Boulder - cobble river channel (S3)
Boulder - cobble river channel communities are mostly found along the high-gradient riverbeds of streams and small to medium-sized rivers common in mountainous and hilly areas. They occur on flashy rivers, flooding rapidly and intensely with snowmelt runoff in the spring and during peak rain events. The floods create a high-energy, erosional environment that minimizes deposition of fine soil particles and leaves only the coarsest substrates behind. Peak flows and ice-scour are also intense enough to prune back and shorten any woody vegetation that becomes rooted among the rocks. The rocky banks often dry up in the summer, at least on their upper margins, but remain moist closer to the river’s edge.
Boulders and cobbles are the dominant substrate, though gravel and sand also occur among the boulders or cobble at some sites. Very little organic litter accumulates. This coarse substrate is sparsely vegetated with tall shrubs (such as alders and willows), tree seedlings, and herbs, particularly grasses and composites. While many examples have only a few isolated plants emerging among the rocks, some examples grade into areas with greater than 25% cover at the transition to other communities. Species composition can be variable between examples, probably due to local variation in propagule availability of stress- and disturbance-tolerant species. This community is defined primarily by the high-energy environment of cobble and boulders, and the presence of species tolerant of flood-scour disturbance.
Characteristic vegetation: Woody seedlings often reflect the composition of the surrounding forest and include red maple (Acer rubrum), white pine (Pinus strobus), paper birch (Betula papyrifera), and red oak (Quercus rubra). Tall shrubs are usually present in low abundance. Willows (Salix spp.) and speckled alder (Alnus incana) or smooth alder (Alnus serrulata) are found in most examples. Other characteristic shrubs may include western black raspberry (Rubus occidentalis), showy mountain ash (Sorbus decora), pin cherry (Prunus pensylvanica), beaked hazelnut (Corylus cornuta), sweet gale (Myrica gale), silky willow (Salix sericea), and brambles (Rubus spp.). Dwarf bilberry (Vaccinium cespitosum), characteristic of alpine areas and northern riverbanks, may occur here or in adjacent riverbank habitat.
Grasses are common and may include bent grasses (Agrostis spp.), reed canary grass (Phalaris arundinacea), little bluestem (Schizachyrium scoparium), common hair-grass (Deschampsia flexuosa), bluejoint (Calamagrostis canadensis),
Rare subalpine plants found in some examples of this community in the White Mountains include mountain avens (Geum peckii), Pickering’s bluejoint (Calamagrostis pickeringii), and New England northern reedgrass (Calamagrostis stricta). The high-energy environment generally prevents the establishment of lichens.
Good examples of this community occur along the headwaters of most large rivers and streams in the White Mountains.
Boulder - cobble river channels usually occur as part of larger high-gradient rocky riverbank systems, and sometimes as part of moderate-gradient sandy-cobbly riverbank systems.
Boulder - cobble river channel along the Peabody River near Gorham (photo by Ben Kimball)
Boulder - cobble river channel along the Peabody River (photo by Dan Sperduto)
Boulder - cobble river channel on the Bearcamp River in Tamworth (photo by Ben Kimball)
Boulder - cobble river channel along the Wild Ammonoosuc River in Landaff (photo by Ben Kimball)