Thanks, Esther! Now we can get going! Kidding, of course, and no smileys on my Blackberry!
My theory is that the same as in aquaria, the gravel houses bacteria that utilize the wastes, and as long as they are AEROBIC, the water quality can remain high. The key is that the gravel layer stay aerobic. The way I handle that is by making sure flow rates are high (16,000 gph in my case) and, similar to my planted aquarium, by rooting extensive plantings in the gravel. This is most important in the lowest areas. My popnd is 4.5 feet at its deepest, and I have found that hardy lilies will acclimate to that depth, and the "anacharis" type plants love it. My koi tear into this stuff, but not faster than it grows!
I do once yearly cleanouts, totally draining the ponds, and I am amazed at how little mulm is in there each spring. And I feed 40 pounds of food a month during the summer to my gluttonous pigs!
Steve