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Theheadwater of ariver orstream is the geographical point of its beginning, specifically wheresurface runoff water begins to accumulate into a flowing channel of water. A river or stream into which one or manytributary 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 wheresurface runoffs fromrainwater,meltwater, orspring water begin accumulating into a more substantial and consistent flow that becomes afirst-order tributary of that river or stream. The tributary with the longestchannel 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 themain stem of the river or stream in question.
TheUnited 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 theMissouri River as a tributary of theMississippi 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.[1] 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 theriver mouth) in thedrainage basin from which water runs year-around (perennially), or, alternatively, as the furthest point from which water could possibly flowephemerally.[2] 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 theSmithsonian Institution, is also used by theNational Geographic Society when pinpointing the source of rivers such as theAmazon orNile. A definition given by the state ofMontana agrees, stating that a river source is never aconfluence but is "in a location that is the farthest, along water miles, from where that river ends."[3]
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 asLake Victoria's outlet where the name "Nile" first appears, which would reduce the Nile's length by over 900 km (560 mi) (dropping it to fourth or fifth on the list of world's rivers), but instead use the source of the largest river flowinginto the lake, theKagera River. Likewise, the source of the Amazon River has been determined this way, even though the river changes names numerous times along its course.[4] However, the source of theThames in England is traditionally reckoned according to the named river Thames rather than its longer tributary, theChurn — although not without contention.[5]
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, followingLewis andClark's naming convention, as the confluence of theMadison andJefferson rivers, rather than the source of its longest tributary (the Jefferson).[3] This contradicts the most common definition,[6] which is, according to aUS 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 theBeaverhead River toRed Rock River, then Red Rock Creek toHell Roaring Creek."
Sometimes the source of the most remote tributary may be in an area that is moremarsh-like, in which the "uppermost" or most remote section of the marsh would be the true source. For example, the source of theRiver Tees ismarshland.
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 ofglacialice.
Headwater areas are the upstream areas of awatershed, 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 theColorado River is at theContinental Divide separating theAtlantic Ocean andPacific Ocean watersheds ofNorth America.
A river is considered a linear geographic feature, with only one mouth and one source.For an example, theMississippi River andMissouri River sources are officially defined as follows:
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 word "source", when applied tolakes rather than rivers or streams, refers to the lake'sinflow.[7][8]
...the Owens River, the source of the lake...
...source of the lake is the River Itambi...
