The correct term is sub. It means that he is built or located under a stream of water. Below a stream or a river.
subflubial is incorrectly written and should be written as subfluvial being its meaning: see: Sub.
subflubial is incorrectly written, and should be written as "Sub" being its meaning: Word of Latin Etymology: sub: below and fluvius fluvii: River, running water; Therefore, as Stephen said, and since it is adjective means what is under River
subfluvial with v which is under the bed of a river.