valley
溪 centers on a valley, especially one with a stream or river running through it.
溪 combines 氵 (water) with 奚, which likely contributes the sound and may relate to a narrow passage, together suggesting a stream-cut valley.
Water 氵 flows through a narrow passage 奚, carving out a valley.
For ケイ, imagine a kayak floating down the valley stream: kayak -> ケイ.