I keep seeing, in books, these ponds where there is a rock wall that barely separates the pond from a dirt filled margin bog when the water overflows.

the margin bog has a liner at the bottom....


doesnt the mud in the bog go "bad" - i guess the pond water level can be reduced when removing and changing the dirt - but it sounds like a bad idea for a small, residential garden pond near the house - more like a big natural pond.

what do you feel?

maybe with gravel ....

Yes, the dirt over time gets stinky and eventually needs replacing but it may take years. Pea gravel is a better idea. You have to weigh the pros and cons of the bog. I think it's a neat idea but have not done one myself. Well, actually the two "Mickey Mouse" ears of my pond as my brother calls them are pea gravel with plants. I've yet to replace that. Don't know that I could!