Before revision: 300x300px|The Mississippi River drainage basin with the main stem highlighted in dark blue In hydrology, a main stem (or trunk) is "the primary downstream segment of a river, as contrasted to its tributaries".
After revision: 300x300px|The Mississippi River drainage basin with the mainstem highlighted in dark blue In hydrology, a main stem (or trunk) is "the primary downstream segment of a river, as contrasted to its tributaries".