Pond Boss Forums AQUATIC VEGETATION Identifying plants American water willow????

This from your link: "Water-willow flowers from May through October. The flowers are on long stems originating from the base of the leaves. Flowers are 5-petaled orchid-like (3/4 inch diameter), white with purple/violet streaks on the lower petals. Water-willow can spread from seeds and forms extensive rhizomes by which it forms colonies and spreads rapidly."

Your plant to me looks like the description on water willow. I would let it grow one more year until it flowers. Flowering will verify the name. I have never seen water willow and do not know much about it. Evidently it will not grow very deep, but can form dense colonies. A glysophate based herbicide should kill it when you decide to get rid of it.