river in Guangdong province
湞 is a rare kanji that refers specifically to a river in Guangdong province. Its meaning is a proper geographic name rather than a general concept.
湞 combines the water radical 氵 with the phonetic component 貞. It was created as a specific river name, and its historical development beyond this phono-semantic structure is uncertain.
The water radical 氵 on the left marks this as a river. The right side 貞 can be thought of as a phonetic label for the specific river name in Guangdong.
For トウ, imagine a tall tower (タワー -> トウ) standing beside the river 湞 in Guangdong. The tower's silhouette cues the reading トウ.
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