I would remove as much topsoil as possible. Topsoil has many uses and can even be sold to recoup some of the building cost. As dale mentioned one does not know what type of soil is actually under the topsoil so removing the topsoil will reveal the subsoil and hints to where likely spots to check if the pond leaks.

If it were my pond I would at least get all the pond bottom down to clay and drive over it several times with a sheepsfoot roller as a form of leak insurance. 8 times or pass with the roller by the book. A leaky pond will be a long term headache for you especially if you try to fix the leak. Quote from Rainman: "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."


With all those surrounding trees that each year will contribute lots of leaf organic matter it is better if you remove topsoil to add as much overall depth as possible. I've seen ponds in wooded areas that gain 1ft of dead leaf matter each year. This not only fills in the pond but reduces water quality and increases chances of fish kill both in summer and winter. Do it right the first time even if you have to make the pond somewhat smaller; you won't regret it.