Re: If we start judging art by "socially-redeeming value," we get into some interesting territory.
The banks and the river bottoms are owned privately on some rivers in Oregon. The water itself is not owned, so people can drift, cannot touch the banks or legally anchor without permission.
Where we differ is that you can consider this an "art project" and I simply don't see it. I think it's more related to litter or graffiti, with the saving grace that it's removed.