Where abouts on the Dan are you thinking about. Big river, lots of places, some public some private but the waterway is public unless posted otherwise. It is a navigable river and the public can use it.
Were it me, I'd go to one of the public accesses near where I wanted to explore. I'd look for any signs that say, "No panning" or "No hunting for mineral wealth" or "No Trespassing".
If there aren't any signs that say "NO" or "Unlawful" I'd start panning or what ever you have in mind.