FYI-The grass you see on Choke is either Hydrilla or American Elodea.
They look very simular and can easily be mistaken for one another. All of them have leaves in whorls around the stem. However, American waterweed has three leaves per whorl, whereas hydrilla almost always have more than three leaves per whorl.
Also, Hydrilla has tubers and Elodea does not. Tubers are little white root balls kind of like a bulb.
Both will survive our mild winter but will go more dormant in temps lower that about 65^. When you start seeing the browning of the leaves you will notice the plants retreating for winter.
Both plants will grow in water from a few inches to over 20+ feet.
Hydrilla is a bad plant because when hydrilla becomes over abundant, fish population imbalances are likely. The dense mats of hydrilla will alter the waters chemistry by raising pH, cause wide oxygen fluctuations, and increase water temperature.
http://www.marinebiochemists.com/elodea.htmJust some info you might have never realized...