I have two ponds that are quite silted over and have created bogs. Great for wildlife. I am slowly digging them out though as above - great source of good quality soil for my beds as I'm on sand / sandstone.
When digging pond soil out, you to need to move it twice, the first time you leave it adjacent to the pond to allow water to drain out and for all resident creatures to crawl out and find their way back to the pond. Then when it's dry to can move it to it's final location.
I'm hoping to dig out some of the clay bottom to do some wild pottery.
