How to get your rivers in the right place

[s0meguy]

edit: I'll just make a thread about this because people probably don't look at these stickied threads much.

Question: Soil around rivers tends to be fertile right? So when making maps of fictional areas I always have a tendency to put a bunch of trees next to the rivers, and some forests sometimes, especially in an area where 2 or more rivers are relatively close to each other and lakes because the river is dumping all the soil nutrients there in the lake, so the soil around it would probably be fertile enough to support a forest. I'm always having a hard time over whether the soil around rivers is fertile, and if it is, what kind of vegetation i should put along side it, and also where to put forests in general. I gathered that they are usually next to mountains, because they trap moist air/rainclouds. Any good rules of thumb?