Natural Communities of NH -- Photo Guide

Riverweed river rapid  (S2S3)

Seed-bearing plants are uncommon in permanently flooded, high-energy parts of river channels. However, in the riverweed river rapid community, riverweed (Podostemum ceratophyllum) forms low mats on submerged rocks in rapids, waterfalls, and other fast-flowing sections of streams and rivers, primarily in central and southeastern New Hampshire. This plant attaches itself to rocks using adhesive-secreting, hold-fast like roots. The community usually occurs as a monoculture of riverweed, though pondweeds occasionally grow in gravel between riverweed covered rocks. This community type is presently described only from the Lamprey River. Examples probably also occur in other rivers that contain riverweed.

River rapids occur where the slope of a river or stream is steep enough to support fast water velocity and whitewater. Water flows all year in at least the deeper portions (thalweg) of the channel. A series of riffles and pools often characterize river rapids. Although river rapids most often occur in upper perennial streams and rivers, they can also be found on steeper gradient portions of lower perennial rivers.   

Characteristic vegetation: Riverweed is the characteristic species of this community. This clonal plant is restricted to river rapids, where it forms a low mat on submerged rocks. Other plants growing directly in the fast flowing waters include white water crowfoot (Ranunculus trichophyllus) and knotty pondweed (Potamogeton nodosus).

Good examples of this community occur in rapids of the Lamprey River (Epping) and the Contoocook River (Henniker).

Riverweed river rapids typically occur in association with moderate-gradient sandy-cobbly riverbank systems.

Riverweed river rapid along the Cocheco River (photo by Ben Kimball for the NH Natural Heritage Bureau)
Riverweed river rapid along the Cocheco River (photo by Bill Nichols) 

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