流域
りゅういき · ryuuiki · JLPT N2 · noun
river basin
🌊 流 (flow) + 域 (area/zone) = the whole land area that drains water into one river
流 means flow and 域 means zone or region, so 流域 is the full land area whose rain and runoff drain into one river. Rain falling anywhere inside that zone ends up in the same river system.
流 — water radical 氵 + 㐬 (phonetic, child being born)
流 is a semantic-phonetic compound. The water radical 氵 gives the meaning, while 㐬 depicts an infant being born head-down with amniotic fluid, suggesting downward flow. Together they convey water flowing, and by extension 'to flow' or 'current'.