Well, I came across even more Butte- a -la Rose information. I have gone from "Huh?" to probably America's foremost expert on the subject, or, "Have I?"
I came across two maps drawn by Union forces after the battle of what they called Fort Butte La Rose. They were done by J. G. Ohmann, sub-assistant to the US Coast Survey. One shows the fort itself, and the other showed vegetation, drainage, houses, fences, roads leading from the fort, positions of US steamers Clifton, Arizona, Calhoun, and Estrella, as well as those of the Confederate transport Anna and gunboat Marytie.
The fort appears to have mounted three cannons, was facing northeast to the Atchafalaya River and with a creek behind it. There was also a pond in the center of it. I read that 60 men and lots of supplies were captured when the fort surrendered.
I would think these maps, which appear to be quite precise should be enough of a source to help locate the remains of the fort.
I also found an 1863 Harper's Weekly sketch by H. Holtz showing the engagement at Butte La Rose in http://americancivilwar.com. It shows the US steamers Estrella, Calhoun, Arizona, and Clifton engaging the CSS J.A. Cotton 20 April 1863.
This also has a nice view of the Confederate fort saying that it was captured later that day.
And the Story Goes On and On. And, I'm Not Finished Yet. --B-Runner