Chas,Waterfalls are usually where to start looking.

Do you have liner up and around the back and sides of your waterfall?
The water will creep back up the undersides of rocks that it runs over and can then leak out of the system.

You can try pouring milk slowly here and there, around the edges of your pond/stream and watch where it flows.

Folds in the liner can act as syphons and conduct water away to the outside of the pond.[capillary action]
Look for wet soil around the perimeter of your pond/stream.
"Horton"

Thanks Horton,
I do have liner behind the falls. I am more concerned about finding the leak in the pond which is 25" from the main falls and the water is turned off so falls aren't on now anyway and I still had a 3" drop.

Are you saying that I would be able to see the white from the milk climing up folds in the black liner. If so, that is exactly the kind of trick I was looking for, Thank you.

chas,

The only thing I can suggest is to let your pond leak down to the point where the water level stops dropping (with your waterfall remaining off of course).
This will give you an indication of the level where the leak is and you can look for any small holes or tears.
Also, any seams that you might have would be a good place to look.

Witz

"Are you saying that I would be able to see the white from the milk climing up folds in the black liner."
Chas, you should see the milk being pulled toward the leak area. Just pour the milk really slowly and watch where it flows.
Good luck with it.
"Horton"

Thanks folks. I only lost a half inch yesterday. I will try the milk after it stops sprinkeling and disturbing the pond (and confusing the water level determination).
I am still interested in the capilary action issue. I have 3 liner sections. The upper falls and short portion of stream is one that drops (2 inches) into a second long stream section that drops (4 inches) into the pond section. If capilary action could cause problems with folds, then I would imagine it could climb up between the upper and lower layers of these sections where they come together. They are not seamed (glued). I had not seen any instructions to glue them. Could that be a problem?

Chas if the drop/overlap is only two or four inches, then yes you could have a problem with capillary action syphoning water out.
Some leaks can be a bugger to find, as with a green water problem,patience is a vitue!

As railheadwitz pointed out, using the process of elimination, can help point out the problem area.
Wind, sun, high temperatures, surrounding soil, plant roots and splashing water all take a toll on water levels.

I remember someone from Arizona posting that they lost over an inch of water a day on "cool days" and more when it was hot! That can be a lot of water to replace in a large pond.
"Horton"

Thought I'd bring this back up to confirm Horton's capilary action theory. I never identified the leak but I removed all rock from the pond edge and lifted liner to remove all folds and removed the stream liner where it fell into the pond. I refilled the 4" of water and it remained stable. That confirmed the capilary action theory. I am not sure that the milk test was useful. I didn't see any movement but I did see a collection of milk in an area adjacent to one of many capillary action points. However that location happened to only be one half inch deep with a little bottom triangular fold that kept the milk from above trapped. Other areas perhaps collected milk but were too deep to be sure.
In any case, I replaced the stream liner draped over a piece of pipe to create an air gap if that was indeed where the leak had been, and re laid the liner with creases but not folds most of the way around. I still have one section that I will struggle to re lay where most of the gathers now reside but at least so far I have no leaks. Thanks again Horton.

Chas, I'm pleased that you found the culprit area. Leaks can be so annoying. Been there done it!
Pond levels dropping, drove me nuts until I pin pointed where I could be losing water out of the pond[s]

What I did if it was at a liner fold that went over onto the ground. I would pump silicone seal into the fold to close up the " syphon tube" it created, to plug it.
That would stop it filling with water, especially after a heavy rain, when it would keep drawing water out from the pond after the pond level rose above normal.
Take care, glad it worked out for you.
"Horton"

I had a similar issue but followed the advice given and it helped greatly.

Here is a link that might be useful: Water Leak Detection