Would you believe that no one at Vicksburg National Park or Port Hudson State Historic Site knew the answer to this question when I asked back in January - they're the ones who steered me to Grabau. I was hoping he and Lloyd would be right on, and they are only 22 miles off, which is probably not that bad when you consider that, depending on how you run the bends - inside, outside, or middle of the channel - that could probably account for twenty miles right there.