Natural Community Systems -- Photo Guide

Moderate-gradient sandy-cobbly riverbank system

moderate-gradient sandy-cobbly riverbank system along the Pemigewasset River near Plymouth (photo by Ben Kimball for the NH Natural Heritage Bureau)
moderate-gradient sandy-cobbly riverbank system 
along the Pemigewasset River near Plymouth 
(photo by Ben Kimball)


Description:  This system is associated with moderate gradient, moderate to high-energy sections of major and minor rivers with frequent sand, gravel, and cobble bar deposits and coarse-textured riverbanks.  Ice and flood scour are important annual disturbances, producing sparse to moderate cover of herbs and shrubs on coarse substrates.  Thus the natural communities in this system on average are sparsely vegetated.  Rapids or riffle sections are common among the depositional bars.  Floodplain forests (primarily silver maple, sugar maple, and balsam fir types) are often adjacent to this riverbank system.  This system lacks extensive deposits of boulders and rock that are characteristic of high-gradient rocky riverbanks.  It has a higher abundance of coarse deposits (gravel to cobble) compared to low-gradient silty-sandy riverbanks.  This system includes extensive alder thickets on large northern streams or small rivers that are flooded at least annually but lack floodplain forests.  This system lacks well-developed emergent marsh vegetation found in low-gradient riverbank systems.

Diagnostic natural communities:

River channels and low riverbanks

   • Boulder - cobble river channel (S3)

   • Cobble - sand river channel (S3S4)

   • Mesic herbaceous river channel (S4)

   • Twisted sedge low riverbank (S3S4)

   • Willow low riverbank (S3)

   • Dwarf cherry river channel (S2)

   • Hudsonia - silverling river channel (S1)

   • Riverweed river rapid (S4)

Medium to high riverbanks and open floodplains

   • Herbaceous riverbank/floodplain (S2S4)

   • Bluejoint - goldenrod - virgin’s bower riverbank/floodplain (S3S4)

   • Alder alluvial shrubland (S3)

   • Alluvial mixed shrub thicket (S4)

   • Acidic riverbank outcrop (S3)

   • Circumneutral riverbank outcrop (S1)

   • Acidic riverside seep (S1)

   • Calcareous riverside seep (S1)

Landscape settings: river channels and riverbanks (below the bankful transition to floodplain) along moderate-gradient sections of rivers and large streams

Soils: primarily alluvial sand, gravel, and cobble; oligotrophic to moderately minerotrophic

Spatial pattern: large patch, extensive narrow-linear (typically 5+ m wide and up to miles long); linear zones parallel to riverbanks or patchy zonation corresponding to intermittent bar deposits

Physiognomy: sparse woodland, tall shrub, medium-height shrub, herbaceous, sparsely vegetated, aquatic

Distribution: broadly distributed throughout the state

Characteristic species:
Species diagnostic of this system (generally not found on low-gradient silty-sandy riverbanks):
      Prunus pumila var. susquehanae (dwarf cherry)
      Prunus pumila var. depressa (dwarf cherry)
      Schizachyrium scoparium (little bluestem)
      Andropogon gerardii (big bluestem)
      Vaccinium cespitosum (dwarf bilberry)
      Calamagrostis pickeringii (Pickering's reed bent-grass)
      Solidago randii (Rand’s goldenrod)
      Hudsonia tomentosa (hairy hudsonia)
      Paronychia argyrocoma var. albimontana (silverling)

Associated natural community systems This system is associated with all three floodplain systems or may occur without a well-developed forested floodplain along upper reaches of large mountain streams with annually flooded shrub floodplains.  Montane/near-boreal floodplain systems are almost always associated with this riverbank system (but not vice-versa).

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