One of these tiles in the river tileset are using the incorrect gfx: river+N+W+land+S+W+NE+mask and river+E+land+S+W+NE+mask.
Both of them use the same gfx, but one of them is supposed to go in the opposite direction.

Also rivers in general tend to break a lot, that is in the middle of the river it starts going in opposite streams.