Statistics for occurrence #1 of “Potomac River” in chapter 3 of The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 4: The Cavalry:

... of three cavalry companies, at Harper's Ferry, in April, 1861. Clarke's cavalry was stationed at the bridge over the Shenandoah River near Harper's Ferry; Ashby's company was at the bridge over the Potomac River at the Point of Rocks, and Drake's company was at the bridge at Brunswick. J. E. B. Stuart was commissioned as lieutenant-colonel and assigned to the command of the cavalry in the district then com...
Max. Freq. Min. Freq.
Entity Corpus Doc Corpus Doc  
Potomac River (United States) 286 0 0 0 0 user votes
Potomac River (Maryland, United States) 2 0 0 0 0 user votes
Potomac River (Maryland, United States) 2 0 0 0 0 user votes

† This entity has been selected by the automated classifier as the most likely match in this context. It may or may not be the correct match.