an affluent of the River Yangtze near Kiukiang
湓 is a rare kanji with a narrow geographical meaning: an affluent or tributary of the Yangtze River near Kiukiang. Its standalone meaning is tied to this specific waterway.
湓 combines 氵 (water) with 盆 (basin, tray), likely as a phono-semantic compound where 盆 contributes the sound and the idea of a water-filled basin or tributary. The exact historical development is uncertain.
The water radical 氵 beside 盆 (basin) suggests a basin-like waterway: a tributary flowing into the Yangtze.
For ホン, imagine a basin (盆, ぼん) overflowing with water, but the reading shifts to ホン as the water rushes into the river: basin -> ホン.