The reference image I used to create the rivers can be found here (external link).
I had to resize the image from 16200×8100 pixels and then applied a posterize effect afterwards, because layer images need to be only black or white.
In the first releases I applied the layer for rivers directly, which led to unsightly effects (especially on large lakes or mountains, where the ground was not even.) Since RELEASE #3 the hightmap was altered and smoothed to fit better with the rivers.