Source - the beginning of a river or the place from which a constant current appears in the riverbed. The source of a river can be springs, glaciers, lakes, marshes.

The source most often corresponds to the place from which the bed of a permanent watercourse emerges or the place from which a permanent flow of water in a channel emerges.

The source is usually the beginning of a stream receiving water from a spring, the end of a glacier, a lake, or a marsh. In wetland rivers, the point from which an open stream with a permanent channel appears is often taken as the source.

In a river system with many sources, the source furthest from the mouth or the one with the most water is considered the main source. The confluence of two rivers with different names is often considered to be the origin of large rivers. For example, the Northern Dvina is formed from the confluence of the Sukhona and the Yuga, the Amur River - from the confluence of the Shilka and the Argun, and the Ob River - from the confluence of the Biya and the Katun. In this case, we should talk about the hydrographic length of the river, which is the sum of the lengths of the main river and that of its constituent rivers, the source of which is farthest from the confluence.