In MO, you do indeed own to the center of the stream, the opposite land owner the same. However, the free flowing water is open to anyone (once they obtain access through a road/bridge right of way or other access. Step on dry land, however, and you are indeed trespassing.
The facts of the situation, as always will make the difference. If the land owner directed the “visitors to leave, and they did not, and even approached and “assaulted” (unwanted physical contact) the land owner may have a defense.
Not enough facts as of yet. Wife states husband “touched” his arm. Landowner may have another story, ad” contact bit more offensive than a congenial “let’s talk this out, friend” contact.
As a MO stream front property owner, I do understand the landowners situation. Trash, piles of feces, noise, etc is not a fun thing to deal with. However, I cannot justify more than fencing the banks proper and placing “Posted” signs. Most canoer’s are polite and careful, but “yutes” and derelicts are there as well.