Quote - ooh that's damn good! :)
I was wondering how ot make a gradient for the river height, that would somehow stick to the height of the river banks, and flow evenly all the way along?

that may be tricky as you'd want the water terrain to be flat and even across the horizontal width of the river, so it would vary (get lower) as the river turned in very fine gradient based on the surroundings not on a mere gradient file or such, dunno how to do that, though, short of laborious painting :/

Thank you. :)

Use the eyedropper to pick up a color form the surface of the water where you want the river or water to be. Then make a path or selection of the river and fill the path with the color you picked. That should give you a leveled water plane.  Then you can use dodge and burn tool to adjust the surrounding area to go along with the river. 

If you want the water to go down hill, use the burn tool to cut through the plain. It takes some time to get this right though.