@#!$%& Leaking Waterfall

Built a pond last year with a small waterfall. The waterfall is just five
limestone slabs stacked on top of each other. I don't have space for
anything more elaborate. The bottom slab juts over the pond. It worked
fairly well last year with a few small leaks, which I fixed by shimming here
and there.

This year I noticed that the ground under the waterfall had settled over the
winter. So I shimmed the whole thing again, with not so great results. I
finally got tired of the moderate leak and shut the whole thing down to
apply some great stuff. I was loosing water in two places, backflow under
the main slab and some trickling off the edge of the bottom slab. So I
foamed the the backflow in the bottom slab and tried to put some artistic
(look like sh*t) edging on the bottom slab to prevent the side trickle. Woke
up this morning to a half empty pond?

So time for a real solution. I know I should have a liner under my
waterfall, but did not relish the idea of digging it all up. Although I did
have it up once last year to hide an overflow pipe under the waterfall. I
was thinking that I could build a tray for my waterfall to sit on. I was
going to use some marine plywood and 1*2 for form a U to hold my liner. This
might be harder to hide than simple liner, but I thought it help the liner
hold it's shape and prevent leaks around the edges?

Any thoughts?