Either Perga affinis or Perga dorsalis. Only females can be visually identified if the details of ovipositor sheath hairs are clear. I'm not confident to call this one.
the third photo was indeed helpful: gave an idea of likely whorl number, and showed overall shape. This species of milfoil has (nearly always) tight crimped hairs on the stem. If wading in is not possible, its worth checking the strandline for milfoil debris (waterfowl can be quite destructive) - these isn't good for photo portraits but can be useful source of species details. Extra photos are really helpful
Jane, I've added a couple more photos but probably still not clear enough. When I've enlarged the photos they look most like M. crispatum. The dam is surrounded by reeds, rushes and grasses so it is very difficult to get a clear photo without getting in the dam. We weren't dressed for that. Lots of lovely native water plants in there. Best retired farm dam that I've ever seen.
Definitely a milfoil, and likely is Myriophyllum crispatum. for certainty, ID to species for milfoils needs other details, most notably number of leaves in a whorl (if whorled, that is).
