Editing River source (section)

==Definition==
[[File:SorgentePo.jpg|thumb|A stone near [[Crissolo]], Italy, is inscribed, "Here is born the [[Po River|Po]]".]]
The [[United States Geological Survey]] (USGS) states that a river's "length may be considered to be the distance from the mouth to the most distant headwater source (irrespective of stream name), or from the mouth to the headwaters of the stream commonly known as the source stream". As an example of the second definition above, the USGS at times considers the [[Missouri River]] as a tributary of the [[Mississippi River]]. But it also follows the first definition above (along with virtually all other geographic authorities and publications) in using the combined Missouri–lower Mississippi length figure in lists of lengths of rivers around the world.<ref>{{cite web|url=https://pubs.usgs.gov/of/1987/ofr87-242/pdf/ofr87242.pdf|publisher=[[United States Geological Survey]]|title=Water Fact Sheet {{!}} Largest Rivers in the United States|year=1992|access-date=12 October 2024}}</ref> Most rivers have numerous tributaries and change names often; it is customary to regard the longest tributary or stem as the source, regardless of what name that watercourse may carry on local maps and in local usage.

This most commonly identified definition of a river source specifically uses the most distant point (along watercourses from the [[river mouth]]) in the [[drainage basin]] from which water runs year-around ([[Perennial stream|perennially]]), or, alternatively, as the furthest point from which water could possibly flow [[Ephemerality#Geographical features|ephemerally]].<ref>{{cite web|url=http://news.nationalgeographic.com/news/2000/12/1221amazon.html|archive-url=https://web.archive.org/web/20010413002040/http://news.nationalgeographic.com/news/2000/12/1221amazon.html|url-status=dead|archive-date=April 13, 2001|work=[[National Geographic Society#Other publications|National Geographic News]]|title=Explorers Pinpoint Source of the Amazon|first=Donald|last=Smith|date=December 21, 2000}}</ref> The latter definition includes sometimes-dry channels and removes any possible definitions that would have the river source "move around" from month to month depending on precipitation or ground water levels. This definition, from geographer Andrew Johnston of the [[Smithsonian Institution]], is also used by the [[National Geographic Society]] when pinpointing the source of rivers such as the [[Amazon River|Amazon]] or [[Nile River|Nile]]. A definition given by the state of [[Montana]] agrees, stating that a river source is never a [[confluence (geography)|confluence]] but is "in a location that is the farthest, along water miles, from where that river ends."<ref name=fwp>{{cite journal|url=https://fwp.mt.gov/binaries/content/assets/fwp/montana-outdoors/l-series/lcinmt_3_missouri.pdf|publisher=[[Montana Department of Fish, Wildlife and Parks]]|work=Montana Outdoors|title=The True Utmost Reaches of the Missouri|first1=Donald F.|last1=Nell|first2=Anthony|last2=Deetriades|edition=July-August 2005|pages=10-13|access-date=12 October 2024}}</ref>

Under this definition, neither a lake (excepting lakes with no inflows) nor a confluence of tributaries can be a true river source, though both often provide the starting point for the portion of a river carrying a single name. For example, National Geographic and virtually every other geographic authority and atlas define the source of the Nile River not as [[Lake Victoria]]'s outlet where the name "Nile" first appears, which would reduce the Nile's length by over {{cvt|900|km|||}} (dropping it to fourth or fifth on the list of world's rivers), but instead use the source of the largest river flowing ''into'' the lake, the [[Kagera River]]. Likewise, the source of the Amazon River has been determined this way, even though the river changes names numerous times along its course.<ref>{{cite press release|url=http://www.ibge.gov.br/english/presidencia/noticias/noticia_visualiza.php?id_noticia=885&id_pagina=1/|url-status=dead|archive-url=https://web.archive.org/web/20110615121345/http://www.ibge.gov.br/english/presidencia/noticias/noticia_visualiza.php?id_noticia=885&id_pagina=1%2F|archive-date=June 15, 2011|publisher=[[Brazilian Institute of Geography and Statistics]]|title=IBGE joins group in search of the true source of the Amazon River|date=May 22, 2007}}</ref>  However, the source of the [[River Thames|Thames]] in England is traditionally reckoned according to the named river Thames rather than its longer tributary, the [[River Churn|Churn]] &mdash; although not without contention.<ref>{{cite news|url=https://www.bbc.co.uk/news/uk-england-gloucestershire-17858944|work=[[BBC News]]|title=Could the River Thames be longer than the River Severn?|first=David|last=Bailey|date=15 May 2012|access-date=12 October 2024}}</ref>

When not listing river lengths, however, alternative definitions may be used. The Missouri River's source is named by some USGS and other federal and state agency sources, following [[Meriwether Lewis|Lewis]] and [[William Clark|Clark]]'s naming convention, as the confluence of the [[Madison River|Madison]] and [[Jefferson River|Jefferson]] rivers, rather than the source of its longest tributary (the Jefferson).<ref name=fwp/>  This contradicts the most common definition,<ref>{{cite web|url=http://infolink.cr.usgs.gov/events/Conferences/mrc04/04Program.pdf|url-status=dead|archive-url=https://web.archive.org/web/20150213023459/https://infolink.cr.usgs.gov/events/Conferences/mrc04/04Program.pdf|archive-date=February 13, 2015|publisher=[[United States Geological Survey]]|work=8th Annual Missouri Rivers Natural Resources Conference 2004|title=Rediscovering Missouri River Connections {{!}} Program}}</ref> which is, according to a [[United States Army Corps of Engineers|US Army Corps of Engineers]] official on a USGS site, that "[geographers] generally follow the longest tributary to identify the source of rivers and streams." In the case of the Missouri River, this would have the source be well upstream from Lewis and Clark's confluence, "following the Jefferson River to the [[Beaverhead River]] to [[Red Rock River (Montana)|Red Rock River]], then Red Rock Creek to [[Hell Roaring Creek]]."