mainstem (hydrology)

A mainstem is defined as the principal river within a given drainage basin, in the case where a number of tributaries discharge into a larger watercourse. Viewed in terms of the Strahler Stream Order system, the mainstem would be the highest order stream amongst the streams in a given drainage basin. The United States National Weather Service considers the mainstem as the principal object of flood forecasting. In detailed analyses of riverine hydrology, mainstem also refers to precise channel mapping of the principal drainage; for example, in a braided channel or system with anabranch elements the mainstem is designated as the principal braid or channel within the overall river.
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