High Quality Content by WIKIPEDIA articles! The main stems of 38 rivers in the United States are at least 500 miles (800 km) long. The main stem is "the primary downstream segment of a river, as contrasted to its tributaries". The United States Geological Survey (USGS) defines a main-stem segment by listing coordinates for its two end points, called the source and the mouth. Well-known rivers like the Atchafalaya,Willamette, and Susquehanna are not included in this list because their main stems are shorter than 500 miles.