Main stem

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".

[3] 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.

[2] In the United States, the Mississippi River mainstem achieves a Strahler number of 10, the highest in the nation.

More than 1.5 million of these small streams, with average drainage basins of only 1 square mile (2.6 km2), have been identified in the United States alone.