Answer: TWO terrains!

Take your terrain where river should be, make river bed in terrain editor, by lowering it along river path very gently, OR doing the same in an image editor (which is even better but requires work using gradients etc) and using that as heightfield.

duplicate this terrian, make it the water one, give water material, remove the terrian outside the river, raise river up slightly so it fills in the river channel
Bingo, river following hillside downstream!

now for accuracy you'd really need to work wih heightfields in Photoshop etc to get it more realistic.

This is just a quick simple test, sorry not very accurate, but you get the jist :)

That's very interesting way. I'll give a a try.  :)

I've been doing a lot of work in Photoshop playing with heightfields with wacom tablet brushes making rivers and roads.

Here is what I am working on:
The base is from world machine and then I added details to the height fields in Photoshop.

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 :/

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 :/

How about this Idea, Does Vue Have Volumetric Materials. If it does it might be possible to apply a Volumetric Water Material to the riverbed ,

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.

Great thread, for some time now I was trying to do  river in mountains in Worldmachine, but  i like this idea with photoshop, give it a try...thanx!