This ecological system of the northern Rocky Mountains and the east slopes of the Cascades consists of deciduous, coniferous, and mixed conifer-deciduous forests that occur on streambanks and river floodplains of the lower montane and foothill zones. Riparian forest stands are maintained by annual flooding and hydric soils throughout the growing season. Riparian forests are often accompanied by riparian shrublands or open areas dominated by wet meadows. Populus balsamifera is the key indicator species. Several other tree species can be mixed in the canopy, including Populus tremuloides, Betula papyrifera, Betula occidentalis, Picea mariana, and Picea glauca. Abies grandis, Thuja plicata, and Tsuga heterophylla are commonly dominant canopy species in western Montana and northern Idaho occurrences, in lower montane riparian zones. Shrub understory components include Cornus sericea, Acer glabrum, Alnus incana, Betula papyrifera, Oplopanax horridus, and Symphoricarpos albus. Ferns and forbs of mesic sites are commonly present in many occurrences, including such species as Athyrium filix-femina, Gymnocarpium dryopteris, and Senecio triangularis.
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