I have been told that the law states that you have to use "hand tools". In NH a shovel is not considered a hand tool. I have also been told that the rangers usually look the other way as long as you are not destroying the river banks. Most people use shovels and most people get away with it. How lucky do you feel? I think a grey area are those two foot shovels. Is it or isn't it? Who knows. I don't think it has been litigated yet.
Good Luck