Homesteading a River

As a response to a thread on reddit, I offered the following to a question about someone homesteading part of a river, and someone else upstream dumping in pollutants, as in: What's happened here? Is there a crime? My answer:

What about this: Bill Murray doesn't appropriate the water, he appropriates the land, the riverbed, and simply allows the water to flow over it, and appropriates whatever water he can capture before it leaves his riverbed boundary. If David Spade upstream drops pollutants into the water, it will eventually trespass over Bill Murray's riverbed. This would justify retaliation by some level of aggression. Trespass is a violation of the NAP, and David Spade is causing pollutants to trespass Bill Murray's riverbed, is he not? It would be akin to air pollutants trespassing over your land, into your house, etc.

If David Spade drops pollutants into the river, but then takes them out again before they reach Bill Murray's boundary, then there's no harm, no foul. If he doesn't, then he's caused a trespass and must answer for that.

I think that's a good answer.