源头 (yuántóu) is the most general word for 'source, fountainhead' — it can refer to the physical source of a river, the root of a problem, or the origin of inspiration. It is broader than 水源 (shuǐyuán, water source) and 发源地 (fāyuándì, birthplace of a culture/movement), and less abstract than 来源 (láiyuán, source/origin of information or things). 源泉 (yuánquán) is more poetic/fountain-like, while 渊源 (yuānyuán) implies a deep historical or inherited connection.