Mudball, the core trench is indeed important, but all the pond basin needs to be well compacted during construction. Don't fall into the myth that a 50,000 pound dozer will compact soils, as there is less PSI of pressure than a typical home refrigerator.

Spending a little on insurance in the way of proper bulding will save huge dollars if the pond leaks and needs to be redone.

I had a dirt pusher build a dam and the original build was $3800. The bills to "fix" all his mistakes topped $32,000.