Dej Potomac hla kev 383 mais ntawm Fairfax Stone, West Virginia rau Point Lookout, Maryland. Nws cuam tshuam ntau dua li 6 lab tus tib neeg uas nyob hauv Potomac watershed, thaj tsam 14,670-square-mile av uas dej ntws tawm ntawm lub qhov ncauj ntawm tus dej. Daim ntawv qhia no qhia txog qhov chaw ntawm tus dej thiab nws thaj chaw uas muaj thoob plaws ntau qhov chaw geological xws li Appalachian Plateau, Ridge & Valleys, Blue Ridge, Piedmont, thiab Nqes Hiav Txwv.
Lub ntsiab qia ntxiv rau tag nrho cov loj tributaries tag nrho 12,878,8 mais ua tus dej Potomac rau lub 21st nyob hauv tebchaws United States. Cov voj voog ntawm Potomac River yog North Branch, Savage River, South Branch, Cacapon, Shenandoah, Antietam Creek, Monocacy River, thiab Anacostia River. Lub Potomac empties rau hauv Chesapeake Bay nqes hav.