noun
Geographical term for the boundary separating drainage basins; used in hydrology and geography.
この山脈が太平洋側と日本海側の分水界になっている。
This mountain range forms the watershed between the Pacific side and the Sea of Japan side.
Compound of 分水 (bunsui, 'dividing water') + 界 (kai, 'boundary'). The derivation is transparent from the kanji components.