Communities in this group are globally and state-rare, consisting of temporarily flooded, sparse shrub and dense grassland vegetation of stabilized outcrop or boulder bars along the shores of major mountain and Piedmont rivers. In Virginia, most of the few known occurrences are located in the Potomac River gorge west of Washington, D.C. and along the James River near the Blue Ridge. Habitats supporting Riverside Prairies are elevated above mean water levels and are flood-scoured at return intervals ranging from greater than annually to about 15 years. Because of rockiness and limited alluvial deposition, soils are relatively shallow and site moisture conditions range from mesic to quite xeric.
The vegetation is a lush assemblage of warm-season grasses and forbs, with scattered, small trees almost always present. In the Potomac gorge, where the greatest environmental and floristic variation is found, prairies occurring on and just above the channel shelf often have a savanna-like growth of stunted green ash (Fraxinus pennsylvanica), swamp white oak (Quercus bicolor), silky dogwood (Cornus amomum and Cornus obliqua), and other floodplain trees. Prairies occurring on a higher, less frequently flooded bedrock terrace contain upland trees such as white ash (Fraxinus americana), post oak (Quercus stellata), and pignut hickory (Carya glabra). Dominant grasses in both variants are usually big bluestem (Andropogon gerardii), indian grass (Sorghastrum nutans), and switchgrass (Panicum virgatum var. virgatum), with little bluestem (Schizachyrium scoparium var. scoparium) also abundant on higher, drier terrace outcrops. Other generally characteristic plants include blue wild indigo (Baptisia australis var. australis), violet bush-clover (Lespedeza frutescens), whorled rosin weed (Silphium asteriscus var. trifoliatum), Culver's-root (Veronicastrum virginicum), freshwater cordgrass (Spartina pectinata), narrow-leaf mountain-mint (Pycnanthemum tenuifolium), flattened spikerush (Eleocharis compressa var. compressa), veiny pea (Lathyrus venosus), and common golden-alexanders (Zizia aurea). Species characteristic of the Potomac River prairies include western sunflower (Helianthus occidentalis ssp. occidentalis), dense blazing star (Liatris spicata var. spicata), tall coreopsis (Coreopsis tripteris), and rough dropseed (Sporobolus clandestinus). Species characteristic of the James River prairies include Sampson's snakeroot (Orbexilum pedunculatum var. psoralioides), northern obedient-plant (Physostegia virginiana ssp. virginiana), and American vetch (Vicia americana var. americana).
Riverside Prairies often occur in patch-mosaics with Riverside Outcrop Barrens, but are readily distinguished by their dense grassland vegetation (vs. sparse vegetation on the outcrops).References: Fleming (2007), Fleming and Coulling (2001), Lea (2000), Rawinski et al . (1996).
Click on the images below to open a larger image in a separate window.
Big bluestem (Andropogon gerardii) and dense blazing star (Liatris spicata var. spicata , with purple flowers) in an elevated bedrock-terrace prairie just downstream from Great Falls, Fairfax County (Great Falls Park). Photo: Gary P. Fleming. Small patches of Riverside Prairie are frequent on benches of the larger Potomac River bedrock terraces downstream from Great Falls, Fairfax County. Catastrophic flood-scouring removes or stunts woody vegetation, allowing the graminoid-dominated herbaceous vegetation to persist. Photo: Gary P. Fleming. A large riverside prairie downstream from Great Falls, Fairfax County. Dominant grasses in this stand are big bluestem (Andropogon gerardii), river oats (Chasmanthium latifolium) , indian grass (Sorghastrum nutans), and rough dropseed (Sporobolus clandestinus). Photo: Gary P. Fleming.
A patch of riverside prairie on the Bedford County side of the James River Gorge (George Washington and Jefferson National Forests). Dominant plants in this stand are big bluestem (Andropogon gerardii), wild blue indigo (Baptisia australis var. australis), and Sampson's snakeroot (Orbexilum pedunculatum var. psoralioides). Photo: Tom Rawinski.