How about this:
P.82
River (Rv) Up to 4 MUs wide, entirely within 6 MUs of the side edge. The placing side dices for its difficulty when putting it down. 1 = uneven, 2,3 = rough, 4,5 =difficult, 6 =impassable. Troops can only move within 45 degrees of straight across. The river cannot have more than 2 bends.
Also, on P.37 the following is needed to prevent troops in a river being forced to conform into a direction which would not then allow them to move:
• Battle wagons, artillery, troops in Orb formation and troops defending field fortifications or a riverbank, or in a river, do not conform to enemy.



