Have you found a pond liner hole? Pond liners may last for decades if they are properly put in with good underlayment. If it isn't exposed to the harsh rays of the sun and weather and isn't allowed to get dry for expended periods of time the pond liner should last a very long time.
If you've done some investigating you might have found a hole. When the hole more than a puncture, and is a roundish hole, take a closer look. A hole 1 to 12 inches from the edge of the pond may be from a rodent. Check for chew marks. When there is a round piece missing from about the size of a quarter to a bit bigger, the evidence suggests you may have a vole or other rodent chewing on the liner!
Next Steps When you Believe you have a rodent chewing the pond liner.
- Identify all holes present
- Patch the holes you've found
- Fill the pond back up without running the waterfall and see if it holds water over 24 hours.
If the problem persists you may have to call an exterminator. Another option is to reline your pond completely and use extra tough geo-fencing that rodents can't chew through.