mountain stream, creek
磎 centers on a small, rocky watercourse in the mountains: a mountain stream or creek.
磎 combines 石 (stone) with 奚, which likely contributes the sound and may relate to a narrow passage or valley. The character is a variant of 溪, which uses 氵 (water) instead of 石, emphasizing the rocky nature of the stream.
The left side 石 (stone) and right side 奚 (a shape associated with narrow valleys) together suggest a rocky mountain stream: water flowing among stones.
For ケイ, imagine a kayak (sounds like ケイ) navigating a rocky mountain creek: kayak -> ケイ, and the stones 石 line the stream.