Check for obstructions. Look for any debris like sticks or leaves in the waterfall area causing a damn or blockage preventing normal water flow. This will cause water to rise higher than normal, spilling up and over the liner.

Check liner edges. Walk all the edges of the pond looking for any obvious wet areas outside of the pond. In a lot of cases, the rubber liner has dropped due to shifting of the earth, an animal or person stepping on the edge or a number of other factors. You may have to turn over some rocks to double check all of the liner. If you notice a spot with low liner and water leaking out, simply pull up the liner and secure it back into place with a rock or backfilling some dirt to support the liner.

Observe plants. Bamboo and Umbrella Palm are known for causing leaks in ponds. If you have either of these not potted, the roots can puncture any nearby liner. Root systems in fast growing plants like water mint can cause damns or blockages causing water levels to rise and leak out over the edge of the liner. Plants outside the pond can grow over the liner into the pond and pull water out through its root systems as well as wicking water it out.

Rodent holes in the waterfall. This is probably the most common cause we see. If you have open areas between rock in your waterfall, check for signs of rodents. You would see rodent droppings, bedding, nesting, crushed up acorns or nuts.

Inspect plumbing. Check all the exposed plumbing, especially elbows and connections for any signs of water leaking.