It's evident that the master of the Minnehaha (Captain David White) was in defiance of a direct order to not offload his prisoners sent down for exchange, but rather to return with them to Helena, Ark., in order to avoid any word leaking out of the Union actions.
At any rate, as we know, he did stop at Greenville.
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Order of Acting Rear-Admiral Porter, U.S. Navy, to Captain Walke, U.S. Navy, forbidding communications with Vicksburg.
U.S. Mississippi Squadron,
[Yazoo River], December 28, 1862.
SIR: Under no circumstances will you permit any prisoners to go to Vicksburg or allow any communication with that place without further orders from me. Keep the Minnehaha under your guns. Allow no communication of any kind with her. Do not allow her to keep up steam enough to move.
Respectfully, your obedient servant,
David D. Porter,
Acting Rear-Admiral, Commanding Mississippi Squadron.
Captain Henry Walke, U.S. Navy,
Commanding U.S.S. Carondelet.
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Order of Acting Rear-Admiral Porter, U.S. Navy, to the master of steamer Minnehaha, declining permission to land prisoners at Vicksburg.
YAZOO RIVER< December 28, 1862.
SIR: Under no circumstances will any communication be allowed with Vicksburg at this moment, much less will public prioners be allowed to land while a battle is going on. You will therefore return to Helena and report to the commanding military officer at that place.
Very respectfully, your obedient servant,
David D. Porter,
Acting Rear-Admiral, Commanding Mississippi Squadron.
Captain David White.
[Commanding Steamer Minnehaha.]
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SOURCE: O.R. Union and Confederate Navies, Series I, Vol 23, Naval Forces on Western Waters, pp. 582-583