Hmm, tough call. I have some river frontage and have built numerous small structures over the last 17 years... all by hand, sometimes with a come-along at most. I would love to pull a Corps permit and get a trackhoe and some big boulders to do some elaborate structures, but haven't had the budget for it. My feeling is that small stuff is cool, but when you start driving equipment in the river, a permit is absolutely in order. I have talked to the Corps office in Albuquerque, and frankly it didn't seem like a big deal to get a permit. As far as ratting the guy out, I don't know. I probably would call the Corps if he was fundamentally changing the channel. If not, I'd shine it on. My 2 cents...
You can never step into the same river; for new waters are always flowing on to you. - Heraclitus of Ephesus