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{{short description|Starting point of a river}}
{{Redirect|Headwater}}
{{refimprove|date=October 2024}}
[[File:wey source farringdon.jpg|right|thumb|upright=1.3|[[River Wey]] near its source at [[Farringdon, Hampshire|Farringdon]], England]]
The '''headwater''' of a [[river]] or [[stream]] is the geographical point of its beginning, specifically where [[surface runoff]] water begins to accumulate into a flowing channel of water. A river or stream into which one or many [[tributary]] rivers or streams flows has many headwaters, these being all of the individual headwaters of its tributaries. Each headwater is one of the river or stream's sources, as it is the place where [[surface runoff]]s from [[rainwater]], [[meltwater]], or [[spring water]] begin accumulating into a more substantial and consistent flow that becomes a [[Strahler number|first-order tributary]] of that river or stream. The tributary with the longest [[channel (geography)|channel]] of all the tributaries to a river or stream, such length measured from that tributary's headwater to its mouth where it discharges into the river or stream, is the [[main stem]] of the river or stream in question.

==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]]|journal=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=https://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]]."

==Characteristics==
[[File:Rhumequelle Rhumspringe.jpg|thumb|[[Rhume Spring]] is the source of the [[Rhume]] river.]]
Sometimes the source of the most remote tributary may be in an area that is more [[marsh]]-like, in which the "uppermost" or most remote section of the marsh would be the true source.  For example, the source of the [[River Tees]] is [[marsh]]land.

The furthest stream is also often called the head stream. Headwaters are often small streams with cool waters because of shade and recently melted ice or snow.  They may also be glacial headwaters, waters formed by the melting of [[Glacier|glacial]] [[ice]].

Headwater areas are the upstream areas of a [[drainage basin|watershed]], as opposed to the outflow or discharge of a watershed.   The river source is often but not always on or quite near the edge of the watershed, or watershed divide. For example, the source of the [[Colorado River]] is at the [[Continental Divide]] separating the [[Atlantic Ocean]] and [[Pacific Ocean]] watersheds of [[North America]].

===Example===
A river is considered a linear geographic feature, with only one mouth and one source.
For an example, the [[Mississippi River]] and [[Missouri River]] sources are officially defined as follows:
*{{cite web | title = Largest Rivers in the United States | publisher = [[United States Geological Survey]] | url = https://pubs.usgs.gov/of/1987/ofr87-242/ }}
*{{Gnis|1629903|Mississippi River}}, Length: {{convert|2340|mi|km}}, Source: {{Coord|47|14|22|N|95|12|29|W|region:US_type:river_source:GNIS}}
*{{Gnis|756398|Missouri River}}, Length: {{convert|2540|mi|km}}, Source: {{Coord|45|55|39|N|111|30|29|W|region:US_type:river_source:GNIS}}

==Related usages==
The verb "rise" can be used to express the general region of a river's source, and is often qualified with an adverbial expression of place. For example:
*The River Thames rises in Gloucestershire.
*The White Nile rises in the Great Lakes region of central Africa.

The word "source", when applied to [[lake]]s rather than rivers or streams, refers to the lake's [[inflow (hydrology)|inflow]].<ref>{{cite web|url=http://www.epa.gov/region09/air/owens/qa.html|url-status=dead|archive-url=https://web.archive.org/web/20170601112510/http://www.epa.gov/region09/air/owens/qa.html|archive-date=June 1, 2017|publisher=[[United States Environmental Protection Agency]]|title=Pacific Southwest, Region 9 {{!}} Air Actions, California {{!}} Owens Valley Particulate Matter Plan Q & A|quote=...the Owens River, the source of the lake... }}</ref><ref>{{cite thesis|url=http://edocs.tu-berlin.de/diss/2004/casallas_jorge.pdf|url-status=dead|archive-url=https://web.archive.org/web/20080625030350/http://edocs.tu-berlin.de/diss/2004/casallas_jorge.pdf|archive-date=June 25, 2008|publisher=[[Technische Universität Berlin]]|title=Limnological investigations in Lake San Pablo|first1=Casallas|last1=Guzmán|first2=Jorge|last2=Enrique|date=January 21, 2005|quote=...source of the lake is the River Itambi...|doi=10.14279/depositonce-1034}}</ref>

==See also==
* [[Source of the Amazon River]]
* [[Source of the Nile]]
* [[Spring (hydrology)]]
* [[Strahler number]]
* [[Water well]]

==References==
{{Commons category|River sources|river sources}}
{{reflist}}

===Bibliography===
* DeBarry, Paul A. (2004). ''Watersheds: Processes, Assessment and Management''. John Wiley & Sons.

{{Rivers, streams and springs}}

[[Category:Water streams]]
[[Category:Fluvial landforms]]
[[Category:Freshwater ecology]]
[[Category:River morphology]]