Hey, without a doubt go with Greg or at least hire a civil engineer to calculate not guesstimate the exact area that will be flooded with a dam. They can usually tell you about the permitting problems/procedures involved. Don't leave a project of this size with any unknown variables. Calculate the cost before jumping in. After you know the exact cubic yards of materials to be moved you can get an accurate bid on building costs, and who knows you may want and get your 100 ac lake afterall! I'm with you big is good in this case!!!