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Defining and delineating the active channel are of interest because in countries
such as the United States, the active channel of navigable rivers comes under regulatory jurisdiction by the national government, whereas the floodplain does not. The
ordinary high water mark defines the boundary between federal jurisdiction and
other property ownership. The ordinary high water mark is the line on the shore
established by the fluctuations of water and indicated by physical characteristics
such as a clear, natural line impressed on the bank; shelving; changes in the character of soil; destruction of terrestrial vegetation; or the presence of plant litter (Wohl
et al. 2016). The ordinary high water mark is thus defined based on field indicators,
rather than magnitude or return interval of a particular flow. Field delineation of the
Fig. 2.3 Aerial view of Buccaneer Creek in the western Brooks Range of Alaska. Although the
margins of the channel-migration zone are vegetated by shrubs and grasses, the edge of the forest
(and the dashed white lines) delineate the channel migration zone of the main channel. Arrow
indicates flow direction
2 Rivers as Ecosystems