Natural Community Systems -- Photo Guide

High-gradient rocky riverbank system

high-gradient rocky riverbank system along the Peabody River near Gorham (photo by Ben Kimball for the NH Natural Heritage Bureau)
high-gradient rocky riverbank system 
along the Peabody River near Gorham 
(photo by Ben Kimball)  

 

Description:  River channels and banks in steep gradients are degradational environments in which fine sediments are transported downstream at high or low river stages, leaving boulders and bedrock as the dominant channel substrate.  Ice and flood scour are pronounced.  Meanders and bars comprised of finer sediments are sparse.  High-gradient rocky riverbanks are extensive along the upper reaches of rivers leading out of the mountains, and scattered on high-gradient sections of other rivers elsewhere in the state.  Sparsely vegetated boulders and some cobble are characteristic channel and riverbank material, often with sand in the interstices.  Exposures of sediments finer than cobble are intermittent or rare.  Riverbank fern glades, alder alluvial thickets, or other herbaceous to wooded vegetation occurs on slightly higher riverbanks.  Outcrops are present in some examples; riverside seeps are rare.  Rare or uncommon northern and subalpine plants are found in this system along northern rivers (not found in low energy settings or southern New Hampshire).


Diagnostic natural communities:

River channels

   • Boulder - cobble river channel (S3)

   • Cobble - sand river channel (S3S4)

Riverbanks

   • Alder alluvial shrubland (S3) 

   • Herbaceous riverbank/floodplain (S4)

   • Acidic riverbank outcrop (S3)

   • Circumneutral riverbank outcrop (S1)

   • Acidic riverside seep (S1)

   • Calcareous riverside seep (S1)


Landscape settings: river channels and riverbanks along high-gradient sections of rivers and large streams; below the bankful flood-stage of river marked by transition to floodplain (when floodplain is present)

Soils: primarily bedrock, boulders, stones, and some cobble with interstitial sand and gravel; 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 cobble bar deposits

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

Distribution: most common in the White Mountains and north, and scattered along upper reaches and intermittent steep-gradient sections of minor and major rivers throughout much of the state


Characteristic species:

   Trees and shrubs
      Alnus incana ssp. rugosa (speckled alder)
      Salix spp. (willows)
      Seedlings and saplings of tree species

   Herbs
      Osmunda claytoniana (interrupted fern)
      Deschampsia caespitosa (blue-green hair-grass)
      Deschampsia flexuosa (common hair-grass)
      Calamagrostis canadensis (blue-joint)
      Danthonia spicata (poverty oat-grass)
      Panicum spp. (panic grasses)
      Aster umbellatus (tall flat-topped white aster)
      Solidago bicolor (silverrod)

   Other composites
      Eupatorium maculatum (spotted Joe-pye-weed)
      Houstonia caerulea (bluets)
      Carex torta (twisted sedge)
      Fragaria virginiana (wild strawberry)

Rare or uncommon northern and subalpine plants: 
      Vaccinium cespitosum (dwarf bilberry)
      Calamagrostis pickeringii (Pickering's reed bent-grass)
      Calamagrostis stricta ssp. inexpansa (pond reed bent-grass)
      Hieracium robinsonii (Robinson's hawkweed)
      Agrostis mertensii (boreal bent-grass)

Riverside seeps:
      Tofieldia glutinosa (sticky false asphodel)
      Drosera rotundifolia (round-leaved sundew)
      Carex garberi var. bifaria (Garber's sedge)


Associated natural community systems:  This system typically borders upland forests on till or high river terraces and does not occur along river sections with well-developed floodplains.  Occasionally it is adjacent to the upper reaches of montane/near-boreal floodplain systems.


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