Natural Communities of NH -- Photo Guide

Calcareous 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. 

Calcareous riverside seeps occur at river narrows of major rivers and below dams (usually at river narrows where riverside seeps were likely natural), on outcrops and occasionally on sediments of steep terraces or cobble bars where there is year-round influence of groundwater seepage. Emergent and subsurface seepage through silty cracks in bedrock, or in cobble, gravel, sand, or silt substrates is evident by the presence of species indicative of cold, fen-like, calcareous conditions. Species include Kalm's lobelia, sticky false asphodel, and grass-of-parnassus, as well as other forbs, grasses, and sedges, and even some midwestern prairie species. Annual flood and ice scour is intense and removes competing woody vegetation. Drier, non-seepy or ledge outcrop areas may be interspersed with seepy spots.

Soils tend to be turfy sands (i.e., sand impregnated with a tightly-woven, fine root mass) wedged in boulders, cobbles and cracks in outcrops. Annual flooding prevents the accumulation of organic matter. Less often, seep vegetation can be found in unconsolidated sediments of steep river terraces or on silty banks. Partial shading from trees and shrubs is common. These seeps appear to be restricted to areas with considerable calcareous bedrock influence, or at least mineral-enriched groundwater. The pH of seepage water ranges from 6.8 to 8.2.

These seeps often contain a number of characteristic species that are rare in the state. Like circumneutral riverbank outcrops, this community only occurs in the Connecticut River valley. 

Characteristic Vegetation: Dominant plant species vary between examples, but many characteristic and rare forbs, grasses, and sedges are present, including midwestern prairie species. Characteristic species include Kalm’s lobelia (Lobelia kalmii), sticky false asphodel (Tofieldia glutinosa), grass-of-Parnassus (Parnassia glauca), Garber’s sedge (Carex garberi), hair-like beak-rush (Rhynchospora capillacea), variegated horsetail (Equisetum variegatum), dwarf ragwort (Senecio pauperculus), muskflower (Mimulus moschatus), shining ladies’-tresses (Spiranthes lucida), (bulrushes) Scirpus spp., and bluets (Houstonia caerulea). 

On drier areas, a mix of big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), blue-green hairgrass (Deschampsia caespitosa), harebell (Campanula rotundifolia), poison ivy (Toxicodendron radicans), and dwarf cherry (Prunus pumila var. depressa) may occur. Shrub border areas include red osier dogwood (Cornus sericea), mountain alder (Alnus crispa), meadowsweet (Spiraea alba), and willows (Salix spp.).


Good examples of this community occur along the Connecticut River.

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


The remains of a calcareous riverside seep community below Bellows Falls (photo by Ben Kimball for the NH Natural Heritage Bureau)
The remains of a calcareous riverside seep community below Bellows Falls (photo by Ben Kimball)

Calcareous riverside seep at Sumners Falls on the CT River (photo by Ben Kimball for the NH Natural Heritage Bureau)
Calcareous riverside seep at Sumners Falls on the CT River (photo by Ben Kimball)

Calcareous riverside seep at Sumners Falls on the CT River (photo by Ben Kimball for the NH Natural Heritage Bureau) The remains of a calcareous riverside seep community below Bellows Falls (photo by Ben Kimball for the NH Natural Heritage Bureau)

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