The vast majority of NCRN lies within the Potomac River watershed. The Potomac is the second-largest tributary of the Chesapeake Bay—America's largest estuary. The parks of the NCRN have a diverse array of natural forest communities determined in part by a wide range of underlying soils, geology, and topography. In the eastern U.S., these foundational groupings typically occur as north-south bands parallel to the Atlantic coast and the Appalachian Mountains.