In relation to hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
, a main stem
is "the primary downstream segment of a river, as contrasted to its tributaries". Another common term for the main stem, the final large channel of a riverine system, is the trunk
. Water enters the main stem from the river's drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
, the land area through which the main stem and its tributaries flow. A drainage basin may also be referred to as a watershed
Hydrological classification systems assign numbers to tributaries and main stems within a drainage basin. In the Strahler system
In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree is a numerical measure of its branching complexity....
, a modification of a system devised by Robert E. Horton
Robert Elmer Horton was an American ecologist and soil scientist, considered by many to be the father of modern hydrology....
in 1945, channels with no tributaries are called "first-order" streams. When two first-order streams meet, they are said to form a second-order stream; when two second-order streams meet, they form a third-order stream, and so on. In the Horton system, the entire main stem of a drainage basin was assigned the highest number in that basin. However, in the Strahler system, adopted in 1957, only that part of the main stem below the tributary of the next highest rank gets the highest number.
In the United States, the Mississippi River
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
main stem achieves a Strahler number of 10, the highest in the nation. Eight rivers, including that of the Columbia River
The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada, flows northwest and then south into the U.S. state of Washington, then turns west to form most of the border between Washington and the state...
, reach 9. Streams with no tributaries, assigned the Strahler number 1, are most common. More than 1.5 million of these small streams, with average drainage basins of only 1 mi2, have been identified in the United States alone.
- Benke, Arthur C., ed., and Cushing, Colbert E., ed. (2005). Rivers of North America. Burlington, Massachusetts: Elsevier Academic Press. ISBN 0-12-088253-1.
- Patrick, Ruth (1995). Rivers of the United States: Volume II: Chemical and Physical Characteristics. New York: John Wiley & Sons. ISBN 0-471-10752-2.