valley
谿 centers on a valley, specifically a deep mountain valley or gorge with a stream.
谿 combines 奚 (a phonetic component with an uncertain original meaning) and 谷 (valley), together expressing a deep valley or gorge.
The left side 奚 looks like a person peering down, and the right side 谷 is a valley. Picture someone standing at the edge of a deep mountain valley, looking into the gorge below.
For ケイ, imagine a deep valley shaped like the letter K: K -> ケイ, and the valley echoes the sound 'kay' as the wind blows through it.