The beaver's work ethic is a part of our national heritage. He's that character we all have a gnawing feeling we ought to keep as busy as. And indeed some of us have kept so busy that we've taken away his job of impeding and impounding the free-flowing waters of America. But when it comes to cost-benefit ratios and environmental impact statements, he's better than the Corps of Engineers by a damsite. So how can you best preserve a pristine stream? Leave it to beavers.