Well I can see both sides of that issue. Doesn't Washington have the "navigable" waters act where private ownership of the adjoining land ends at the high water mark of the river ? If that is the case then he does not have ownership of the river along his property. Some states do not have this. This was discussed earlier this year if you put navigable into the search tool it will bring up those previous threads.
I empathize with you. A new owner just bought a large prime area of my favorite steelhead river, has posted all of the river bank but anglers can still walk into and out of it as long as they stay in the river only getting out when the water is to deep to walk around the deep spot, then must get back into the water. In Michigan the land owners do not own the river only up to the high water mark.
Good luck, I guess I would find a new area to fish since I would not want to have worry about confrontations with the owner each time out there.
What does he say to drift boaters, the same I suppose.