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

Definition

More specifically, a source is defined as the most distant point (from the river mouth) 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 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 actual source of rivers such as the Amazon or Nile.

Other authoritative geographic definitions agree, stressing 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."

Thus, neither a lake (except in rare cases in which it has no inflows) 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 where the name "Nile" first appears, but as the source of the largest river flowing into the lake, the Kagera River.

The official source of the Mississippi-Missouri is not where either river begins in name, but is defined by the USGS as the source of the longest tributary, the Jefferson River The official source of the Amazon River is a similar example.

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.