My father has built several ponds here in MidMichigan. He built our pond in the winter. It was actually much more efficient since the surrounding top soil was frozen well enough to allow him to drive his 50 foot reach excavator farther into the pond area without the need to lay mats. The digging itself was much faster and cleaner. The area where our pond is used to be a bog. Summer digging would have been a wet, sticky mess and the clay tends to really stick in the bucket of the excavator. Digging in winter meant that each scoop could go directly into the dump truck without having to pause while the water drained from the bucket nor having to stop repeatedly to scrape the bucket. It should be noted that the frost only penetrated down about 3ft. so there was no problem with smoothing the bottom with clay since it wasn't even frozen at that depth.
