RiverNSDirection= and RiverWEDirection= the direction that the water flows along the river. Valid values are 0=north, 1=east, 2=south, 3=west. These settings MUST be used in conjunction with isNOfRiver or isWOfRiver. EG:
isNOfRiver RiverWEDirection=3 # This will create a river on the bottom edge of the tile flowing from right to left. isWOfRiver RiverNSDirection=0 # This will create a river on the right edge of the tile flowing from bottom to top.