You really have to troubleshoot where the leak is before you start making any changes. Here is how you do it.
(1) You start by turning off the falls and wait. Does the pond level still drop? If No, then your leak is in the falls somewhere and you'll have to tear it apart. If Yes then Proceed to (2)
(2) Wait some more, At what level does the water level stop dropping. When the water level stops dropping then your leak is at that level somewhere. Proceed to step (3)
(3) Start pulling your rocks off while looking closely to see where the water is running out. When you find out where the water is leaking out proceed to step (4)
(4) Patch leak and fill the pond up above the spot where it was leaking and wait. If the water doesn't go down anymore proceed to step (5)
(5) Put rocks back and fill the pond again and turn the waterfall back on. If water continues to drop, go back to step (1)