Its important to keep in mind what the map is for. With satellite images the rivers don't show up much. The Nile is about thousands of miles long but at its max is only 1 mile wide. The main issue is tho that for any RPG player character you cant cross a river if its wide and deep - esp if laden with horses and so forth. So the position of a river is vital to plotting a course across the map never mind that its trivially thin in comparison to the whole journeys distance in total. Large scale maps probably need to be more accurate about the sizes than regional and world maps.