First
According to the O.R.s those guns were not part of the orignal compliment of weapons found on the CSS Ponchartrain. Those two weapons were picked up from the guns at Fort Randolph north of Memphis in late May of 1862 as the northern forts along the Mississippi river were being abandoned due to the movements of Grants army after Shiloh and Island #10.
Second
The CSS Ponchartrain and Maurepas were ordered to Arkansas in responce to Governor Rector's threat to Seceed from the Confederatacy due to Van Dorn abandonment of Arkansas and moving the Army of the West to Mississippi. Those guns and extra ordinance were picked up at Fort Randolph in responce to Rectors demand that the Confederacy support Arkansas. In other words President was in some ways engaging in "Gunboat Diplomacy". This is why The Ponchartrain and Maurepas missed the naval Battle of Memphis June 6th, 1862.
Third.
I have a problem with any mention of the CSS Ponchartrain ever being in the "White River" or anywhere close to St. Charles. She simply had too deep a draft to steam up the White River. The Ponchartrain's Boilers and running gear were housed below desk in the Hull of the ship. This made for excellant protection of that equipment from shell and shot during battle, but hindered the manuveribility of the ship because she had an 10 foot plus draft when empty and required deep water to sail. She was designed to only sail in the deeper waters of the Mississippi River. She was only able to cross the bar of the lower Arkansas River between Pine Bluff and Arkansas Post, which was only reportedly normally 5 feet deep. The Ponchartrain was so overloaded with these extra guns and ordinance, which may have included a battery of field artillery besides her own five guns and the two 32 pd Fort Randolph guns, that she was so overloaded that her decks were almost awash as she steamed up the Arkansas R. in May of 1862.
Union City Class Gunboats reported that after the battle of St Charles June 1862 the White River was falling and that they were scrapping bottom at 6 feet as they withdrew. The White Rivers normal depth would have been less than that. This information comes from the Naval OR's reports of the Battle of St Charles.
Union Naval Commander Thomas O Selfridge who have commanded the USS Cairo and was in command of the USS Conestoga assigned to picket the mouth of the Arkansas and White Rivers kept a constant watch and recieved regular reports of the Water levels on the Arkansas R. from Fort Smith in order to determine if the Ponchartrain was a threat of breaking out into the Mississippi River, and for the purpose of launching his own attack up the Arkansas against Pine Bluff and Little Rock.
The water levels were the reason for the building of a Fort at Arkansas Post was as a deep water secure landing for boats running the Union Navy Patrols of the Mississippi. Possibly even as a home port for the Ponchartrain after it had been refitted as an Ironclad.
Clearly If the Ponchartrain ever went Up the White River, If there ever was enough water to get over those same bars, the rapid rising and falling of these rivers would have seriously endangered the Ponchratrain of being grounded and stuck forever being useless, which it pretty much was at Little Rock anyway. But in any case the Arkansas River above Pine Bluff was the only place deep enough to float the Ponchartrain reasonable and at times even there it was stuck in the mud from time to time.
Guns from the Ponchartrain were transported by Train to DeVal's Bluff and the floated downstream by boats like the "Blue Wing" to St. Charles. This is the way that two 8 inch Columbiads were transported to St. Charles and emplaced by the 24th Arkansas Infantry in December of 1862. These guns were dismounted and taken back to Devals Bluff as the Battle of Arkansas Post was going on. They were captured by the Union Gunboats having been loaded onto flatcars to be taken back to Little Rock. Again this Information is found in the Naval OR's reports of the Battle of Arkansas Post.
There is still one gun unaccounted for. The CSS Maurepas was reported to have two guns. One mounted on the bow, and one of the stern. These heavy guns were reportedly NOT removed when the Maurepas was sunk. The Union Naval reports the recovery of only one gun from the bow of the Maurepaus. It could have been that the stern gun was the 12 pd mountain howitzer that was removed by the Confederates for use on land. But that is unclear.
Just my 2 cents worth from what I have researched.