Before revision: upright=1.35|The Mississippi River drainage basin with the mainstem highlighted in dark blue In hydrology, a mainstem (or trunk) is "the primary downstream segment of a river, as contrasted to its tributaries".
After revision: upright=1.35|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".