Tidal Freshwater Marshes
This is a diverse group of herbaceous wetlands subject to regular diurnal flooding along the upper tidal reaches of inner Coastal Plain rivers and tributaries. Ranging from New York to North Carolina, freshwater marshes occur in the uppermost portion of the estuarine zone, where the inflow of saltwater from tidal influence is diluted by a much larger volume of freshwater from upstream. In Virginia, tidal freshwater marshes are best developed on sediments deposited by large meanders of the Pamunkey and Mattaponi Rivers, although outstanding examples also occur along the Potomac, Rappahannock, Chickahominy, and James Rivers. Strictly speaking, freshwater conditions have salt concentrations < 0.5 ppt, but pulses of higher salinity may occur during spring tides or periods of unusually low river discharge.
The most common species are arrow-arum (Peltandra virginica) dotted smartweed (Persicaria punctata), wild rice (Zizania aquatica var. aquatica), pickerelweed (Pontederia cordata var. cordata), rice cutgrass (Leersia oryzoides), tearthumbs (Persicaria arifolia and Persicaria sagittata), and beggar-ticks (especially Bidens laevis and Bidens trichosperma). Locally, sweetflag (Acorus calamus), tidal-marsh amaranth (Amaranthus cannabinus), marsh partridge-pea (Chamaecrista fasciculata var. macrosperma), and southern wild rice (Zizaniopsis miliacea) may form dominance patches. Species diversity and vegetation stature vary with salinity, duration of inundation, and disturbance; the most diverse marshes occupy more elevated surfaces in strictly freshwater regimes. Mud flats that are fully exposed only at low tide support nearly monospecific stands of common spatterdock (Nuphar advena), although cryptic submerged aquatic species may also be present.
Tidal freshwater marshes provide the principal habitat for the globally rare plant sensitive joint-vetch (Aeschynomene virginica) and are important breeding habitats for a number of birds, e.g., the least bittern (Ixobrychus exilis) . Chronic sea-level rise is advancing the salinity gradient upstream in rivers on the Atlantic Coast, leading to shifts in vegetation composition and the conversion of some tidal freshwater marshes into oligohaline marshes. Tidal Freshwater Marshes are also threatened by the introduced invasive plant marsh dewflower (Murdannia keisak). Several communities in this group are chiefly restricted to the Chesapeake Bay drainage basin and are likely globally rare or uncommon.References: Ahnert (1960), Coulling (2002), McCoy and Fleming (2000), Megonigal and Darke (2001), Parker and Wyatt (1975), Perry and Atkinson (1997), Perry and Hershner (1999).
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Freshwater Tidal Marsh composed primarily of common spatterdock (Nuphar advena) along Accokeek Creek, Stafford County. Photo: Gary P. Fleming / © DCR Natural Heritage.
A rare type of Freshwater Tidal Marsh dominated by American lotus (Nelumbo lutea). Potomac Creek, above its confluence with Accokeek Creek and the Potomac River, Stafford County. Photo: Irvine Wilson / © DCR Natural Heritage.
Wild rice (Zizania aquatica var. aquatica) and pickerelweed (Pontederia cordata var. cordata) in a Freshwater Tidal Marsh along Baileys Creek, near its confluence with the James River, Henrico County. Photo: Irvine Wilson / © DCR Natural Heritage.
REPRESENTATIVE COMMUNITY TYPES:
Nine community types have been recognized, based on the analysis of > 170 plots (map). The overwhelming majority of samples, however, come from only two river systems, the Pamunkey and Mattaponi. Freshwater marshes in the James, Rappahannock and Potomac watersheds are especially underrepresented and warrant further inventory, which is also needed to determine global and state conservation ranks more confidently. Quantitative information on the riverine marshes of the bayside of the Eastern Shore (Pocomoke River watershed, Accomack County) are also sorely lacking. However, most of the types listed below are relatively robust units, classified in a comprehensive regional analysis of all available Maryland and Virginia tidal data (NatureServe, in prep.) Click on any highlighted CEGL code below to view the global USNVC description provided by NatureServe Explorer.>