This sighting shows a smaller plant with basal leaves, which are different to the other leaves. See comments on:- http://canberra.naturemapr.org/Community/Sightings/Details/3376647
It is quite unusual to look at, and I have only noticed it this year after visiting the area many times. There was just a small dense patch about 5-10 metres across. However, I can't speak for other sections of the river. Seeing your concern I have checked it out and changed its status to noxious weed.