As the nation's largest estuary, the Chesapeake Bay contains a diverse collection of habitats and salinity regimes. In order to address the diversity of habitats in the southern Chesapeake Bay subregion, the NERRS established a multi-component system along the salinity gradient of the York River estuary.
The York River estuary is the Chesapeake Bay’s fifth largest tributary in terms of flow and watershed area - on the order of 6900 km2 (2662 mi2). The York River basin is located within Virginia’s Coastal Plain and Piedmont physiographic provinces, and includes all of the land draining into the Mattaponi, Pamunkey and York rivers. The York River system is classified as a microtidal, partially mixed estuary. The mean tidal range is 0.7 m (2.3 ft) at its mouth, 0.9 m (1.0 ft) at West Point, and increases to more than 1 m (3.3 ft) in the upper tidal freshwater regions of the Mattaponi and Pamunkey Rivers. Salinity distribution along the York River estuary ranges from tidal freshwater to polyhaline regimes.