Mysterius Spice

[Smash]

spice makes 3 food and 4 trade in my game.

Only thing I can think of is the city square terrain itself or governent type.

[colossus]

I have long noticed that a spice tile normally generates 1 food & 3 arrows, but when I build a city on it, or the spice happens to be rivered, it generates 3 food & 3 arrows. I could understand that builing a city on spice triggers some 'farming' on the tile so there is increased food. But I could not explain why river increase food. Anyone can explain to me?