Elodea-Click for more info

American waterweed-Elodea- is usually fairly easy to distinguish from its more notorious relatives, like Brazilian elodea and hydrilla. All of them have leaves in whorls around the stem. However, American waterweed has three leaves per whorl, whereas hydrilla and Brazilian elodea almost always have more than three leaves per whorl. Brazilian elodea is also a much larger, bushy plant with longer leaves. American waterweed lives entirely underwater with the exception of small white flowers which bloom at the surface and are attached to the plant by delicate stalks. It produces winter buds from the stem tips which over winter on the lake bottom. It often over winters as an evergreen plant in mild climates. In the fall leafy stalks will detach from the parent plant, float away, root, and start new plants. This is American waterweed's most important method of spreading, with seed production playing a relatively minor role. 

Washington State Dept of Ecology