Do you have a liner under your waterfall? Check along the edges of the waterfall to see where the ground is wet. If no liner the waterfall is just soaking into the ground under the waterfall. How big is your pond? If my pond did that my basement would be flooded!

When I rebuilt my pond I made sure I had enough liner for continuous pond and waterfall. On the previous pond where waterfall liner was separate and overlapped pond I always had problems with the waterfall finding it way underneath the join. Unless joins are well sealed water will find a way out even the smallest hole.

Jennearl, kinda hard to help without a picture; Jalal may be on to something... do you have any buried pipeline?

What kind of pond? Liner?

How big is your pond and how big is your waterfall?
A picture would be great. It must not be a very big pond, otherwise your yard would be really wet. I hope its not going under your pond liner.

A picture would definitely help, if you can provide one.

In the meantime, I'm assuming you have shut off the waterfall and refilled the pond to make absolutely certain that the leak is there. It must be a pretty major leak if you're losing that much water in such a short time.

Is your piping or hose buried? Could be a joint that isn't sealed. Also, the ribbed black flexhose is notorious for leaks, and they can get some pretty big rips in them.

Splashing is also a major source of water loss, and can be easily detected by wet rocks or soil. Sometimes the stream, if you have one, can be the source of leaks. That's where most of my own leaks occur.

If the liners were not properly overlapped (pond liner under, stream or waterfall liner over and draped over the edge of the pond), water can escape there.

I had a major leak as well. Same thing, I would loose most of my water in a matter of hours. I always suspected the seam in the liner at the waterfall but could never find a tear or wet spot. It wasn't until I bypassed the piping altogether buy running another pump and pipe to the waterfall that I found the problem. I critter of the R.A.T. kind had chewed a hole in the pipe leading to the filter. I have repaired it and so far so good. If you can, try to bypass the plumbing to see if the leak continues. It could be a simple fix. Now if I can just find that rat!

I have a waterfall going into a small hard-sided pond, and had a similar problem. The falls are only about a foot high, and maybe 18 inches wide, but I would lose about 4 inches of water in 6 hours. The pond itself held water for days with no loss, but as soon as I turned on the falls...poof, water gone. It was driving me nuts!

After doing a lot of reading here, and elsewhere, I discovered the concept of "wicking", or capillary action. Apparently, water CAN run uphill, under the right circumstances. If a waterfall liner runs over the edge of a pond and is tight up against the side of the pond, the water in the pond can be pulled up between the underside of the liner and the pond side, and be sucked into the soil. I found no other possible explanations, so I came up with a plan to possibly stop this, IF that was what was happening.

First, I laid down a thick seam of expanding foam where the underside of the falls liner came over the edge of the pond. This was not intended to seal the seam so much as it was intended to just lift the liner away from the side of the pond. Once the foam was cured, I went back and "painted" the foam with a thorough coat of aquarium-grade silicone. I extended the silicone out from the foam about an inch in every direction, getting the liner AND the side of the pond. This WAS intended to seal every tiny crack and seam, and I did a thorough job of it. It looked terrible, but the liner covered all of it, and no one will ever see it!

Then I trimmed off some excess liner where it draped into the pond, on the theory that the turbulence from the waterfall was causing the underside of the liner to be wetter, which could cause or worsen the wicking. (A shorter liner might "catch" fewer bubbles and splashes on the backside.) Then, as a final step, I added a bead of silicone to the underside of the bottom rocks of the waterfall, as a "drip edge". I put the bead about one inch back from the edge of the rock. I let the silicone cure for 24 hours, turned on the waterfall, and waited...

Total success. It has been running for 2 days now, and has lost maybe a 1/4 inch...mostly from splash-out and evaporation I suspect. I know the drip edge is working because I can see water coming off the underside of the bottom rocks. I do not know if the shorter liner is having any effect. But the liner itself is most definitely being held away from the pond side by the foam, and the silicone is certainly blocking the crevices. Not sure what part is doing most of the work, but the combination is doing the job! What a relief.