source

The source or headwaters of a river or stream is the furthest place in that river or stream from its estuary or confluence with another river, as measured along the course of the river.
There is no universally agreed upon definition for determining a stream’s source. 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”. 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.
This most common definition of a river source specifically utilizes the most…