Hydrology (from Ancient Greek, Greek wikt:ὕδωρ, ὕδωρ, ''hýdōr'' meaning "water" and wikt:λόγος, λόγος, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and ...
, a mainstem (or trunk) is "the primary downstream segment of a
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
, as contrasted to its
A tributary, or affluent, is a stream
A stream is a body of water
(Lysefjord) in Norway
Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål
Bokmål (, ; literally "book tongue") ...
Water enters the mainstem from the river's
A drainage basin is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. The drainage basin includes all the surface water from surface runoff, rain runoff, snowm ...
, the land area through which the mainstem and its tributaries flow.
A drainage basin may also be referred to as a ''watershed'' or ''catchment''.
Hydrological classification systems assign numbers to tributaries and mainstems within a drainage basin. In the
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
, a modification of a system devised by Robert E. Horton
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 mainstem 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 mainstem below the tributary of the next highest rank gets the highest number.
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
The Mississippi River is the second-longest river and chief river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and b ...
mainstem achieves a Strahler number of 10, the highest in the nation. Eight rivers, including the
The Columbia River (Upper Chinook
Upper Chinook, endonym Kiksht, also known as Columbia Chinook, and Wasco-Wishram after its last surviving dialect, is a recently extinct language of the US Pacific Northwest. It had 69 speakers in 1990, of w ...
, 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 , have been identified in the United States alone.
Outside of the United States, the
The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America
South America is a entirely in the and mostly in the , with a relatively small portion in the . It can also be described as the southern of a single con ...
reaches a Strahler number of 12, making it the highest-order river in the world.