cow, In your first pic, what is that long rectangular gutter looking thing on the wall behind the pond?

Chas, thanks for your comment. I did dig up and expose the entire return pipe (last two pictures) and though the soil is generally moist, there are no wet spots that would indicate an active leak. Same for the housing connections at the top and origin/botton.

Re evaporation, I don't remember it losing water this quickly vs last year (it's our 2nd year in the house), that's why i started investigating. But according to an online "drip calculator," losing 1.5 gph is equivalent to 6 "drips" a second. Not hard to imagine that splash and evaporation would generate this?

Re liner - is it possible that these liners just harden and crack over time? I'm estimating it is ~10 yrs old.

Butterfly, if you are referring to the vertical member, it is in fact a downspout/gutter. it empties into the green drain box, which in turn empties into the 6" corrugated tube, away from the house.

My liner is only eight years old, but it is still completely flexable. I have never heard of them hardening. The normal use for this stuff is on roofs. Hot roofs!

Re leaking: Your point about a difference from last year is certainly a good one although I suppose you might consider questioning the clarity of your memory. I have a long stream and because I like to make it appear natural, the liner edges are close to spilling in various areas. They occasionally get pressed down by the dog or clumsy me and with all my plants, they are often hard to detect. When I get desperate, I attach lengths of pipe to the outlet at the top of the stream, either part way or all the way down to the pond so that part or all of the stream is out of circulation. If you could connect something just before the spillway that gets directed back to the pond, you could determine if the loss is in your falls etc.

I was just thinking the same thing as chas045. Last winter was our first with our new pond. We'd just completed it in fall. We didn't know if we should keep the waterfall running over the winter, but before we could make other plans we got lots of snow & sub-freezing weather that didn't stop until there was ice on the sides of the waterfall stream. By then it was too late to drain the filtration system so we just continued to run it all winter, and there was nothing we could do about the ice build-up. Anyway, in spring we noticed the pond losing water much faster than normal. What we discovered was the ice (or combination of ice & dirt settling on the new waterfall structure) had cracked the mortar around the rocks that formed the sides of the waterfall on one side, right at the top, and also right under the waterfall box (see close-up pic). The dirt-covered structure in the upper right is the waterfall spillway.

Ice formed mostly on one side, as you can see in the photo, and that's the side that eventually leaked. We are in VA so you could have it even worse in MN, even without your waterfalls running in winter.

This allowed water to seep behind the rocks. Once it got behind those rocks, it made it's way down behind all the rocks on that side - between the rocks & the liner. But then near the bottom of the waterfall we had a steeper section & the water made it's way over the top of the liner & out of the pond. Once some of the rocks hiding the liner were removed, we could see where the water was running over the top. We never could tell it was leaking by looking around the landscaping there either, as it never appeared wetter than elsewhere.

Chas, your suggestion to divert ended up being the right one. By diverting outflow to the bottom spillway I was able to confirm no meaningful leak over ~6 hrs (picture for entertainment purposes only):

Once I narrowed down where to look, it turns out that the liner is in fact discontinuous, and the lip of the upper liner was glued using black foam to the flat stone below. In the picture below, the Rorschach appearance is from liner pulled back, with stone and foam below. As I pulled it back, it was evident that in some areas there had been regular water (not just wet, but some sediment, etc) Just upstream of where the foam ends is exposed fabric liner and dirt underneath:

So, my extremely helpful gardenweb friends, the question now is: How should I repair this? I would prefer to not go to the trouble of replacing and relining the entire liner in one piece (as it should have been done before), so is there a better bonding agent that I should use to glue the liner down to the stone? Or should I just use the same black foam stuff (Great Stuff?? and accept that I will periodically have to redo?

Thanks!!

Never mind - started new thread on how to rebuild (more) correctly. Thanks!