Info on the original 2010 pond is aprox 8 X 12 and is just shy of 3 feet at its deepest (shallows out as it moves away from the house. I have 4 inch PCV pipe that runs underneath the stairs so as the water rise in the new pond it overflows into the pipe and drains into the lower pond. I have tubing run from the lower pond to the higher one to have water pumped up into the higher one so there is constant water flowing between the 2. The lower pond is edged as you can see with river rock, has a pebble gravel base in it . Shallow hardy plants with 2 lillies in it. The lower pond is great as you can easily see it through the big picture window so when eating meals in our dinning room we can see the pond and when windows are open enjoy the sound of the waterfalls. During the winter I shut down the waterfalls and redirect my pumps to create a bubbler effect in order to prevent the pond from freezing solid and keep an opening. I've had very good luck the last several years at not losing any fish although this was the first winter for the new pond (and a very harsh one at that) so will be another week before I see for sure the koi survived (so far the signs look good). I will also note that house itself shields both ponds from the westerly winds which helps. I live on the outskirts of a village so anyone wishing to pull up and try and make off with my koi would stand out like a sore thumb. I'm more concerned with last summer's Heron returning to try and steal them, LOL. I started with goldfish,comets and shuburkins the first year, 2nd year I added 3 koi which started my obsession with ponds. I added several more koi last year when the new pond went in and my goal is next month I will round up a bunch of the goldfish (especially all the baby ones that were born in the last 2 years) and relocated them to my brother in laws pond as he loses a lot to racoons. One thing I love about ponds is that you are constantly learning and fiddling with stuff (you don't become and expert overnight) so if I lose some due to not having it deep enough I'll need to make some adjustments, will suck if it comes at the expense of losing some koi but it will give me an excuse to tinker. As soon as I was finished I was thinking yep I should have gone an extra foot deeper.
Also if anyone is wonder I used 15m rebar that go at least 3 feet deep into the ground. Each corner is overlapped (interlocked) with rebar going through. I think I spaced rebar about every 2 1/2 feet and each layers of ties is nail spiked into to the 2 layers below it.