溪 is often combined: 小溪 (xiǎoxī, small stream), 溪流 (xīliú, brook), 溪水 (xīshuǐ, stream water). It refers to smaller waterways than 河 (river) or 江 (large river).
memory
The left radical 氵(water) plus 奚 (xī, phonetic component). Water + the sound = a stream.
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 jī (chicken, traditional 雞), 蹊 xī (path) — all sharing the -ī rhyme from old Chinese.
