The source or headwaters of a river or stream is the place from which the water in the river or stream originates.

Definition

There is no universally agreed upon definition such that any stream's source can be systematically determined. 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 identified as the source stream". Thus, for example, the USGS considered the Missouri River a tributary of the Mississippi River, despite the Missouri being longer.

By another definition a source can be defined specifically as the most distant point (from the river mouth) in the drainage basin from which water runs year-around (perennially, or, alternatively, as the furthest point from which water could possibly flow (ephemerally. 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 or Nile. A definition given by the state of Montana agrees, stating that a river source is never a confluence but is "in a location that is the farthest, along water miles, from where that river ends." 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 all other major geographic authorities and atlases define the source of the Nile River not as Lake Victoria's outlet where the name "Nile" first appears, but as 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 several times along its course.

This definition, however, is not used in many cases. For instance, the longer Missouri River (2540 miles long) is considered a tributary, while the shorter but much more voluminous Upper Mississippi River (1250 miles long) belongs to the main stream of Mississippi River, rather than the other way around. In its turn, Missouri River's source is located by the USGS (and other federal and state agencies, following Lewis and Clark) as the confluence of the Madison and Jefferson rivers, rather than the source of its longest tributary (the Jefferson).

Characteristics of sources

Often the source, or start 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 marshland.

The furthest stream is also often called the headstream. Headwaters are usually small streams that are often cool waters, because of shade and recently melted ice or snow. They may also be glacial headwaters, waters formed by the melting of glacial ice.

The source is the farthest point of the river stream from its estuary, mouth, or its confluence with another river or stream, regardless of what name that watercourse may carry on local maps and in local usage. Where a river is fed by more than one source, it is customary to regard the longest as its source, with other sources considered tributaries. Often, however, the manner in which streams are named is not consistent with this convention. Many rivers change names numerous times over their length.

Near its source, a river or stream may have a modest flow rate, but the flow increases as more surface runoff and tributaries drain into the subject stream.

Headwaters are the most extreme upstream areas of a watershed. The end point of the watershed is called an outflow or discharge. A watershed is an area of land that is drained by a body of water. The river source is generally on or quite near the edge of the watershed, or watershed divide.

Example

A river is considered a linear geographic feature, with only one mouth and only one source.For an example, please note how the Mississippi River and Missouri River sources are officially defined:

• , Length: , Source:
• , Length: , Source:

Related usages

The verb "rise" can be used to express the idea 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 lakes rather than rivers or streams, refers to the lake's inflow.

See also

• Spring
• Water well

References