Sometime during the Civil War, Colonel L.G. DeRussy, a Mexican American War veteran (1846-1848) and Chief Engineer for Louisiana during the Confederacy (1861-1865), surveyed the falls in the Red River by Alexandria. As Chief Engineer, he worked on projects to improve navigation on the Red River. In this map drawn prior to 1864, he noted the elevation changes and depths at the natural barriers that made up the "Alexandria rapids" in the Red River at Alexandria, Rapides Parish. These waterfalls made traversing the Red River at low water dangerous.
Somehow, DeRussy's map landed in the hands of the Union army. In 1864, map maker F. H. Gerdes superimposed DeRussy's map to depict the dams built by Lieutenant Colonel Bailey of the U.S. army as Porter's fleet fled from their losses at Grand Ecore just north of Natchitoches.
I don't have the date for when DeRussy's map came into Union Army's possession. That Lt. Bailey was able to construct two dams at the rapids relatively quickly in 1864 leads me to suspect that this map was obtained secretly, and used for military intelligence to help Porter's fleet navigate through the rapids.
Now, both maps of the rapids at Alexandria during the Civil War reside in the Library of Congress.
DeRussy's Map: https://www.loc.gov/item/99446417/
Gerdes's Map: https://www.loc.gov/item/99446418/