[agracier]

The second attempt is indeed the right way to proceed, you do have the waterways at the right height and all ... but to get rid of the straight, geometric look that the river now has, you ought to blur my_mapC.tga after you have finished making/drawing the rivers on, but before you use the coast align tool.

Both GIMP and Paint Shop Pro and PS have blur tools. Sometimes you need to run the blur tool several times on my_mapC.tga to achieve a good natural look for coast lines and rivers ...