ha! I'm more confused instead of less now, lol.
This is an above-ground pond: the lower pond goes 4 feet in and 2 feet up, and the upper pond that I'm building will be wholly above ground, with the top sitting at 3 feet above ground.
The pond is slightly higher on the side that butts up against the house. Did it this way on purpose so that when it overflows, it flows away from the house and I don't wind up with water against the foundation.
There's no proper overflow built in. I put in the liner, folded it over the edges as best I can (because it's almost a cube - 8' x 8' x 6' deep - there was a lot of bunching up and gathering in the corners), stapled it down, then screwed on the cap boards (1 1/2"x10" cedar). Because of the bunching/folding, water is able to seep out between the cap boards and the liner in the spots where there wasn't bunching/folding, if that makes sense.
Now thinking on all of this, I've realized I'm a bit of an idiot, lol. I'd planned to try and put in the overflow in the upper pond - obviously that won't work! The water will flow down the water fall and into the lower pond before it overflows out an overflow drain.
There's a 2" gap in the cap boards that I've left to "hide" plumbing in in the back right corner of the lower pond. Idea being that when the upper pond/falls/bog is built, the pump tubing will go there and be strapped down to keep a low profile. Conveniently, there's a drain coming down from the roof and going to the yard's drainage system (runs through a pipe to the road out front) right there - literally inches away.
I'm thinking I could install an overflow and plumb it straight into the drain pipe? Kind of a shame to not use any of the water for the gardens (waste), but I can't think of any other convenient and discreet way to install an overflow. The two ponds will basically form a backwards "L", wrapping around the corner of the house. The current pond runs along the wall of the house from the back door of the garage to the corner of the house. The upper pond that I'm building will run from the side of the house half way out the length of the current pond (the new pond, which is in progress, is 8' long, 4' wide, and will likely be 3' deep at it's deepest point). I'd like to hide any plumbing by having it either against the house, or on the back of the upper pond, against the fence (there's a 2 1/2" gap between the end of the pond and the fence). I guess my other option would be to use some flexible tubing for the overflow, still put it in the back corner of the lower pond, then run it around the corner of the house, between the pond and the wall (I've kept a 4" gap for plumbing).
Hmmm...I could buy a rain barrel and hook it up to the bottom of that, then hook a second overflow to the top of it. Run the rain barrel's overflow to the drain in the driveway, but use the rain barrel contents to water the garden...