I think since this CC3 drawing style is iso style, so your rivers should be drawn this way aswell (with more stretched curves, carefully following the hills/plains shapes). So I pretty much agree with Neyjour. I also agree that the white outline isn't appropriate there. A thin dark green/dark brown outline would fit better. I would also go for some hand drawn white inner lines on the borders of the rivers to match the hand drawn CC3 style.