I pretty much know everything about this book except for the title and author. Basically this guy inherits a farm from his Uncle that has a portal thing in the barn that links to alternate dimensions/realities. It's stuck on a version where humans don't exist/haven't discovered the Americas so everything is untouched, including natural resources and fauna. He and his friends use it to first go back and capture a few passenger pigeons, which they sell to zoos to fund a gold mining operation. They set up a runway in the alternate reality and use a plane to fly to a river full of gold which they dredge up and take back to our reality to sell and make money. Some shit goes down and the government comes in and tries to take the portal. I read it around 2005, so it's at least that old. IIRC it was marketed at teens