Source, n.
Etymology: [OE. sours, OF. sourse, surse, sorse, F. source, fr. OF. sors, p.p. of OF. sordre, surdre, sourdre, to spring forth or up, F. sourdre, fr. L. surgere to lift or raise up, to spring up. See Surge, and cf. Souse to plunge or swoop as a bird upon its prey.]
1. The act of rising; a rise; an ascent. [Obs.] Therefore right as an hawk upon a sours Up springeth into the air, right so prayers . . . Maken their sours to Goddes ears two. Chaucer.
2. The rising from the ground, or beginning, of a stream of water or the like; a spring; a fountain. Where as the Poo out of a welle small Taketh his firste springing and his sours. Chaucer. Kings that rule Behind the hidden sources of the Nile. Addison.
3. That from which anything comes forth, regarded as its cause or origin; the person from whom anything originates; first cause. This source of ideas every man has wholly in himself. Locke. The source of Newton's light, of Bacon's sense. Pope.
Syn. -- See Origin.
Shades of meaning
Fernald has no entry of its own for source; he lists it under beginning, cause.
Where he writes about it:
A source is that which furnishes a first and continuous supply, that which flows forth freely or may be readily recurred to; as, the source of a river; a source of knowledge; a source of inspiration; fertile land is a source (not an origin) of wealth. — from beginning
A rise is thought of as in an action; we say that a lake is the source of a certain river, or that the river takes its rise from the lake. — from beginning