Re: John Day River bank access.
There is a dirt road that acceses the West bank just above the boat camp, little less than half way to the top. I have not been there, but had seen people there years ago as we ran up to the head of the arm. Not sure what water depth in the area would be like though, as the channel up to the narrows runs (or did) along the EAST bank above the boat camp.
Google earth is your friend here, that is where you will see what access you have to the West bank. I would think that you should be aware that rain may make those (Primitive) roads a problem.