The headwater of a river or stream is the farthest point on each of its tributaries upstream from its mouth/estuary into a lake/sea or its confluence with another river. Each headwater is considered one of the river's sources, as it is the place where surface runoffs from rainwater, meltwater and/or spring water begin accumulating into a more substantial and consistent flow that becomes a first-order tributary of that river. The tributary with the longest course downstream of the headwaters is regarded as the main stem.
River Wey near its source at Farringdon
A stone near Crissolo, Italy, inscribed: Here is born the Po
Rhume Spring, source of the Rhume river
A river is a natural flowing watercourse, usually a freshwater stream, flowing on the Earth's land surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland, or another river. In some cases, a river flows into the ground or becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to by names such as creek, brook, and rivulet. There are no official definitions for these various generic terms for a watercourse as applied to geographic features, although in some countries or communities, a stream is customarily referred to by one of these names as determined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and Northeast England, and "beck" in Northern England. Sometimes a river is defined as being larger than a creek, but not always; in English the language is vague compared to some langauges like French, where a fleuve flows into the sea and a rivière is a tributary of another rivière or fleuve.
Elwha River in the Olympic Peninsula
Melting toe of Athabasca Glacier, Jasper National Park, Alberta, Canada
The Colorado River at Horseshoe Bend, Arizona
Nile River delta, as seen from Earth orbit. The Nile is an example of a wave-dominated delta that has the classic Greek letter delta (Δ) shape after which river deltas were named.