riverweed

[ riv-er-weed ]
/ ˈrɪv ərˌwid /

noun

any of several chiefly tropical submerged aquatic plants of the genus Podostemum and related genera, growing in rapidly moving streams.

Origin of riverweed

First recorded in 1665–75; river1 + weed1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019