riverweed
‘an eastern North American plant (Podostemum ceratophyllum) having olive-green foliage resembling seaweed and growing on rocks in rapidly flowing streams’
Lexical development traced from the Tamil root:
river < A.F. rivere, O.F. riviere ‘river, riverside, river bank’ < Vulgar Lat. riparia ‘riverbank, seashore’ < ripa ‘(steep) bank of a river, shore’ + weed < ri < Tamil aruvi