headwaters

The source or 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 flow 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 runoffs from rainwater, meltwater, or spring water begin accumulating into a more substantial and consistent flow that becomes a first-order tributary of that river or stream. The tributary with the longest 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.