Thanks for the detailed response. I guess the least I can do is give you a breakdown of Lloyd's river mileage. Maybe your Corps friend can look at it and see where he thinks it might be off.
River Miles from St. Louis, Missouri, to
Vicksburg, Miss. 840
Grand Gulf, Miss. 890 (50)
Rodney, Miss. 910 (20)
Natchez, Miss. 950 (40)
Mouth of Red River 1015 (65)
Bayou Sara 1050 (35)
Port Hudson 1062 (12)
And now that I look at these numbers laid out like this, they do look very neatly rounded. I would be curious to hear what your Corps friend thinks of them.