Before revision: 300x300px |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 mainstem (or trunk) is "the primary downstream segment of a river, as contrasted to its tributaries".