Filtering a pond in winter?

We are constructing a 21' diameter by 3' deep pond or ducks we plan on getting in the spring. We live in the Colorado Front range and experience pretty consistent freezing temps all winter and are looking at ways to keep the water circulating and filtering without letting it freeze over completely. Basically, we are tired of bringing water out to them a few times a day as we have done with geese that we no longer have.

I am also an avid gardener and love the idea of using aquaponics/duckponics to filter the water from the pond thru some grow beds filled with gravel and lava rock. I figure we will stop using the grow beds during this time, as nothing would survive the cold temps and the slow-moving water would freeze. Therefore, we are wondering if we should go to a more "mechanical" filtering system, which we would keep insulated.

Any ideas on how to keep a pond this size from freezing? And how about filtering it?