This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瀇 combines 氵 (water) with 廣, which likely contributes the sound. The exact historical development is uncertain.
The water radical 氵 beside 廣 suggests something related to water, but the exact meaning is obscure. Imagine a vast, expansive body of water hinted at by the broad shape of 廣.
For オウ, picture an old man saying 'Oh!' as he gazes at the wide water: Oh -> オウ.
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