river in Henan province
洹 is a rare kanji that refers specifically to a river in Henan province. Its standalone meaning is tied to that geographic name.
洹 combines 氵 (water) with 亘, which likely contributes the sound. The character was created to name a specific river.
The water radical 氵 beside 亘 suggests a river. Think of the 亘 shape as a winding path, like the river's course through Henan.
For エン, imagine the river winding through a green valley: the sound 'en' in 'green' cues エン, and the water radical 氵 keeps the river image.
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