IPA: /ˈhɛdˌwɔːtə/
KK: /ˈhɛdˌwɔtər/
The place where a river or stream begins, often in the mountains or hills.
The hikers followed the trail to the headwater of the river, where the water flowed gently from the rocks.
Headwater is formed from "head" (from Old English "heafod", meaning the top or source) and "water" (from Old English "wæter", meaning water). The term refers to the source or beginning of a river or stream, where the water originates.
Think of the 'top' ('head') of a river where the 'water' begins to flow — that's what headwater means.