flexible altitude mask

[Dune]

I can't seem to get this working, though I thought I had it once. So, a nice challenge for you, though it may be very simple and just overlooked.
When you have a stream going downhill, it would be good to have an altitude mask that can adjust its altitude to the existing terrain slope, so you can have, say stones, or shores, get their reflection up to a certain distance above that particular waterlevel.
Here's a file I started working on. It seems so obvious, but it just doesn't work 

[Kadri]

Not sure if you want something like this Ulco?

[Dune]

No, sorry, that's not the solution. It's now simply showing the SSS mask from the upper surface layer. I need an altitude within the gully to be adjusted and sharpened at will.

[WAS]

I don't think I understand this.

An altitude won't work, cause it has a defined altitude, and fuzzy zone if used. You have a slope. So unless your altitude limit is at the top of the hill, it'll never fill the river.


Can't you just use a large Simple Shape Shader that's soft, allowing for shore masking, and than just apply Slope Limits for rocks and such?

[Dune]

No, that won't work, as the tops of rocks in water on a sloping river would still be wet. And it won't work for rivers made by Daniil's shader. That's what I'm after. It should be possible to redirect/displace/warp a certain height/altitude with the terrain. The slope is actually not important, just the differences in altitude at each XZ location.

So I don't understand why it does work here, but not when the altitudes are derived from the terrain itself. Brain failure