(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 "
If you derive something such as pleasure or benefit from a person or from something, you get it from them. Mr Ying is one of those happy people who derive pleasure from helping others