Hi
This is not going to be much help I think, but I did manage to draw a river. I think I did it by putting the mouse over text box at top of screen & choosing and roads .. from the menu. There might be a differentb option, but you can deff draw rivers. I did this before any hills. I got most of the map with a red overlay Which would not allow me to place a river in it, so I could only spot along a river or across The Edge corners.
I seem to remember forum posts from long ago, that this is by design. So The Game does not have a river crossing center of map & one bridge, Which would be an AI problem, let alone a player one. I may be wrong, but the red overlay Seemed to follow this idea.
The facility is there. you just have to find it.
Cheers