Main stem

Common card · No. 294864

In hydrology, a main stem or mainstem (also known as a trunk) is "the primary downstream segment of a river, as contrasted to its tributaries". The mainstem extends all the way from one specific headwater to the outlet of the river, although there are multiple ways to determine which headwater (or first-order tributary) is the source of the mainstem. Water enters the mainstem from the river's drainage basin, the land area through which the mainstem and its tributaries flow. A drainage basin may also be referred to as a watershed or catchment. Hydrological classification systems assign numbers to tributaries and mainstems within a drainage basin. In the Strahler number, a modification of a system devised by Robert E. Horton in 1945, channels with no tributaries are called "first-order" streams. When two first-order streams meet, they are said to form a second-order stream; when two second-order streams meet, they form a third-order stream, and so on. In the Horton system, the entire mainstem...

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