Thanks for the discussion. Sorry to get into such "deep water", but the "shallow" explainations for such a blockade in the past just don't seem, to me, to make much military sence to allocate such valuable resources to such meaningless duty where other avenues of handleing the problem were not only available, but appearently according to the OR's discussed and dismissed.
I am not entirely sure that the containment of the CSS Ponchartrain was entirely the reason for that blockade. Certainly it wasn't to deny the use of the Arkansas and White Rivers to the Confederates. The Blockading boats were too easy to evade.
My thesis is, First that the Conastoga and Gen Bragg were there as picket boats to give warning should the Ponchartrain make a run into the Mississippi. Not necessarily there to engage and destroy the Ponchartrain, because certainly they, by themselve, weren't powerful enough to do so. The Union navy command appeared to be quite content for the Ponchartrain to remain idle at Little Rock, except for Selfridge himself. Their mission was to keep tract of the Ponchartrain and give warning of her breakout, should the confederates choose to use her in that manner, as a raider, to the other river traffic on the Mississippi.
Second, by not totally closing the Arkansas and White Rivers, with Chain Booms or Garrisons, left open the possibilities that the Union navy could make a lighting dash attack up the Arkansas River. Just like at Arkansas Post. This would force the Confederate to build and rebuild defences and maintain garrisons along the Lower Arkansas thereby preventing the troops in those garrisons from going to the relief of Vicksburg, which was the Union high command first priority.
While low water levels on the Lower Arkansas did play a part in such matters the water levels on the Lower Arkansas weren't low all the time, especially during the normal spring flooding season. So that certainly doesn't explain totally why no assualt was ever made by the Union Navy up the Arkansas River.
Henry Merrill, in his autobiography, observed, and rightly so, that had Pine Bluff been captured, Little Rock would have been abandoned without a shot being fired, because the Confederate Supply lines to Texas at Benton, south of Little Rock, would have been threatened. This in itself is a compelling argument for the Union navy to have advanced up the Arkansas River far enough to have assisted in the capture of at least Pine Bluff. This would have effectively taken almost the entire State of Arkansas out of the War for the Confederacy as much as a year before these events actually transpired. But, they chose not to attack by this avenue when they finally did capture Little Rock.
So there must have been a more compelling reason than just low water levels for U.S. Grant and ADm. Porter to pass up such oppertunities as this. If you argue that the Union Army and Navy were overextended as a reason for not making such an assualt then that makes the use of the Conastoga and Gen Bragg for Blockadeing detail even more questionable, unless there was a threat of something coming out of the Arkansas River on a raid.
The Ponchartrain certain had the run of the Arkansas River as far south as Pine Bluff. And while she wasn't that powerful without some guns, could she in conjunction with the fortifications, defences and guns at Pine Bluff been dangerous enough to have threaten, as a Ram, a Union navy fleet especially troop transports?
As you say I guess we will never know without some more detailed study. I just wanted to see if anyone else had some ideas, or information on the Ponchartrain, about this beside me.
Sorry for the lenght of the posting.