Best way to stabilize rocks

[Bread & Water]

I have two places within my 11 foot high waterfall which have vertical spill, on is 12 inches and one is three feet. My concern is how to secure the vertical stones that are behind the waterfall to the epdm liner. I'm considering the black spray foam or mortar or pressure-grouting. Any thoughts or experience out there?

 

[jgrandmont]

I don't know how applicable this is to your situation, but when I built my pond, I made a vertical drop of about 4 feet from the base of the waterfall to the deepest part of the pond (a water depth of 3 feet with about a foot drop to the water's surface). I believe I mortared in the base of the waterfall, but the rocks were just stacked beneath it, angling them backwards as I went. I don't believe I used any mortar in the stack, assuming that the angle would prevent them from falling forward into the pond. I probably did use some spray foam to try to lock them into position. In the picture below, the waterfall is on the far right-hand side. You can see how the rocks start further out into the pond and are angling outwards to a spot under the waterfall.