Basically it's a simple shape that makes the river/fall area, and you use it as a mask to not have strong displacements (like the strata). You have to make it so it sits on the place where your fall will be of course. So, make a simple soft terrain, find a nice place for a fall, put your simple shape there and add more differentiation inversely masked by that simple shape and you have a soft riverbed in the rough terrain. Use the same mask through a color adjust (to decrease and harden coverage) to add waterlike colors and structures (like the strata, blue whites and shine in my example). In my case it's all in one line to the planet, no water shader. You can warp the simple shape if you like, but it isn't necessary.
This should do it...
