Also if the province is invaded from more than one province I believe that the battle depends on where the leadr of the invasion comes froms, for example whenever I invaded Valencia from Aragon I always got a bridge battle, but not when I invaded from Castile. So what I often do is move in the multi-starred genral unit alone from Castile to Valencia, and the rest of the invasion army from Aragon to Valencia, and avoid fighting on a river.