When the cost of rebuild becomes too expensive for your wallet, IMO keep downsizing it until it gets within your wallet $$. Get a contractor that knows how to use a pull behind double barrel sheepsfoot roller or sheepsfoot mechanical compactor to compact the pond bottom assuming you have good clay. If he says he doesn't need it then switch contractors. This is very good leak insurance that is relatively cheap insurance. I would never spend good money building a pond without a sheepsfoot roller. The risk and cost is too high to get leaks with just dozer compaction unless you have excellent clays. Hundreds of members here ask questions about fixes for their leaky pond.
Read through this thread for an education especially the part of compaction.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92487#Post92487As esshup said - Trees on the pond banks or dam is like having small drain straws surrounding your pond!
Rule: all dirt leaks even the very best clays; the variable is how much is it leaking / seeping / wicking? I know well sealed northern rock/stone lined ponds that only loose 12" of water (mostly evaporation) during a drought year.
