but if you decide you want that pantheon, you simply settle your cities intentionally near those particular resources. Same thing with rivers.
Of course this might work better for players who are comfortable settling their cities far apart (I will do this for natural wonders, even if I am not Spain). I do realize that some people like to clump all their cities next to each other, in which case they are far more limited in terms of geography!