[delvpier]

For opacity in Gimp :



For me it's important to use brushes a little bigger than rivers width and not only placing black color on river... in one way, setting rivers at sea level in map_h is like cuting in relief... for low heights it is not really important... but for higher heights you have to cut softly with smooth transition between actual relief and color you paint...

As illustration from map above :wink: :roll:

Durance river in a real relief at 700m :



River in Corse, all length, start in relief at 1400m :



Both are at sea level and integrated smoothly in high relief...

Anyway, you can't follow sea level river into highest heights... differences are unrealistic...