What is a watershed?

A watershed is the geographic area of land where all the water that is under it or runs off of it drains to a shared destination.  Every stream, tributary, or river has an associated watershed.  Watersheds come in all shapes and sizes.  Smaller watersheds are contained in larger watersheds.  As an example related to Allamakee County, the Ludlow Creek Watershed is part of the Yellow River Watershed.                                                                                                                                     Image from Watersheds.org
The Yellow River then flows into the Mississippi River and so is part of the upper Mississippi River watershed.  The Upper Mississippi watershed joins with the Missouri River watershed, the Ohio River watershed, the Tennessee River watershed, the Lower Mississippi River watershed, and the Arkansas-Red-White Rivers watershed to create the Mississippi River watershed.