氵 is left-side water and locks 溪 into the river/stream family - a brook is a small mountain watercourse. Same radical in 河 river, 江 large river, 湖 lake. 溪 specifically denotes the smallest, most intimate of these.
奚 gives the sound xī exactly, same tone. The literary 'what/why' meaning has no role here. Same phonetic in 鸡 (chicken, traditional 雞), 蹊 (path) - all sharing the -ī rhyme from old Chinese.
