Natural Communities of NH -- Photo Guide

Acidic riverside seep  (S1)



Riverside seeps occur on open bedrock, cobble, sand, or silt substrates of flood scoured shores along our larger rivers, frequently in narrows below dams. Cold groundwater emerges from bedrock cracks and generates these fen-like communities where otherwise competing woody species are swept away by floodwaters and ice floes. 

Acidic riverside seeps are rare in New England, but occur infrequently throughout New Hampshire. This community occurs on seepy, open bedrock or on the cobble, sand, or silt substrate of flood-scoured shores of larger rivers where cold groundwater emerging from bedrock generates fen-like conditions. It is distinguished from calcareous riverside seeps by the absence of rich-site indicators and from acidic riverbank outcrops by the presence of acidic, wet-site indicators.

Characteristic Vegetation: Characteristic species include leatherleaf (Chamaedaphne calyculata), large cranberry (Vaccinium macrocarpon), gerardias (Agalinis spp.), lance-leaved violet (Viola lanceolata), rhodora (Rhododendron canadense), male berry (Lyonia ligustrina), round-leaved sundew (Drosera rotundifolia), silvery sedge (Carex canescens), ostrich fern (Matteuccia struthiopteris), swamp candles (Lysimachia terrestris), woolly bulrush (Scirpus cyperinus), bluets (Houstonia caerulea), meadowsweet (Spiraea alba), alders (Alnus spp.), St. Johns-worts (Hypericum spp.), mosses, and liverworts. Northern occurrences may have such species as black spruce (Picea mariana), creeping snowberry (Gaultheria hispidula), and Labrador tea (Ledum groenlandicum).

When present, drier outcrop areas often support such species as little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), panic grasses (Panicum spp.), asters (Aster spp.), grass-leaved goldenrod (Euthamia graminifolia), and numerous other graminoids and composites.



Good examples of this community occur along the Ammonoosuc and Pemigewasset Rivers in the White Mountains.

Acidic riverside seeps often occur as part of high-gradient rocky riverbank systems and moderate-gradient sandy-cobbly riverbank systems.


Acidic riverside seep along the Merrimack River in Concord (photo by Ben Kimball for the NH Natural Heritage Bureau)
Acidic riverside seep along the Merrimack River in Concord (photo by Ben Kimball)

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