Who owns water? The US landowners putting barbed wire across rivers

I'm one of those people who thinks there's a land crisis going on and it's being fueled by wealthy, doomsday preppers. People are putting up private property signs on public (re: federal lands) every where they can and are trying to claim rivers as their own. I think a lot of it started with Michael Burry convincing wealthy people to do it after he got success by betting on the housing crash, combined with the political landscape.

But regardless, as a non-wealthy person who enjoys the outdoors it's very concerning. I don't like that I may be out one day on a river or a trail (particularly when on a mountain bike, dirt bike, or in my Wrangler) and may encounter barbed wire or "private property" signs when it's definitely not private property.

I canoed and kayaked 150 miles through the Adirondacks and I remember a lot of private property signs but it was all in regards to the shore and actual land; signs put up to keep people from camping on private land. I get that. But blockading a river isn't much different than blockading a road to me.

They're getting away with it right now because it's small tributaries and such, but what if some jackass pulled this off on the Mississippi or something equivalent? Logistics of that aside, I bet some governments would be far more concerned. And depending on how court cases regarding this go they may be setting someone up to do just that.