One time I punched a hole clear through the return tubing that leads from the bottom pof the pond to the top to the waterfall...sheeeeesh...what a royal mess to dig up and find the leak. So much for yard art on a stick! I feel for you.
Shut off your waterfalls first and see if your water level goes down (you will have to aerate your pond separate from your waterfalls). If it does not, then your problems/leaks are confined to your falls and stream. If your water level goes down after you have shut down your waterfalls, then there is a leak in your liner - the water level should drain to the level where the leak is - if you dribble drops of milk by the edge of the water and the liner and wait, you will see the milk moving towards the leak.