I think the last FAQ, dated Q1 2019 (which can be found in the link below) answers this one quite well:-
“Rivers: Q: When crossing a river for the first time, do I have to test at its near bank? A: Yes, you test when you first touch the river.
Q: If I don’t have the movement to finish crossing a non-paltry river or I had to stop because I had moved more than 1 BW when I contacted it , do I have to finish the crossing next turn? A: No.
Q: If an element flees into a river, it is destroyed. Is an element that recoils into a river destroyed? A: No.
Q: May I deploy in a river? A: No.
Q: The river in our battle was rolled to be “paltry”. I have an element that needs to move down it in order to get on the flank of an enemy element. Can I move laterally down the river? A: No. The river counts as good going for tactical move distance purposes. Group moves are allowed. The limitations concerning troops crossing at the same angle to its course as they enter and troops not being able to shoot while in a river still apply. Please note that a fleeing element that enters a paltry river is still destroyed as well.”
I like to paraphrase Phil Barker’s rules and re-word them so that my feeble brain can understand them. So to me, troops in a river can only move straight forwards, straight backwards, or to conform in combat. (i.e. they cannot turn, not even make an 180° about-turn, as that would not be "moving at the same angle to the river as they entered it")
Thanks stevie, that is the way I had my nephew do it, but the FAQ does say - "The limitations concerning troops crossing at the same angle to its course as they enter and troops not being able to shoot while in a river still apply."
Technically reversing course would reverse the angle and didn't want to run into a 'rules lawyer' without having confirmation of 'our' interpretation.