IPA: //ˈhɛdstriːm//
KK: /hɛdstrim/
A stream that is the starting point or source of a river.
The headstream of the river flows down from the mountains.
Headstream → It is formed from "head" (from Old English "heafod", meaning the top or leader) and "stream" (from Old English "stream", meaning a flow of water). The word "headstream" refers to the source or beginning of a stream or river, essentially the leading flow of water.
Think of the 'top' ('head') of a 'flow' ('stream') — that's why headstream means the source of a stream.