Mully, you've got yourself a crop of variable-leaf milfoil (Myriophyllum heterophyllum) - not coontail.
BTW, I'd much rather do battle with milfoil than coontail.
The transient water may pose an issue, but probably not something that can't be overcome - but that'll depend upon some specific flow-characteristics.
To what extent does the pond's water exchange/discharge over a 5-day period? 10%? 20%? 50% transition?
Is there any desirable aquatic vegetation present that you want to keep?
