Photos don't show the essential details needed for ID milfoil (leaves per whorl, flowers or fruits). However the habit, and the habitat and personal experience of milfoils along local rivers means I am fairly confident it is Myriophyllum variifolium. There is another species (M. simulans) which is very very similar (need details of leaves, flowers, fruits) to tell these apart but it doesn't seem to occur in rivers. It is well out of range for M. alpinum which is a high country species.