[riv-er-weed] 
| any of several chiefly tropical submerged aquatic plants of the genus Podostemum and related genera, growing in rapidly moving streams. |
| riv·er·weed (rĭv'ər-wēd') n. An eastern North American plant (Podostemum ceratophyllum) having olive-green foliage resembling seaweed and growing on rocks in rapidly flowing streams. |
ərˌwid