moat, wetland, name of Chinese river
湟 centers on a body of water: a moat, a wetland, or a specific Chinese river. The common thread is water in a defined or low-lying area.
湟 combines 氵 (water) with 皇, which likely contributes the sound. The character is used for water-related features such as moats, wetlands, and a specific Chinese river.
Water 氵 beside 皇, which looks like an emperor's crown over a base. Picture the emperor's moat filled with water, protecting the palace.
For コウ, imagine a koi fish swimming in the moat: koi -> コウ, and the water 氵 surrounds the emperor's 皇 domain.