flu·vi·al \ˈflüvēəl, -vyəl\adjective Etymology: Latin fluvialis, from fluvius river (from fluere to flow) + -alis -al — more at fluid 1. a.: of or relating to rivers < fluvial law > b.: conforming to the changing course of a stream < a fluvial boundary > 2.: growing or living in streams < fluvial vegetation > 3.: produced by river action < a fluvial plain >