It is actually there, just hard to realize it when you are looking at it. I almost made the same request (as others have), until I read another player's query here somewhere. I'll try to do the author a favor and point it out for you.
When you are in EDIT mode, click on a land tile and look at the image of it in the left-hand section of the editor screen. If it already has a river it has a blue line and an arrow indicating which sides of the Hex it occupies, and the direction of flow.
To remove a section of existing river, click the blue line on the side of the hex where you do not want it; to add a section of new river click just outside the hex on the side you want it located on and the blue line and arrow appear. The system is logical, so it will determine direction as long as you do not confuse it somehow.
You do have to carry the logic (the course of the river you want) over to connect a series of tiles, paying attention to which sides you want it to travel down, but (if I recall) there is a blue dot in the corner of any hex that touches an river adjacent to it to help you. I might have this part wrong, but you will see it and figure it out when it occurs.
Did I get that right DonQuiche?
BTW I love this Mod, it is priceless when it comes to testing out ideas, creating on-the-fly scenarios and customizing maps for use later.
