As far as riding on muddy trails: most people don't respond kindly to being told they are doing the wrong thing... you have to show people the consequences of their actions. Signs at all the trailheads explaining trail erosion (with photos) will gradually have an impact on how people think...
I first noticed beaver activity last spring. They started building this dam over the summer. There was a huge tree that fell across the creek and they built off of that. I agree with J- as of now... it doesn't really effect anything. But... I enjoy riding by and seeing the changes week to week.
Great call as far as letting the park naturalist decide the proper plan of action.
I actually enjoy the fact that we have a dam. I would hate to see it removed. If anything- the dam will help prevent flooding.
Hope nobody is seriously thinking about ripping out a beaver dam... hard to image the park people would sign off on that. If the trail is getting soggy- move the trail.