This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瀯 combines the water radical 氵 with 營, which likely contributes sound. The exact historical development is uncertain.
With 氵 (water) beside 營 (a shape associated with managing or operating), imagine water flowing in a managed channel, like a small stream. This fits the kun-reading おがわ (small stream).
For エイ, picture a small stream (おがわ) shaped like the letter A: A -> エイ, and the water flows through it.