Couple of things I would consider here, first, if the water flow is constant the pond walls could be reduced to 3 feet. Three foot walls require a lot less re-enforcing than 4. So with Bill Cody's rebar from slab to wall idea the strength problem is solved. Next you cold weather basement guys know that there are coatings that waterproof basement walls, which if applied to the outside of the pond walls would solve many of the leak problems, especially if you planned on another coating on the inside.
Concrete has come a long ways and adding the right supplements will increase the elasticity a great deal so freeze/thaw factors can be reduced.
The biggest problem I see is in making a silk purse out of a sow's ear and I am wondering why that is a goal? Wouldn't an above ground pond work as well? Plus it would give you an opportunity to evaluate an entirely different way of managing a pond.
I believe that you could grow some big fish really fast in a high water flow system, but that type of system would require a lot of management and better to do it standing up than bending over. Just a thought.
