I'm still bothered by this, so here's a series of experiments, none of which works. Some even work in strange ways. Water level is at -5.8m. River at right, dunes I need on the left part without the holes in ground.
Setting a minimum altitude for unclamped displacement doesn't work either (it wasn't as simple as that), as the negative displacement values are starting above water level (which you allow) but extend downward. So that's off.
I tried multiplying by a black (fuzzy/non-fuzzy) distribution shader with a minimum altitude, which (I'd say) would blacken the unclamped low blacks, thus cancelling those negative displacements out. Apparently not. I added a white before, just to make sure the rest is still present. But with that white off, it's just annihilating every 'displacement color'.
Distribution shader, even without any restriction seems to smooth both top and low end, strangely.
Btw. color adjust is also a hard clamp just like in pf's, and that's exactly what I don't want. I want a smooth low end only and and open unrestricted top end.
I finally used another approach, which works, but still want to see if there's a way to get this to work.
So any ideas are welcome.