Clearing thorny locust trees

I have approximately 50 acres of bottom land on my farm that is covered with 20 year old thorny locust trees. They are from 6"- 12" in diameter. What is the best/most economical way for me to remove them? I have received a wide range of opinions, but would like to hear some experienced advice. I don't have a lot of topsoil, so I would prefer to disturb the ground as little as possible. I don't have a lot of money, but I have quite a bit of experience on large tractors, skidsteers, and other medium duty equipment. I would like to build a decent size pond as well, if I can get it done with the same piece of equipment. Thanks.