wetlands, marsh
湫 centers on low-lying, waterlogged land: a wetland or marsh. The meanings are closely related, both describing soggy, swampy ground.
湫 combines the water radical 氵 with 秋, which likely contributes the sound. The character is used for wetlands or marshes, but the exact historical development is uncertain.
The water radical 氵 beside 秋 (autumn) suggests a place where water collects in the fall, like a marsh or wetland after autumn rains.
For シュウ, imagine a shoe sinking into the muddy marsh: shoe -> シュウ, and the waterlogged ground swallows it.
wetland; marsh