It doesn't quite make sense to me why more leaders can cross a river border than a clear border during the impulse phase. Is there a playability reason for this or is this historically accurate? Thanks.
The rivers were prime transportation byways during the period. What might be confusing you is that the river runs through the border (you can see them lightly on the map) - the blue border is just to let you know that you are using it. Thus, the area border runs perpendicular to the river itself.