This rare kanji has no clear standalone meaning. Its given reading おおみず suggests 'great water' or 'flood', but treat its meaning as uncertain.
潡 combines 氵 (water) with 敦, which likely contributes the sound トン. The exact historical development is uncertain.
Water 氵 beside 敦 suggests a large, forceful flow. Picture a great flood (おおみず) surging past a sturdy mound.
For トン, imagine a ton of water crashing down: ton -> トン, and the great flood rushes in.