Herbaceous riverbank/floodplain  (S4)

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Herbaceous riverbank/floodplains are wet to mesic meadows on silty or sandy soils along the banks of large rivers, small rivers, and large streams. They are dominated by various combinations of tall herbs, including reed canary grass, big bluestem, goldenrods, robust ferns, and various sedges. Shrubs and tree saplings mix with herbs in some examples, but amount to less than 25% cover. These medium to high riverbank communities sometimes extend onto adjacent floodplain settings, and can be similar in appearance to meadow marshes. In contrast to meadow marshes, however, there is little or no organic matter accumulation due to flood and ice scour, and rapid decomposition occurs during low water periods. This community is found statewide.

Characteristic Vegetation: Dominated by various combinations of tall herbs, including reed canary grass (Phalaris arundinacea), big bluestem (Andropogon gerardii), goldenrods (Solidago spp.), robust ferns, and various sedges.

Variants: The community is broadly defined, and five variants are described:
1.) Reed canary grass variant 
   This variant is dominated by reed canary grass (Phalaris arundinacea). Several other species occur with a cover of less than 1%. Soils are fine sand with some silt. This variant ranges from 1.0-1.8 m above midseason water levels. Its full distribution and abundance are not well documented. 
   Good examples of this variant occur along the Merrimack River, Cocheco River, and Little Cohas Brook.

2.) Bluejoint variant 
   The description for this variant is based on one site along the Exeter River where several acres of open floodplain are dominated by bluejoint. A thin organic fibric layer (1 cm) covers a mesic to saturated silt layer. A layer of marine clay exists below the silt (>20 cm). This variant may be limited to coastal New Hampshire. A site along the Cocheco River is transitional between this variant and the reed canary-grass riverbank/floodplain variant. 
   This variant is dominated by a dense cover (60%) of bluejoint (Calamagrostis canadensis). Associates include tussock sedge (Carex stricta), lake sedge (C. lacustris), reed canary-grass (Phalaris arundinacea), meadowsweet (Spiraea alba), silky dogwood (Cornus amomum), northern arrowwood (Viburnum dentatum), nannyberry (Viburnum lentago), and scattered red maple (Acer rubrum), swamp white oak (Quercus bicolor), and American elm (Ulmus americana) saplings. 
   A good example of this variant occurs along the Exeter River.

3.) Goldenrod variant 
   A dense cover of Canada goldenrod (Solidago canadensis) dominates this variant. Common associates include rough goldenrod (Solidago rugosa), silverleaf grape (Vitis aestivalis), box elder (Acer negundo), poison ivy (Toxicodendron radicans), Morrow’s honeysuckle (Lonicera morrowii), Virginia creeper (Parthenocissus quinquefolia), common blackberry (Rubus allegheniensis), and Japanese knotweed (Fallopia japonica). Soils are fine sand and loam over 50 cm deep. 
   Good examples of this variant occur along the Pemigewasset and Connecticut Rivers.

4.) Northern herbaceous variant 
   This variant occurs along several major streams and minor rivers north of the White Mountains. It is characterized by graminoids or a mix of graminoids and forbs on intermittently flooded silt or fine, sandy alluvial soils of moderate-energy environments. It occurs at slightly higher elevations in the riparian zone than the cobble - sand river channel but lower than alder alluvial shrublands. Species composition includes both wetland and moist-meadow species generally <1–1.5 m tall. Non-Sphagnum mosses may be abundant. Microtopography may undulate slightly due to the presence of abandoned, intertwining stream channels. 
   Characteristic species include fringed brome grass (Bromus ciliatus), grass-leaved goldenrod (Euthamia graminifolia), broom sedge (Carex scoparia), Rudge’s sedge (C. debilis), tussock sedge (C. stricta), follicled sedge (C. folliculata), rough cinquefoil (Potentilla norvegica), Mexican muhly (Muhlenbergia mexicana), soft rush (Juncus effusus), neglected reed bent-grass (Calamagrostis stricta var. inexpansa), bent grasses (Calamagrostis spp.), currants (Ribes spp.), and moss species. Medium-height shrubs such as meadowsweet (Spiraea alba) and steeplebush (Spiraea tomentosa) are occasional or locally abundant.
   A good example of this variant occurs along the Upper Ammonoosuc River.

5.) Inflated sedge variant 
   This variant is dominated by inflated sedge (Carex vesicaria) with lesser amounts of woolly bulrush (Scirpus cyperinus), rattlesnake mannagrass (Glyceria canadensis), reed canary grass (Phalaris arundinacea), tussock sedge (Carex stricta), and meadowsweet (Spiraea alba). Infrequent plants are bulbiferous water hemlock (Cicuta bulbifera), arrow-leaved tearthumb (Persicaria sagittata), false nettle (Boehmeria cylindrica), field bindweed (Convolvulus arvensis), and other species. This sedge-dominated meadow also supports a low shrub cover. The soil is silt and fine sand. Moderate-sized swales supporting this variant are sometimes intermixed in a mosaic dominated by silky dogwood. 
   A good example of the inflated sedge variant occurs along the Winnicut River.

6.) Big bluestem variant
   Big bluestem (Andropogon gerardii) is diagnostic and is usually the most common herb. Woody plant richness is moderate, while cover is low to moderate. Scattered mature trees typically average around 5% cover.
   Good examples of the big bluestem variant occur at Manchester River Outcrops (Manchester), Garvins Falls (Concord), and Bellows Falls (Walpole).


Herbaceous riverbank/floodplains often occur as part of larger low-gradient silty-sandy riverbank systems, moderate-gradient sandy-cobbly riverbank systems, temperate minor river floodplain systems, montane/near boreal floodplain systems, and major river silver maple floodplain systems.

Herbaceous riverbank/floodplain along the Merrimack River in Canterbury (photo by Dan Sperduto)

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