main stem

The Mississippi River drainage basin with the mainstem highlighted in dark blue

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.