The simplest advice I can give is the rock is going to be what helps hold the sand in place both above and below the liner. The sand just give you the elevation for a falls. The rock is what gives it stable form.

I would not do shelves inside the pond. They usually will fail and will give 'coons an easy way in or out. I have built ponds (4) at each home and (2) for friends. We have clay soil which holds up better than sand. After the shelves slumped the 1st time I quit building them. You can set plants on top of plastic milk crates and this will also give fish a place to hide from predators.

I do not use soil to build up for the waterfall. I have used cement block with liner over it and rocks stacked on top. The cement block were used as a foundation with many layers of stair stepped stacked flat rock on top with large rocks on top so the water sheets off instead of trickling between the cracks in smaller rocks.

If you lay a row of double rocks just below the water line with the liner coming under the front rock and laying on top of the back rock, topped off with a large rock you will not have the liner at the top edge of the pond. (You can do this for more than 1 layer.)

Good luck, you can do this! You may have to re-stack, run the hose over areas until you get it the way you want it. Ponds are very rewarding (relaxing sounds, tame fish, tad poles, little frogs, dragonflies, observing nature). They do have their trials (string algae, green water, bull frogs, snakes) but you can overcome.

I'm in the process right now of redoing a small pond/water fall for the 3rd time in 12 years and why I'm visiting today,, to find ideas.

I agree with frankieynnsie, the shelves slumping are a huge pain. I wanted a higher waterfall this time and first decided to use concrete blocks, but changed my plan to commercial 24 ga. roofing panels coated with Galvalume and a special paint. The two will last 100 yrs on a roof, so I think I can get by with 20 years in the ground.

It's about 6" into the ground, and where the 2 big silver squares are in the middle of the panels I attached two 4" wide pieces of heavy aluminum going back 3' into the ground with a large cleat at the end to keep it from slumping. The waterfall will be at the black line I made in the middle and after I throw the liner over the metal I'll build the wall, pull the liner back over and cut off the excess metal to the contour I want. It's not very high just looks high in the picture, with the waterfall about 1.75' above the water line once done.

I think this will give me more flexibility over the design and not confined to the straight lines of concrete blocks. If if fails, I'll use block.

I'll build 90% of the inside wall, and work on the outside buildup of dirt next before I cut the panels, cover the sharp ends and fold over the liner. From there I'll place the final inside and outside with rocks. Third times a charm, right?

I'm not saying to do this my way and thought I'd throw it out, finished the metal install yesterday.