Inhydrology, amain stem ormainstem (also known as atrunk) is "the primary downstream segment of ariver, as contrasted to itstributaries".[1][2] The mainstem extends all the way from one specificheadwater to the outlet of the river, although there are multiple ways to determine which headwater (or first-order tributary) is the source of the mainstem.[clarification needed] Water enters the mainstem from the river'sdrainage basin, the land area through which the mainstem and its tributaries flow.[3] A drainage basin may also be referred to as awatershed orcatchment.[3]
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Hydrological classification systems assign numbers to tributaries and mainstems within a drainage basin. In theStrahler number, a modification of a system devised byRobert 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 theStrahler system, adopted in 1957, only that part of the mainstem below the tributary of the next highest rank gets the highest number.[2]
In the United States, theMississippi River mainstem achieves a Strahler number of 10, the highest in the nation. Eight rivers, including theColumbia River, 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 square mile (2.6 km2), have been identified in the United States alone.[2] Outside of the United States, theAmazon River reaches a Strahler number of 12, making it the highest-order river in the world.[4]
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