Our duckpond is only a dog-digged hole under a plumtree, and moles make drains in it, which are quite easy to manage - either stomp on it to close, or suppress the hole by punching down. And the lake is bigger and bigger everyday - ducks are very good in excavating, and after all, it is their pond, not mine, so it is completely right that they work on it (with other animals, like in a fairytale). 
Because their legs and beaks get very (and I mean VERY) muddy sometimes in the hard work of pulling out earthworms and filtering mudsoup I always have them a babytub of clean water near where they can wash off. Someday maybe they will have a real pond too.