Fascinating. Is there such a thing in Europe? I am not aware...
@hanswolters FYI if you took these maps and overlaid them on a topographic map you'd find there is a very close relationship between them. In all likelihood the boundary between the 4 main (largest) colored areas would have the ridge of a mountain running along it which divides the waterflow down either face. The larger rivers will almost always exist in the lowest lying areas as well, the valleys below the mountains.
@hanswolters In short, the rivers will tend to lie perpendicular to the altitude lines on a topographic map and generally grow larger the more lines they pass through.