I've believed this plant to be watercress ever since it was included (with warnings of its aggressive behavior) with the plant package by the company that installed our pond. I just noticed that the leaves are rather different from those in most pictures of watercress (see Watercress (Nasturtium officinale)), and the flowers don't look quite the same either. The leaves do taste a bit peppery, so perhaps it's a relative? The photo below was taken just yesterday, giving an idea of flowering time.

It could be Water Celery/Water Parsley/Oenanthe javanica.

Absolutely, that's it. Thanks! Hard to find information online that's not for the 'Flamingo' variegated cultivar.

It's a fine plant IF you do not let it escape captivity.

I had planned to say that the invasive potential of this plant is less of a problem in the colder climate of the northeast but can be quite invasive in warmer climates, but...I see that only recently have folks begun to realize that this plant can survive cold winters and grow on to spoil our natural waterways. Here is something from Wisconsin:

http://host.madison.com/news/l...