Based on what I am seeing in your latest image, it looks as though you have run an incise flow procedure in Wilbur. A simple way to get rivers from that point would be to use the texture-other-river flow procedure in Wilbur. Once it has done it's thing, you can set the river colors (both of them) to white, use a solid background color (black) and then save it as a .bmp or .png file. That will isolate your rivers enough, hopefully, that you can get your paths in gimp together a little easier. I'm not proficient with gimp so can't be certain that it will be of any help, but maybe?