The major barrier to the union Gunboat using the Arkansas River was the uncertain water level and the shoals at the lower Arkansas between Arkansas Post and Pine Bluff. That however was not a great obstacle however. Commander T.O. Selfridge of the USS Constatoga was constantly wanting to command an expedition up the Arkansas to Capture the Ponchartrain and destroy it while it was at anchor at Little Rock. So an assualt up the Arkansas River was NOT an impossibility and was not unthought of, or unplanned.

The main reason for the Union navy not to come up the Arkansas was tactical. The Union really wanted control of the Arkansas River Valley because with the Arkansas in their control it would solidify their control of Missouri and force the Confederate back to the Red River. But they did not have the resources to do everything until after the fall of Vicksburg and Port Gibson.

Becuase of the Union's inability to pursue everything at once doesn't deminish the importance of Arkansas operations. For one thing the necessary need for the Confederates to protect such important points as Arkansas Post and Pine Bluff and St Charles reduced the numbers of Confederates which would be available to fight at places like Praire Grove. So in fact from a Union standpoint NOT attacking up the Arkansas River was a Military plan also. It tied up Confederate resources.

The fact that the Confederates defended the Lower Arkansas as well as they did likewise made it necessary for Steele to attack Little Rock from the north rather than using the Arkansas River. Etc. etc. etc.

It is this kind of "chess game" warfare that influenced the military operations along the Lower Mississippi, Arkansas and White Rivers much more than the attitude of "lets attack and wipe them all out". That does not say the concerns, the advantages, and the disadvantages were any less significant. Lets say that If the Confederates had not defended say Arkansas Post or Pine Bluff, Would you have expected the Union Commander to pass up such important plums when they were so freely offered?