Public land is your property? Right. He looted a grave? Right.
And what pray tell would you have him do? Or any other person that finds artifacts on "your property"?
I'll bet he'll probably donate the artifact.
He should have reported his find to the Army Corps of Engineers, so they could bring in bulldozers to bury the gravel bar.
Tinian is a bit over the top in his presentation, but he does have a point. If Henley found this hook on public land, it isn't his. Artifacts found in the US generally belong to the landowner. If the landowner in this case is the state or federal government, Henley has no right to keep or sell it. He may have no CHOICE but to donate the artifact.
If he was on private land and had permission from the landowner to be there, then he has himself a nice (and likely valuable) artifact. If it was public land, it needs to be turned over to whatever agency manages the land.