Looking at this picture, and reading what the article described as to the law in Missouri, it seems to my feeble mind that the small trees at the edge of the river bank and along the sand bar are probably within the easement, but the woods beyond those trees are not. That is what is probably confusing to people, because the exact line is indefinite, and we as property owners usually like to have a boundary.
Another confusion you may have is regarding the legal definition of navigible. You see, in Missouri, navigable does not mean the same as in the Delaware River or the Hudson. A canoe that can make its way down most of the year probably makes a river navigable. A creek that only has running water after a storm is not.
The picture also makes it appear as though the guy who was shot was shot out in the middle of the sand bar. If he shot the guy there, it's a jail he'll be goin'. The easement is definitely going to extend onto that sand bar. Which means he acosted people who had a right to be where they were. Unless your knowledge of Missouri case law indicates otherwise, in which case, I am happy to hear your legal argument.