E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Derive (2 syl.)
means back to its channel or source (Latin, de rivo). The Latin rivus (a river) does not mean the stream or current, but the source whence it flows, or the channel through which it runs. As Ulpian says, Fons sive locus per longitudinem depressus, quo aqua decurrat.