[MaOs]

It's impossible to modify rivers (there's a way to "delete" rivers adding new land masses above them) or make new ones. If you look the map from under it, you will see that there are two types of models of floor (water&terrain), and pieces of terrain are on the water-type , modeling them. And, because you can't modify the default map (as longer I know), you can't delete or make new rivers.