It's hard to image that one square foot of chiffon, nylon or organza is what is necessary to create a full lush waterfall.
Trial and error has taught me there is no such thing as too much material.
The material manipulates best if it sits on the diagonal with widest part corresponding roughly to the widest part of the falls.
As the top is very narrow I trimmed the upper corner into a rough rectangle.
The edges are tucked just under the "rock" and anchored with a zigzag. I used invisible thread but a neutral would also work.
Anchor one side, then the other leaving the tops and bottoms un-sewn.
A photo from the side shows how the chiffon balloons out from the surface, in this case almost 4 inches.
Now comes the fun part, and it's what shapes all that material into a lovely waterfall.
Gather the excess and pin it. A line of stitches does not have to run from top to bottom. Anchoring the material with a dozen or so stitches is all that's necessary.
This way you can manipulate the material back and forth, upwards and downward to give it a natural position. It takes about 5 passes, alternating from one side to the other to tuck in all that material.
I like to take a few final passes with a pure white, or sometimes and shiny metallic. But lines seldom run the full length of the "water".
The excess at the top was all tucked in rather than cutting it away. This makes the beginning of the falls very dense and very white.
The bottom is left unfinished as it will be covered with trees and other rocks.
I usually put in the rock detail before the water, saving the best? for last, but as there is already a lot of definition in the rock by my cutting and repositioning, I'm undecided as yet how much I will add.
The shadows need to be dealt with so that will be a good measure of how much stitching will be needed to balance everything from top to bottom..