source (e.g. of a river); headwaters
origin of something continuous (language, culture, etc.)
この川の源流は山間の湧き水だ。
The source of this river is spring water in the mountains.
ジャズの源流をたどるとブルースにたどり着く。
Tracing the origins of jazz leads you back to the blues.
源流から流れ落ちる水は透き通るように澄んでいた。
The water flowing from the headwaters was crystal clear.