Thanks for the input and well wishes from all of you. We have had a
slight(?) change of plans. We've switched the 10-12,000 gal pond
w/stream and header pond to a 20" high (5 foot deep) rectangular pond
with a 30" high (6 foot deep) circular pond (in the rear center of the
rectangle), containing a waterfall flowing into the rectangular pond.
One half of the circle (the waterfall half) will be within the
rectangle and the other half will extend behind it. Basically, we've
gone from informal in ground (which our old pond was) to formal above
ground.

I estimate the bottom pond at around at around 12,000 gal and the
upper pond at 3,000 gal. I want to load the lower pond with koi and
use the upper pond as a plant filter and baby koi incubator pond (I
guess I'll put some kind of screen above the waterfall so the babies
don't get washed into the lower pond?). We want to build the ponds
with bottom drains, vortex/biofilters (which I learned about at
pondtechnology.com), concrete block below ground and Unilock Block
above ground. We found a local Unilock dealer and picked the style of
stone (Roman Pisa) from unilock.com.

I still hope to do a lot of the planning and building myself, but I've
decided to hire this local dealer to build the walls, since I don't
have a clue about how to build this wall so it looks right. Has
anyone built a pond with Unilock or similar self-locking block?
Almost all of the above ground ponds I have found online are made with
concrete block and cemented brick, so I'm kind of running blind here.
Any input on planning or pitfalls to watch out for would be much
appreciated. Thanks in advance to all!
Dennis