The headwaters of a river or stream is the place from which the water in the river or stream originates. More specifically, a source is defined as the most distant point in the drainage basin from which water runs year-around, or, alternatively, the furthest point from which water could possibly flow. This latter definition includes 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.

Thus, neither a lake nor a confluence of tributaries can by definition ever 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, but as the source of the largest river flowing into the lake, the Kagera River.

Often 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. 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.

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.

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