Sorry for the late response! been a while since I was here last.
The rivers are made up of several layers of various thinkness of lines, which are then merged together to create the final river with smaller and smaller rivers coming off it. You then delete away to suggest the rivers are getting thinner and thinner so they are too small to see from 'space' or the distance on the map.
Sorry I think I missed out a step where if you have black lines, then you want to invert the black to white and then softly erase away. (Image>Adjust>Invert )
you should then have a white river over your black land. (as in the first image under 2.4) but to fade it so it softens as you get in land you erase away with a fall off brush. (so you get the second image under 2.4)
hope that helps.